Acoustic device for achieving zero static bulk modulus, and method

Abstract

An acoustic device for achieving a zero static bulk modulus, and a method. The acoustic device may comprise: an acoustic cavity, having a compressible fluid medium therein; a composite plate, arranged to vibratably close an opening on at least one side of the acoustic cavity; and an instability introducing member, coupled to the composite plate, and arranged to provide negative stiffness to the composite plate during vibration of the composite plate, so as to counteract positive stiffness of both the acoustic cavity and the composite plate, thereby achieving a critical state of an acoustic unit, the critical state indicating that a static bulk modulus of the acoustic unit approaches or equals zero.

Description

Acoustic apparatus and method for achieving zero static bulk modulus

[0001] This application claims priority to U.S. Provisional Application No. 63 / 670,118, filed July 12, 2024, entitled “Acoustic Blackbody”; U.S. Provisional Application No. 63 / 712,467, filed October 27, 2024, entitled “High-efficiency Speaker based on Acoustic Blackbody”; and U.S. Provisional Application No. 63 / 787,918, filed April 13, 2025, entitled “Acoustic Devices with Zero Static Modulus”, the entire contents of which are incorporated herein by reference. Technical Field

[0002] This disclosure relates to the field of acoustics, and more specifically to an acoustic apparatus and method for achieving zero static bulk modulus. Background Technology

[0003] Acoustic absorption and radiation technologies have crucial applications in architectural acoustics, noise control, audio equipment, underwater communications, and medical imaging, among others. An ideal sound absorber, often called an "acoustic blackbody," is similar to an electromagnetic blackbody and can theoretically absorb 100% of incident sound energy across the entire wavelength range. Similarly, an ideal acoustic radiator can effectively convert input energy into sound radiation across the entire frequency spectrum. Summary of the Invention

[0004] One of the purposes of this disclosure is to enhance the absorption or sound radiation performance of existing acoustic devices.

[0005] According to a first aspect of this disclosure, an acoustic unit is provided. The acoustic unit includes: an acoustic cavity having a compressible fluid medium therein; a composite plate arranged to vibrately close an opening on at least one side of the acoustic cavity; and an instability-introducing member coupled to the composite plate and arranged to provide negative stiffness to the composite plate during vibration to counteract the positive stiffness of both the acoustic cavity and the composite plate, thereby achieving a critical state of the acoustic unit, wherein the critical state indicates that the static bulk modulus of the acoustic unit approaches or equals zero.

[0006] It will be understood that the instability mechanism introduced by the aforementioned instability-introducing component allows the acoustic unit to reach a critical state where the static bulk modulus approaches or equals zero. In particular, under this critical state, the acoustic unit can achieve absorption performance close to that of an acoustic blackbody or an acoustic radiation efficiency close to 1 over a low-frequency and wide bandwidth range.

[0007] In some embodiments, in the critical state, the fundamental frequency of the acoustic unit is less than or equal to 50 Hz. According to the definition of this disclosure, this fundamental frequency can be used to characterize the aforementioned critical state.

[0008] In some embodiments, the composite plate includes: a vibratory and rigid central portion to which the instability-introducing member is coupled; a flexible transition portion surrounding the central portion; and an attachment portion located around the transition portion and arranged to secure one end of the transition portion opposite to the central portion to a housing structure defining the acoustic cavity. It will be understood that a composite plate of this design can suitably serve as a carrier for the aforementioned instability-introducing member.

[0009] In some embodiments, the central portion is provided with reinforcing ribs on at least one side. In this way, the central portion structure can be strengthened, thereby forming a better rigid connection with the instability-introducing member.

[0010] In some embodiments, the central portion covers more than 90% of the area of ​​the opening of the acoustic cavity. In this way, it can vibrate more effectively in response to sound waves from the external environment or external driving forces such as acoustic actuators.

[0011] In some embodiments, the instability-introducing component includes an electromagnetic structure comprising: a current-carrying coil integrated near the edge of the composite plate and adapted to receive current; and a magnet structure arranged around the coil, wherein the magnitude of the current is adapted to be tuned such that the current through the current-carrying coil interacts with a magnetic field generated by the magnet structure, producing a negative stiffness force on the composite plate against the restoring force of the acoustic cavity and the composite plate caused by the current-carrying coil deviating from its equilibrium position. It will be understood that this electromagnetic structure scheme can very effectively introduce instability.

[0012] In some embodiments, the magnet structure comprises magnets of a Halbach array for generating a quadrupole field.

[0013] In some embodiments, the magnet structure generates a magnetic field distribution of one of a quadrupole field, a hexapole field, or an octapole field, and the current-carrying coil is located at the center of symmetry of the magnetic field distribution when in the equilibrium position.

[0014] In some embodiments, the magnet structure includes four magnetic poles for generating the quadrupole field, wherein any two adjacent magnetic poles have opposite polarities, and the current-carrying coil is positioned at the center of the quadrupole field. It will be understood that this provides an alternative implementation of the magnet for the Halbach array described above.

[0015] In some embodiments, the acoustic unit further includes a current control device adapted to maintain the magnitude of the current at or slightly below a critical current level to achieve a critical state of the acoustic unit. It will be understood that this can advantageously maintain the stability of the acoustic unit.

[0016] In some embodiments, the instability-introducing component includes a mechanical structure comprising: a plurality of elastic beams, one end of each beam converging to form a convergence portion attached to the center of the composite plate, and the other end of each elastic beam attached to a sidewall structure defining the acoustic cavity. The plurality of elastic beams are radially distributed, each beam being mounted in a pre-compressed state and adapted to generate a negative stiffness force on the composite plate resisting the restoring force of the acoustic cavity and the composite plate caused by the composite plate's deviation from its equilibrium position during vibration. It will be understood that this mechanical structure can very effectively introduce the aforementioned instability.

[0017] In some embodiments, the forces exerted by the plurality of elastic beams on the composite plate are balanced in the planar direction of the composite plate. It will be understood that this advantageously maintains the stability of the composite plate during vibration.

[0018] In some embodiments, the acoustic unit further includes a pre-compression mechanism disposed near the other end of each elastic beam and adapted to tune the pre-compression level of each elastic beam when the elastic beams are mounted to the acoustic unit. In this way, adjustment of the pre-compression of the elastic beams can be conveniently provided.

[0019] In some embodiments, the acoustic unit is a sound-absorbing unit, and the acoustic unit further includes a sound dissipation component for dissipating sound, which is positioned within or outside the acoustic cavity, adjacent to or integrated with the composite panel. In some embodiments, the sound dissipation component is the microperforated plate (MPP), which is positioned adjacent to the composite panel and arranged substantially parallel to the composite panel within or outside the acoustic cavity. In some embodiments, the central portion of the composite panel is formed by the microperforated plate, which serves as the sound dissipation component. It will be understood that this provides different implementations of the sound dissipation component.

[0020] In some embodiments, the acoustic unit is a sound-absorbing unit, and in the critical state, the sound absorption rate of the sound-absorbing unit increases with increasing wavelength.

[0021] In some embodiments, under the critical state, the sound-absorbing unit exhibits a sound absorption rate of 90% for sound wavelengths that are 100 times or more greater than the unit thickness d, wherein the unit thickness d is defined as the thickness from the upper surface of the composite plate to the surface of the acoustic cavity facing the composite plate.

[0022] In some embodiments, the acoustic unit is a sound radiation unit, wherein the sound radiation unit further includes a sound driving device configured to drive the composite plate to vibrate to generate sound; wherein, in the critical state, the sound radiation efficiency of the sound radiation unit approaches 1 as the wavelength of the generated sound increases.

[0023] In some embodiments, the acoustic driving device is a voice coil or piezoelectric component, which is coupled to the composite plate.

[0024] In some embodiments, the acoustic unit further includes at least one sensor configured to monitor one or more of acoustic response, composite plate displacement, and acoustic impedance. It will be understood that this is highly advantageous for real-time adjustment of the acoustic characteristics of the acoustic unit.

[0025] According to a second aspect of this disclosure, a noise reduction device is provided. The noise reduction device includes two or more acoustic units as sound-absorbing units as described above, and the two or more acoustic units are arranged in series and / or parallel.

[0026] According to a third aspect of this disclosure, a sound radiation device is provided. The sound radiation device includes two or more acoustic units as sound radiation elements as described above, and the two or more acoustic units are arranged in series and / or parallel.

[0027] In some embodiments, the plurality of acoustic units in the noise reduction device or the plurality of acoustic units in the sound radiation device are arranged in series and / or in parallel.

[0028] According to a fourth aspect of this disclosure, a method for fabricating an acoustic unit is provided, comprising: providing an acoustic cavity having a compressible fluid medium therein; providing a vibrating composite plate to close an opening on at least one side of the acoustic cavity; providing an instability introduction member and coupling it to the vibrating composite plate such that the instability introduction member is adapted to provide a negative stiffness to the composite plate during vibration of the composite plate, the negative stiffness being able to counteract the positive stiffness of both the acoustic cavity and the vibrating composite plate, thereby achieving a critical state of the acoustic unit, wherein the critical state indicates that the static bulk modulus of the acoustic unit approaches or equals zero.

[0029] According to a fifth aspect of this disclosure, an acoustic device is provided, comprising: an acoustic cavity having a compressible fluid medium of a certain volume; a composite plate enclosing at least one surface of the acoustic cavity, the composite plate being configured to vibrate in response to changes in sound pressure; a magnetic structure for generating a multipole magnetic field, the magnetic structure being fixedly positioned with respect to the acoustic cavity; and a current-carrying coil attached to the composite plate and located within the multipole magnetic field, wherein the interaction between a current through the coil and the magnetic field generates a negative stiffness force resisting the restoring force of the acoustic cavity and the composite plate caused by the composite plate deviating from its equilibrium position, thereby reducing the effective bulk modulus of the acoustic device.

[0030] According to a sixth aspect of this disclosure, an acoustic device is provided, comprising: an acoustic cavity having a compressible fluid medium of a certain volume; a composite plate enclosing at least one surface of the acoustic cavity, the composite plate being configured to vibrate in response to changes in sound pressure; and a plurality of elastic beams arranged in a radial pattern, each elastic beam being mounted in a pre-compressed state and coupled to the composite plate, the elastic beams being configured to generate a negative stiffness force resisting the restoring force of the acoustic cavity and the composite plate caused by the composite plate deviating from its equilibrium position when operating in a post-buckling state, thereby reducing the effective bulk modulus of the acoustic device.

[0031] According to a seventh aspect of this disclosure, a method for achieving broadband sound absorption is provided, comprising: providing an acoustic cavity enclosed by a composite plate, the acoustic cavity having a volume of compressible fluid medium; coupling an instability-introducing member to the composite plate, the instability-introducing member being configured to provide negative stiffness to the composite plate; and adjusting the negative stiffness to achieve a critical state in which the static bulk modulus of the acoustic cavity, the composite plate, and the instability-introducing member as a whole approaches or equals zero, thereby achieving broadband sound absorption that increases with wavelength.

[0032] According to an eighth aspect of this disclosure, a method for achieving broadband acoustic radiation is provided, comprising: providing an acoustic cavity enclosed by a composite plate, the acoustic cavity having a volume of compressible fluid medium; coupling an instability-introducing member to the composite plate, the instability-introducing member being configured to provide negative stiffness to the composite plate; adjusting the negative stiffness to achieve a critical state in which the static bulk modulus of the acoustic cavity, the composite plate, and the instability-introducing member as a whole approaches or equals zero; and applying an external driving force to the composite plate via an acoustic driving device to generate acoustic radiation with an efficiency close to 1 over a wide frequency band.

[0033] According to a tenth aspect of this disclosure, a sound absorber for an airflow environment is provided, comprising: an acoustic cavity having a composite plate enclosed by a smooth outer surface, the acoustic cavity containing a volume of compressible fluid medium; an instability-introducing member coupled to the composite plate, the instability-introducing member being configured to provide negative stiffness to the composite plate, thereby reducing the effective bulk modulus of the acoustic cavity, the composite plate, and the instability-introducing member as a whole; and a sound dissipation component coupled to the acoustic cavity or the composite plate, wherein the smooth outer surface of the composite plate reduces interference with airflow, and the reduced effective bulk modulus enables broadband acoustic absorption.

[0034] According to the eleventh aspect of this disclosure, an acoustic device is provided, comprising: a high-frequency sound absorption structure; and a low-frequency sound absorption structure, the low-frequency sound absorption structure comprising the acoustic unit as a sound-absorbing unit as described above, an acoustic device as a sound-absorbing apparatus, or a sound absorber as described above.

[0035] It should also be understood that the description in the Summary of the Invention is not intended to limit the key or essential features of the embodiments of this disclosure, nor is it intended to restrict the scope of this disclosure. Other features of the embodiments of this disclosure will become readily apparent from the following description. Attached Figure Description

[0036] The above and other features, advantages, and aspects of the embodiments of this disclosure will become more apparent from the accompanying drawings and the following detailed description. In the drawings, the same or similar reference numerals denote the same or similar elements, wherein:

[0037] Figure 1 schematically illustrates the principle of how instability can be used to improve the absorption limit;

[0038] Figure 2 shows a schematic diagram of an acoustic unit using an electromagnetic structure as an instability-introducing component according to a first exemplary embodiment of the present disclosure, wherein (a) shows a three-dimensional structural schematic diagram of an acoustic unit equipped with a micro-perforated plate (MPP) as a sound-absorbing unit; (b) shows a magnet structure of a Halbach array for generating a quadrupole field, wherein the central circle with an x-shaped band represents a coil; (c) shows the quadrupole field distribution corresponding to the magnet structure of the Halbach array; (d) shows a schematic diagram of an acoustic unit used as a sound-radiating unit under the action of an external driving force, such as an acoustic driving device; (e) shows a partial structural schematic diagram of the composite plate of the acoustic unit; and (f) shows a schematic diagram of the parameter η of the acoustic unit as a function of the coil current I; and (g) shows a typical configuration of the magnets of the Halbach array relative to the coil and its magnetic field distribution.

[0039] Figure 3 shows a schematic diagram of each intrinsic mode and its frequency corresponding to the electromagnetic structure scheme of the first example embodiment described above;

[0040] Figure 4 shows a related schematic diagram of a modified acoustic unit according to a first exemplary embodiment of the present disclosure, wherein (a) shows a three-dimensional structural schematic diagram of the acoustic unit as a sound-absorbing unit; (b) shows an enlarged schematic diagram of the electromagnetic structure for generating a quadrupole field; (c) shows a distribution diagram of the position of the quadrupole field relative to the current-carrying coil; and (d) shows a more detailed magnetic field distribution of the quadrupole field.

[0041] Figure 5 shows a schematic diagram of an acoustic unit using a mechanical structure as an instability-introducing component according to a second exemplary embodiment of the present disclosure, wherein (a) shows a three-dimensional structural schematic diagram of the acoustic unit equipped with a micro-perforated plate (MPP) as a sound-absorbing unit; (b) shows an arrangement of multiple elastic beams; (c) shows a schematic diagram of the acoustic unit as a sound-radiating unit under the action of an external driving force, such as an acoustic driving device; (d) shows a partial structural schematic diagram of the combination of the composite plate and elastic beams of the acoustic unit; and (e) shows a schematic diagram of the variation of the parameter η of the acoustic unit with the displacement δL of the beam;

[0042] Figure 6 shows a schematic diagram of the various intrinsic modes and their frequencies corresponding to the mechanical structure scheme of the second example embodiment described above;

[0043] Figure 7 shows the acoustic performance corresponding to the electromagnetic structure scheme of the first example embodiment described above, where (a) shows the effective bulk modulus B under different η parameter values. eff (a) shows the variation of the absorptivity α with wavelength λ / d for different η parameter values; (b) shows the variation of the absorptivity α with wavelength λ / d for different η parameter values; (c) shows the variation of the acoustic radiation efficiency with wavelength λ / d for different η parameter values, where circles ○ represent simulation results and curves represent theoretical predictions.

[0044] Figure 8 shows the acoustic performance corresponding to the electromagnetic structure scheme of the second example embodiment described above, where (a) shows the effective bulk modulus B under different η parameter values. eff (a) shows the variation of the absorptivity α with wavelength λ / d for different η parameter values; (b) shows the variation of the absorptivity α with wavelength λ / d for different η parameter values; (c) shows the variation of the acoustic radiation efficiency ε with wavelength λ / d for different η parameter values, where circles ○ represent simulation results and curves represent theoretical predictions.

[0045] Figure 9a shows the relationship between the normalized absorption integral and the ratio of current I to the critical value Icr according to the first example embodiment described above.

[0046] Figure 9b shows the normalized absorption integral as a function of the pore size critical value of a conventional back-cavity MPP scheme (i.e., the scheme of the first example embodiment described above without the instability-introducing component). Aperture value of MPP A graph showing the relationship between the ratios, where

[0047] Figure 10a illustrates the process by which the sound absorption rate approaches the ideal blackbody behavior under different sound absorber thicknesses d; and

[0048] Figure 10b shows a schematic diagram of the relationship between the acoustic absorption rate of the acoustic unit according to a first exemplary embodiment of the present disclosure, verified by numerical simulation, the deviation Δ of the ideal blackbody behavior, and the thickness d of the acoustic absorbing device. Detailed Implementation

[0049] Embodiments of this disclosure will now be described in more detail with reference to the accompanying drawings. While some embodiments of this disclosure are shown in the drawings, it should be understood that this disclosure can be implemented in various forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided to provide a more thorough and complete understanding of this disclosure. It should be understood that the accompanying drawings and embodiments of this disclosure are for illustrative purposes only and are not intended to limit the scope of protection of this disclosure.

[0050] As mentioned earlier, acoustic absorption and radiation technologies have crucial applications in architectural acoustics, noise control, audio equipment, underwater communications, and medical imaging, among others.

[0051] However, the inventors discovered that fundamental physical constraints limit the performance of traditional acoustic devices. The most important of these constraints is the principle of causality, which imposes a mathematical limit on the absorption bandwidth of any passive acoustic system. This constraint can be expressed as:

[0052] Where Σ represents the total energy loss across the entire spectrum, α(λ) represents the absorptivity at wavelength λ, d represents the absorber thickness, ρ represents the fluid density, c represents the speed of sound, and B ∞ This represents the static bulk modulus of the system.

[0053] The above-mentioned relationship (1), derived from the principle of causality, shows that broadband absorption in a system with finite thickness is fundamentally limited. Traditional materials struggle to achieve ideal blackbody absorption, and their fundamental limitation lies in the fact that the absorption bandwidth is fundamentally limited by the ratio of the device thickness to the system's static bulk modulus. In traditional materials and structures, reducing the bulk modulus faces significant challenges.

[0054] For example, for an acoustic system using an inflatable cavity, the static bulk modulus B ∞It depends on the thermodynamic process. Under near-adiabatic conditions, B ∞ =γp atm Where γ>1 is the gas adiabatic index, p atm At atmospheric pressure. Under effective heat exchange, B ∞ Under isothermal conditions, it can approach p atm However, this represents the fundamental lower limit of inflatable cavity design.

[0055] While adding solid materials into the cavity could be considered, this would inevitably increase the static bulk modulus. Furthermore, due to causality, this would limit the absorption bandwidth. Moreover, although near-zero effective bulk modulus can be achieved using open-back cavity systems, they require impractically large spaces, thus limiting their practical applications.

[0056] As a further example, traditional acoustic absorption methods mainly rely on the following types of materials and structures, each with its own drawbacks:

[0057] ● Porous materials. Porous materials such as foams, fiber structures, and perforated panels have been widely used for sound absorption. These materials typically absorb sound by converting sound energy into heat energy through viscosity and thermal loss as sound waves propagate through the porous structure. While they are effective at higher frequencies, their performance degrades significantly at lower frequencies because the wavelengths at low frequencies are much larger than the material thickness.

[0058] ● Resonant Structures. Resonant absorbers, including Helmholtz resonators, membrane absorbers, and microperforated plates (MPPs), utilize resonance to achieve high absorption rates at specific frequencies. These structures can be tuned for specific frequency bands but typically exhibit narrow bandwidth absorption centered on the resonant frequency.

[0059] ● Metamaterials. Acoustic metamaterials are engineered structures designed to exhibit properties not found in natural materials, and have shown potential for enhanced absorption performance. Various designs, including spatially coiled, labyrinth structures, and membrane acoustic metamaterials, have demonstrated improved absorption characteristics. However, these structures remain constrained by fundamental causal relationships and often rely on resonance phenomena, thus limiting their broadband performance.

[0060] Therefore, despite extensive research, the fundamental trade-off between absorber thickness and absorption bandwidth remains an ongoing challenge. Causal constraints suggest that achieving truly broadband absorption / radiation in compact devices requires a fundamentally different approach to controlling the static bulk modulus of the system.

[0061] Recent theoretical and experimental studies have explored time-varying systems, active control schemes, and negative stiffness materials as potential pathways to overcome these fundamental limitations. However, a practical passive system capable of approaching the behavior of an ideal blackbody in both absorption and radiation remains elusive.

[0062] Therefore, there is an urgent need for a new method that can effectively reduce causal constraints, which can eliminate the need for active components, impractical large sizes, or complex time-varying characteristics.

[0063] As shown in formula (1) above, the absorption bandwidth is fundamentally limited by the absorber thickness d and the static bulk modulus B. ∞ The ratio. Unlike the approach of reducing the absorption bandwidth by decreasing the thickness d of the absorber, this disclosure considers reducing the static bulk modulus B at the critical state. ∞ Reduced to zero, thus achieving unprecedented broadband absorption.

[0064] To this end, this disclosure proposes a novel system and method that achieves the aforementioned static bulk modulus B through a controlled instability mechanism. ∞ The critical state is close to or equal to 0. By precisely balancing the negative stiffness provided by the component introduced by the instability with the positive stiffness of the system itself, an acoustic cavity with unprecedented broadband absorption and radiation capabilities is achieved, fundamentally removing the traditional limitations imposed by causal constraints.

[0065] To better understand the principles of this disclosure, Figure 1 schematically illustrates how instability can be used to improve the absorption limit.

[0066] As shown in Figure 1(a), let's assume a typical acoustic system 10, which includes a cylindrical acoustic cavity 11 containing a certain volume of compressible fluid medium; a composite plate 12 that closes (e.g., seals) one side of the acoustic cavity 10; and a micro-perforated plate 15 (MPP), adjacent to the composite plate 12 and positioned on the opposite side of the acoustic cavity 11, which can serve as a sound dissipation component, where d can represent the distance from the upper surface of the composite plate 12 to the lower surface of the acoustic cavity. As shown in Figure 1(b), this acoustic system 10 can be equivalent to a spring system with positive stiffness k. In this case, we have: k = SB ∞ / d (2)

[0067] Where S is the opening area of ​​the cylindrical acoustic cavity 11.

[0068] According to the concept of this disclosure, as shown in Figure 1(c), a controlled instability mechanism is introduced by coupling an instability-introducing member that provides a negative stiffness k' to the composite plate 12. The negative stiffness introduced by this instability-introducing member can be designed to at least partially or completely cancel the positive stiffness of the original acoustic system 10, thereby reducing the static bulk modulus of the entire acoustic system. In some embodiments, by adjusting the value of the negative stiffness k', it is possible to allow the value of the negative stiffness k' to be close to or exactly equal to the value of the positive stiffness k, thereby achieving a critical state where the static bulk modulus is close to or equal to 0. It should be understood that complete cancellation of the positive and negative stiffnesses is the most ideal state, where the static bulk modulus is 0 and the fundamental frequency of the acoustic unit is 0H. Z However, in reality, when the negative stiffness roughly cancels out the positive stiffness, causing the static bulk modulus of the acoustic unit to approach zero, the fundamental frequency of the acoustic unit can be reduced to below 50Hz. At this point, the acoustic unit can also be considered to have reached a critical state. It should also be understood that at the fundamental frequency of the acoustic unit, sound waves will be completely absorbed or radiated at maximum. Outside the fundamental frequency, the overall absorption or radiation spectrum of the acoustic unit will increase with increasing wavelength in frequency bands above the fundamental frequency, and decrease with increasing wavelength in frequency bands below the fundamental frequency.

[0069] To better understand, the composite plate 12 can be compared to the small ball in Figure 1(d) and (e). The positive stiffness k allows the small ball to travel along the upper curve in Figure 1(d), while the negative stiffness k' allows the small ball to travel along the lower curve in Figure 1(d). Under the above critical conditions, the two can be close to or always completely cancel each other out in the entire acoustic system, thereby achieving the state of random equilibrium (or critical equilibrium or neutral equilibrium) of the small ball as shown in Figure 1(e).

[0070] It should be noted that the term "instability" used in this article should be understood as the characteristic that any disturbance (such as displacement or deformation) deviating from the equilibrium state of a component (e.g., the aforementioned composite plate, when not coupled to the outside) will cause its potential energy to decrease, thus evolving in the direction of amplifying the disturbance. For example, when the aforementioned composite plate deviates from its equilibrium position and undergoes displacement due to a response to sound waves, the positive stiffness of the composite plate itself tends to evolve in the direction of stability towards the equilibrium state, while the aforementioned instability-introducing component tends to resist the evolution in the direction of stability caused by positive stiffness by, for example, providing negative stiffness, i.e., evolving in the direction of amplifying the disturbance.

[0071] Specifically, when the negative stiffness provided by the instability-introducing component is exactly equal to the aforementioned positive stiffness, the composite plate can achieve so-called "adaptive equilibrium." It should be understood that the term "adaptive equilibrium" here refers to the state where an object in equilibrium, when subjected to a small disturbance by an external force and deviating from its equilibrium position, can still achieve equilibrium in its new position; such equilibrium is called adaptive equilibrium.

[0072] To further study this acoustic system, its total potential energy function U(x) can be expressed as: U(x) = U stable (x)+U unstable (x) (3)

[0073] Among them U stable (x) represents the potential energy of the stable component (usually a quadratic function with positive curvature), U unstable (x) represents the potential energy of the unstable component (with a region of negative curvature).

[0074] At the critical state, the second derivative of the total potential energy at the equilibrium position will approach zero:

[0075] This state creates a virtually "flat" potential energy map around the equilibrium position, corresponding to a static stiffness of 0. For small displacements away from the equilibrium position, the restoring force approaches zero, resulting in a zero static modulus within the acoustic cavity.

[0076] To provide the mathematical basis for this method, we will, without loss of generality, consider using a potential energy of... A composite plate (or elastic plate) encloses (e.g., seals) a cavity, wherein λ iklm It is the elastic modulus tensor. u l This represents local displacement. When coupled with an instability-introducing component, the composite plate (or elastic plate) experiences an additional potential energy term, which can typically be expressed as:

[0077] Therefore, the total potential energy E is as follows:

[0078] Formula (6) reveals two fundamental mechanisms that introduce instability: (i) negative elastic contribution (-λ') iklm (ii) Position-dependent force terms (-κ) il These are forces used to generate instability. In some embodiments, these two mechanisms can be physically implemented through one or more of the following: mechanical, electrical, magnetic, or electromagnetic.

[0079] For harmonic motion with frequency ω = 2πc / λ, the equation of motion can be expressed as:

[0080] Where ρ pl This indicates the density distribution of the plate.

[0081] For those that are close The frequency, where ω0 is the natural resonant frequency of the elastic plate, and The frequency introduced for instability, when the response is composed of an eigenmode When dominant, the effective bulk modulus B of the entire acoustic system eff It becomes:

[0082] It should be understood that, at zero frequency (infinite wavelength), the effective bulk modulus B eff It will approach the static bulk modulus B. ∞ Therefore, in the critical state, ω = 0, B eff =B ∞ =0, then:

[0083] Formula (9) forms the theoretical basis of the method disclosed herein: unstable potential energy in the mode The intrinsic components of the system precisely cancel out the elasticity of the system at zero frequency.

[0084] For the piston-like motion of a composite plate (or elastic plate) (where S and σ are the surface area and thickness of the plate, respectively), Z l (where the unit normal direction is used), the effective bulk modulus becomes: B eff ≈(1-η)ρc 2 -ω 2 md / S (10)

[0085] in It can be used to characterize the energy ratio between unstable potential energy and intrinsic elastic potential energy. In the static bulk modulus B... ∞ It is zero (or equivalent to the effective bulk modulus B). eff In the critical state of η=0, we have η=1, which can be physically interpreted as force balance, where the unstable force exactly cancels out the restoring force from the composite plate and the acoustic cavity.

[0086] The following examples will illustrate how to achieve the above static bulk modulus (or effective bulk modulus B). eff (a) The critical state that is close to or equal to 0.

[0087] Figure 2 shows a schematic structural diagram of an acoustic unit using an electromagnetic structure as an instability-introducing component according to a first exemplary embodiment of the present disclosure.

[0088] As shown in Figure 2(a), the acoustic unit 20 includes at least: an acoustic cavity 21, a composite plate 22, and an instability introduction component 23.

[0089] The acoustic cavity 21 serves to contain a certain volume of compressible fluid medium. As an example, the acoustic cavity 21 can be generally cylindrical. It should be understood that acoustic cavities of other shapes are also possible.

[0090] The composite plate 22 serves to vibratoryly close (e.g., seal (including tight and loose seals)) the opening on at least one side of the acoustic cavity and is adapted to receive sound waves transmitted from the corresponding side or radiate sound waves toward the corresponding side. In Figure 2, the composite plate 22 only closes (e.g., seals) one side of the opening of the acoustic cavity; however, this is not a limitation. In other embodiments, it is possible to arrange additional composite plates 22 to close (e.g., seal) other sides of the acoustic cavity. In particular, for example, two composite plates 22 may close (e.g., seal) both sides of the acoustic cavity 21, or three composite plates 22 may close (e.g., seal) three sides of the acoustic cavity 21, or four composite plates 22 may close all four sides of the acoustic cavity 21. In a more particular embodiment, all sides of the acoustic cavity 21 may even be closed with corresponding composite plates 22.

[0091] As an example, Figure 2(e) shows a schematic diagram of the structure of a composite plate according to an exemplary embodiment of the present disclosure.

[0092] As shown in Figure 2(e), the composite plate 22 includes a vibratory and rigid central portion 221, a flexible transition portion 222, and an attachment portion (not shown). The composite plate 22 serves to couple the instability-introducing component to the compressible fluid medium (or acoustic medium) within the acoustic cavity while maintaining the required mechanical properties.

[0093] The central part 221 is used to vibrate in response to sound waves transmitted from the external environment or external driving forces such as sound-driven devices.

[0094] In some embodiments, the central portion is reinforced to form a rigid connection with the instability-introducing member. The central portion will move as a whole in response to the force and sound pressure of the instability-introducing member. As an example, reinforcing ribs may be provided on at least one side to enhance the rigidity of the central portion, wherein the at least one side may be the side facing the acoustic cavity and / or the side facing outwards from the acoustic cavity. In yet other embodiments, the reinforcing ribs may be formed in a honeycomb structure (or a hexagonal lattice frame structure).

[0095] In some embodiments, the central portion 221 may be a planar structure. In yet other embodiments, the central portion 221 may also be a dome-shaped shell structure or a dome structure. For example, the dome-shaped shell structure or dome structure may resemble a portion of an eggshell. Furthermore, the dome structure may protrude either outward or inward from the acoustic cavity.

[0096] In this way, the rigidity of the central part 221 is enhanced while the weight is reduced, which may be very advantageous for specific applications that require faster response speed or reduced mass.

[0097] In some embodiments, the central portion may cover more than 50% of the opening area of ​​the acoustic cavity, for example, 50%, 60%, 70%, 80%, 90%, 95%, 99%, or 100%. In particular, in embodiments where the central portion is a planar structure, the area of ​​the central portion 221 may be substantially the same as the opening area of ​​the acoustic cavity 21, thereby maximizing the vibration response to sound waves from the external environment or external driving forces such as acoustic actuators.

[0098] The transition portion 222 is flexible and arranged around the central portion 221. In some embodiments, the transition portion may include pleats or a flexible pattern with pleats to achieve controlled deformation while minimizing forces from the edges as the composite plate moves in a piston-like motion. This region is crucial for allowing the central portion to move freely while maintaining its connection with the outer ring.

[0099] An attachment portion (not shown), positioned around the periphery of the transition portion 222, primarily functions to seal and secure the structure. Specifically, this attachment portion secures the composite plate 22 to the periphery of the acoustic cavity while maintaining an acoustic seal. This portion must strike a balance between flexibility and sealing performance to prevent acoustic leakage without significantly affecting the movement of the central portion. As an example, the end of the transition portion 222 opposite to the central portion 221 can be fixed to the shell structure (e.g., wall structure) defining the acoustic cavity 21. In particular, the attachment portion can be annular.

[0100] As a material for implementation, in some embodiments, the rigid central portion may be made of PMMA, for example. In some embodiments, the transition portion 222 may be made of PDMS, for example. However, many other materials may be suitable depending on specific application requirements. For example, for the rigid central portion, alternatives include, but are not limited to, lightweight metals (aluminum, titanium alloys), carbon fiber composites, or engineering plastics such as polycarbonate and ABS. For the transition portion, alternatives may include silicone elastomers of varying hardness, thermoplastic elastomers (TPEs), polyurethane elastomers, or specially formulated flexible epoxy resins. It should be noted that mechanical properties, durability, environmental stability, and manufacturing compatibility should be considered when selecting materials.

[0101] As previously described, it will be understood that the instability-introducing member 22 serves to couple to the composite plate 22 and to provide negative stiffness to the composite plate 22 during vibration, thereby at least partially offsetting the positive stiffness of both the acoustic cavity and the composite plate, and thus reducing the static bulk modulus of the acoustic unit. In some embodiments, the magnitude of the negative stiffness can be tuned to completely or nearly completely offset the positive stiffness of both the acoustic cavity and the composite plate, thereby achieving a critical state of the acoustic unit, wherein the critical state indicates that the static bulk modulus of the acoustic unit 20 approaches or equals zero.

[0102] This disclosure does not limit the specific implementation of the instability-introducing component 23, which may be selected from one or more of electrical, magnetic, electromagnetic or mechanical structures, as long as it can at least partially offset the positive stiffness of both the acoustic cavity and the composite plate.

[0103] As an example, the instability-introducing component 23 in Figure 2 is an electromagnetic structure, which includes a current-carrying coil 231 and a magnet structure 232.

[0104] The aforementioned current-carrying coil 231 can be integrated near the edge of the composite board 22 and is adapted to receive current. In some embodiments, the current-carrying coil 231 can be a single-turn or multi-turn coil.

[0105] The magnet structure 232 is arranged around the current-carrying coil 231, wherein the magnitude of the current is adapted to interact with the magnetic field of the magnet structure to generate a negative stiffness force on the composite plate that resists the restoring force of the acoustic cavity and the composite plate caused by the composite plate deviating from its equilibrium position.

[0106] In some embodiments, the magnet structure 232 can generate a magnetic field distribution of one of a quadrupole, a hexapole, or an octupole field, with the current-carrying coil located at the center of symmetry of the magnetic field distribution in its equilibrium position. In some embodiments, as shown in FIG2(b), the magnet structure 232 can be a magnet of a Halbach array for generating a quadrupole magnetic field. FIG2(c) shows a schematic diagram of the field distribution of the quadrupole magnetic field, and (g) shows a more detailed configuration of the Halbach array magnet and its magnetic field distribution.

[0107] As can be seen from Figure 2(g), the Hellbeck array can be composed of 16 toroidal magnets with systematic magnetization directions (as indicated by the white arrows in (g)). This structure generates a near-perfect quadrupole magnetic field in the central region of the toroids, with the local magnetic field direction indicated by the black arrows, and its length proportional to the field strength. The central circle represents a current-carrying coil located within this magnetic field.

[0108] In this example, the outer and inner radii of the Hellbeck array's annular column are designed to be 1.5 mm and 0.325 mm, respectively, maintaining a 50-micrometer gap with coil 231—sufficient to withstand displacement equivalent to 96 dB of airborne sound above 10 Hz. A 100-micrometer vertical gap within the magnet's annular column prevents collisions between the composite plate and the magnet during vibration, while maintaining the integrity of the internal magnetic field.

[0109] The magnets in a Heilbeck array can be arranged such that the magnetization directions of adjacent magnets differ by π / 8, generating a quadrupole magnetic field, characterized by B. r =2Kz, B z = -2K(r-r0), where (r0,0) represents the center of the loop. When current flows through the coil, any vertical displacement u z Both will generate Lorentz force F z =2KJu z ∫dV, this force amplifies the displacement. This generates unstable potential energy:

[0110] This corresponds to κ in the theoretical framework. 33 = 2KJ, where all λ′ iklm =0.

[0111] In some embodiments, the aforementioned current-carrying coil can be designed with specific geometry and current density to optimize its interaction with the Hellbeck array. As an example, a copper wire coil (0.55 mm thick, single turn, 26.5 mm radius) can be bonded to the surface of a composite board. When the coil is located at a specific distance from the array and carries a current I, the coil experiences a force: F = I∮dl × B (12)

[0112] Within the aforementioned quadrupole field, the force-displacement relationship is almost linear within the working region, with a negative slope, thus generating the controllable instability required for the zero static bulk modulus condition.

[0113] It should be understood that by precisely adjusting the current through the coil, the magnitude of the negative stiffness supplied to the composite plate can be adjusted, thereby allowing the entire acoustic unit to be tuned to a critical state, i.e.: k negative +k positive =0 (13)

[0114] The above adjustments can be performed manually or through an automatic control system that monitors the system's response and adjusts the current accordingly. In practice, the current is typically maintained slightly below a critical value to ensure stability and maximize performance.

[0115] In some embodiments, the acoustic unit 20 may further include a current control device (not shown) adapted to maintain a current slightly below a critical current level to ensure the stability of the composite plate 22 while maximizing the force used to counteract the negative stiffness of the positive stiffness. By precisely controlling the current flowing through the current-carrying coil, the entire acoustic system can be tuned to a critical state where the negative stiffness precisely counteracts the positive stiffness of both the composite plate and the acoustic cavity, thereby achieving a static bulk modulus of zero. It should also be understood that the embodiment of FIG2 has advantages in terms of dynamic adjustability, precise control, and integration with electronic systems.

[0116] As previously mentioned, η can be used to characterize the energy ratio between unstable potential energy and inherent elastic potential energy. Therefore, in this example embodiment, this parameter η can be precisely controlled by adjusting the coil current I. Specifically, η = 0 represents a conventional acoustic system without the aforementioned unstable instability-introducing component, while η = 1 represents that the unstable potential energy is exactly equal to the inherent elastic potential energy. This also means that the negative stiffness generated by the unstable potential energy can exactly offset the positive stiffness of the inherent acoustic system, thereby achieving a critical state where the static bulk modulus is close to or equal to 0. Figure 2(f) shows a diagram of η as a function of the current in the current-carrying coil.

[0117] In some embodiments, the acoustic unit 20 may be a sound absorption unit or a sound radiation unit.

[0118] Specifically, in embodiments where the acoustic unit 20 is a sound-absorbing unit, the acoustic unit 20 may further include a sound-dissipating element 24 designed to dissipate sound. According to the design of this disclosure, the sound-dissipating element may be positioned within or outside the acoustic cavity 21, adjacent to the composite plate 22, and / or integrated with the composite plate 22. In some embodiments, the sound-dissipating element 24 may be selected from the group consisting of microperforated plates (MPPs), composite plate materials having intrinsic viscosity, and sound-damping materials. In particular, when the composite plate itself is formed of a microperforated plate (MPP) or a material having intrinsic viscosity, the composite plate itself may serve as a sound-dissipating element, i.e., integrated with the composite plate.

[0119] As an example, the aforementioned sound dissipation element 24 can be a micro-perforated plate positioned adjacent to the composite plate and arranged substantially parallel to the composite plate within or outside the acoustic cavity. For instance, as illustrated in FIG2(a), the micro-perforated plate can be positioned adjacent to the outer side of the composite plate and substantially parallel to it. As another example, the central portion of the composite plate can be formed by the micro-perforated plate, in which case the central portion can simultaneously function as a sound dissipation element and vibrate in response to sound waves.

[0120] In embodiments where the acoustic unit 20 is a sound radiating unit, the acoustic unit 20 may not have the aforementioned sound dissipation element, and alternatively, the acoustic unit 20 may also include a sound driving device (not shown) designed to apply an external driving force P to the composite plate. drive This drives the composite plate to vibrate to produce sound, as shown in (d) of Figure 2.

[0121] As an example, the sound driving device could be a voice coil positioned in a magnetic field, coupled to a composite plate. Applying a current to the voice coil would cause the composite plate to vibrate and radiate sound. As another example, the sound driving device could be one of a piezoelectric component, an electrostrictive element, or a magnetostrictive element, coupled to the composite plate. Applying a voltage to the piezoelectric component or electrostrictive element, or a magnetic field to the magnetostrictive element, would apply an external driving force to the composite plate, causing it to vibrate and radiate sound.

[0122] The effectiveness of the electromagnetic scheme described above is verified by analyzing the eigenmodes. Figure 3 shows the eigenmodes of the electromagnetic implementation described above. Figures (a)-(f) in Figure 3 show the first six eigenmodes and their corresponding characteristic frequencies. The frequency of the first eigenmode is close to zero (0.257Hz), which approximates the theoretically expected zero-frequency mode generated by the neutral equilibrium state, corresponding to the critical state. The higher eigenmodes (≥1197Hz) have weaker coupling with the external plane wave, further proving the effectiveness of the design. This approximate piston-like motion over a wide frequency range verifies the accuracy of the predictions made by the design scheme.

[0123] Figure 4 shows a related schematic diagram of a modified acoustic unit according to a first exemplary embodiment of the present disclosure, wherein (a) shows a three-dimensional structural schematic diagram of the acoustic unit as a sound-absorbing unit; (b) shows an enlarged schematic diagram of the electromagnetic structure for generating a quadrupole field; (c) shows a distribution diagram of the position of the quadrupole field relative to the current-carrying coil; and (d) shows a more detailed magnetic field distribution of the quadrupole field.

[0124] The structure in Figure 4 is similar to that in Figure 2, but the difference is that the magnet structure used to generate the quadrupole field can be simply implemented with four magnetic poles.

[0125] As shown in Figures 4(a) and (b), any two adjacent magnetic poles of the four magnetic poles have opposite polarities, and the current-carrying coil 231 is positioned at the center of the quadrupole field. Specifically, in this example, the four magnetic poles include a first pair and a second pair. The first pair of magnetic poles has opposite polarities (e.g., N and S, respectively) and is provided by a first pair of annular magnets positioned on either side of the composite plate. The second pair of magnetic poles is located inside the first pair and also has opposite polarities (e.g., S and N, respectively), and is provided by a second pair of annular magnets positioned on either side of the composite plate. In some embodiments, the second pair of annular magnets can alternatively be provided by a pair of iron rings positioned on either side of the composite plate, wherein the transverse interface of each iron ring can be designed as L-shaped, having a transverse portion and a vertical portion. The transverse portion of each iron ring is attached to opposite magnetic poles in the first pair of magnets, which are the opposite magnetic poles of the first pair of magnetic poles furthest from the composite plate. In this manner, it should be understood that the arrangement and configuration cost of the quadrupole field are simplified.

[0126] Figure 5 shows a schematic structural diagram of an acoustic unit using a mechanical structure as an instability-introducing component according to a second exemplary embodiment of the present disclosure.

[0127] The structure in Figure 5 is similar to that in Figure 2, but the difference is that, instead of the electromagnetic structure described above, the acoustic unit 20 uses a mechanical structure 30 to constitute the aforementioned instability-introducing component. For the sake of brevity, the same components in Figures 5 and 2 will not be described again below; instead, the focus will be on describing the aforementioned mechanical structure.

[0128] As shown in Figure 5(b), the mechanical structure 30 may include: a plurality of elastic beams 31, one end of each elastic beam converging to form a converging portion 32, the converging portion 32 being attached to the center of the composite plate 22 (or the central portion), and the other end of each elastic beam being attached to a sidewall structure defining the acoustic cavity 21. The plurality of elastic beams may be radially distributed. Furthermore, the elastic beams 31 are installed in a pre-compressed state and are adapted to generate a negative stiffness force resisting the restoring force of the acoustic cavity and the composite plate caused by the composite plate deviating from its equilibrium position during vibration.

[0129] It will be understood that the aforementioned mechanical mechanism will utilize the post-buckling behavior of the pre-compressed elastic element to achieve the desired controllable instability.

[0130] This disclosure does not limit the number of the plurality of elastic beams 31, as long as the forces exerted by the plurality of elastic beams 31 on the composite plate 22 are balanced in the planar direction of the composite plate. Therefore, in some embodiments, the number of the plurality of elastic beams 31 may be even, for example, including four, six or eight elastic beams. In still other embodiments, the number of the plurality of elastic beams 31 may be odd, for example, including three, five, seven, nine, etc.

[0131] In some embodiments, the plurality of elastic beams 31 may be radially distributed. In some embodiments, the plurality of elastic beams 31 are symmetrically distributed. As an example, three elastic beams 31 may be radially distributed at an angle of 120 degrees to each other. As another example, six elastic beams 31 may be radially distributed at an angle of 60 degrees to each other.

[0132] In some embodiments, the aforementioned elastic beams can be uniformly compressed from their ends (i.e., the ends abutting against the walls of the acoustic cavity), introducing unstable potential energy into the composite plate via the converging portion 32, thereby enhancing its sensitivity and producing the desired softening effect. In still other embodiments, the cross-section of each elastic beam can be T-shaped, arcuate, or H-shaped to suppress lateral buckling modes. In some embodiments, the converging portion 32 of the aforementioned elastic beams can be axially connected to the center of the composite plate 22 via a thin hollow rod.

[0133] As an example, in the embodiment shown in Figure 4, six spoke-shaped PMMA beams with T-shaped cross-sections are arranged at 60-degree equiangular intervals. The beams are 0.35 mm thick and 2 mm wide. These T-shaped cross-sections are specially designed to suppress unwanted transverse modes, ensuring that principal buckling occurs in the desired direction.

[0134] In some embodiments, the pre-compression state (or buckling effect) of the aforementioned elastic beam 31 can be achieved by a pre-compression mechanism, which can precisely control the pre-compression state of the beam to adjust it to a critical state. As an example, the pre-compression mechanism can be implemented by various mechanical adjusters, including but not limited to screw adjustment mechanisms, lever systems, or thermal actuators. For instance, the pre-compression mechanism can be located near one end of each elastic beam (e.g., near the wall of the acoustic cavity) and is adapted to tune the pre-compression degree of each elastic beam when it is installed into the acoustic unit.

[0135] As previously mentioned, η can be used to characterize the energy ratio between unstable potential energy and inherent elastic potential energy. In this example embodiment, this parameter η can be precisely controlled by adjusting the displacement δL at the ends of each beam. Specifically, η = 0 indicates that this embodiment does not have any unstable introducing components, while η = 1 indicates that the unstable potential energy is exactly equal to the inherent elastic potential energy. This also means that the negative stiffness generated by the unstable potential energy can offset the positive stiffness of the inherent acoustic system, thereby achieving a critical state where the static bulk modulus is close to or equal to 0. Figure 2(e) shows a diagram of how η changes with displacement δL.

[0136] It will be understood that when the pre-compression force exceeds the critical buckling load, these beams will generate unstable potential energy:

[0137] In this publicly available design framework, this corresponds to λ′ iklm =(1 / 2)σ lm δ ik δ lm , of which all κ il =0.

[0138] The aforementioned pre-compression degree is adjustable and can be adjusted to reach a critical state, where the negative stiffness of the buckling beam exactly offsets the total positive stiffness of the composite plate and the acoustic cavity, i.e.: k beams +(k plate +k cavity )=0 (15)

[0139] In some embodiments, such adjustments can be made during manufacturing and may include on-site adjustments to accommodate changes in environmental conditions or specific application requirements. To achieve optimal stability and performance, the beam is typically kept pre-compressed to provide a negative stiffness slightly below a critical value.

[0140] Similar to the embodiment in Figure 2 above, the acoustic unit in the embodiment of Figure 5 can also be either a sound absorption unit or a sound radiation unit. When the acoustic unit is a sound absorption unit, it can be similarly arranged with sound dissipation elements; while when the acoustic unit is a sound radiation unit, it can also be driven by an acoustic driving device to apply an external driving force to the partition to achieve sound radiation (i.e., sound emission) of the acoustic unit, as shown in (c) of Figure 5.

[0141] Similarly, the effectiveness of the above mechanical scheme can be verified by analyzing the eigenmodes. Figure 6 shows the eigenmodes of the mechanical implementation scheme, where (a)-(f) of Figure 6 show the first six eigenmodes and their corresponding characteristic frequencies. The frequency of the first eigenmode is close to zero (0.181Hz), similar to the electromagnetic implementation scheme, confirming that the system almost achieves the required elastic cancellation through instability, i.e., achieving the so-called critical state. The higher eigenmodes (greater than or equal to 782Hz) have weaker coupling with the external plane wave, further proving the effectiveness of the above mechanical structure design. This approximate piston-like motion over a wide frequency range enhances the accuracy of the predictions made by this design scheme.

[0142] The following parameters will be used to measure the performance of the two implementation methods in terms of broadband absorption or radiation.

[0143] 1. Effective bulk modulus B eff

[0144] As shown in Figure 7(a) and Figure 8(a), when the normalized unstable energy (η) approaches 1, the effective bulk modulus B eff / (ρc 2 It approaches zero over a wide frequency range. This zero-modulus condition is crucial for achieving ultra-wideband acoustic performance.

[0145] Considering the higher-order modes of the cavity, the theoretical prediction of the effective bulk modulus is as follows:

[0146] Both implementation schemes show a high degree of consistency between theoretical predictions and simulation results. At shorter wavelengths, the deviation is small due to the influence of transverse higher-order cavity modes.

[0147] 2. Absorption rate

[0148] As shown in Figures 7(b) and 8(b), the absorption spectra of the two embodiments are almost identical at equivalent η values. When η approaches or equals 1 (i.e., the critical state), the absorption bandwidth of the sound-absorbing unit expands dramatically, reaching 100% at infinite wavelength. Most notably, unlike conventional materials, the absorption rate increases with increasing wavelength, and in particular, when λ is greater than 100d (e.g., 114d), the absorption rate can exceed 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, or 98%. Even more specifically, when λ is greater than 200d (e.g., 261d), the absorption rate can even exceed 99%, where d is defined as the thickness from the upper surface of the composite plate to the surface of the acoustic cavity facing the composite plate.

[0149] To achieve sound absorption, the acoustic cavity is coupled with a microperforated plate (MPP) to provide the necessary acoustic dissipation and minimize dispersion (Figures 2 and 5(a), i.e., the sound absorption scheme). The acoustic impedance characteristics of the MPP can be expressed as:

[0150] Where τ is the thickness of the microporous plate, The pore size of the microplate is... Let τ be the porosity and v be the kinematic viscosity of air. By optimizing these parameters (τ = 0.5 mm), This allows the real part to be equal to ρc, satisfying the impedance matching condition for maximum absorption.

[0151] Specifically, the absorption rate can be given by the following formula:

[0152] 3. Radiation efficiency

[0153] Figures 7(c) and 8(c) show that when η approaches or equals 1 (i.e., the critical state), both implementation schemes achieve near-perfect radiation efficiency, and as B... eff Approaching zero, the acoustic radiation efficiency ε of the acoustic radiation unit approaches 1 over an increasingly wider bandwidth (or with increasing wavelength). This characteristic, possessing both absorption and radiation properties, confirms the principle of absorption-radiation reciprocity.

[0154] It should be noted that, regarding radiation, we can remove the MPP and directly apply driving pressure P to the composite board using a sound-driven device. drive (Figure 2(d) and Figure 5(c), i.e., acoustic radiation schemes). Radiation efficiency is defined as the ratio of radiation pressure to driving pressure, as shown below:

[0155] In contrast, traditional systems exhibit poor radiation efficiency at long wavelengths because their effective modulus is not zero.

[0156] To more clearly demonstrate the advantages of the present disclosure over conventional structures, Figure 9a shows the normalized absorption integral as a function of current I and critical value I according to the example embodiment of Figure 2. cr Figure 9b shows the relationship between the ratios; Figure 9b shows the normalized absorption integral as a function of the pore size critical value of the conventional back cavity MPP scheme (i.e., the scheme of Figure 2a without the instability-introduced component). Aperture value of MPP A graph showing the relationship between the ratios.

[0157] As shown in Figure 9a, when the current approaches the critical value (I) cr When ), the normalized absorption integral Σ / (4π²d) increases infinitely, eventually due to B ∞ It disappears and diverges. This behavior contrasts sharply with traditional back-cavity MPP absorption, where, as shown in Figure 9b, the pore size is reduced... There is a maximum limit to improving the absorption effect. For apertures below the critical value, Σ reaches saturation at this limit. Note: In Figure 9(a), the theoretical curve derived from the absorption equation shows good agreement with the simulation data (circles) at lower currents, but the difference at high currents is due to the limited frequency band of the numerical simulation, which cannot fully capture the extremely wide broadband absorption behavior.

[0158] Furthermore, the superiority of the technical solution disclosed herein can also be demonstrated by comparing the absorption behavior of this disclosure with that of a blackbody.

[0159] Figure 10a illustrates the process by which the sound absorption rate approaches the ideal blackbody behavior under different sound absorber thicknesses d. For example, for simplicity, the absorption rate can be simplified as follows:

[0160] According to the above formula (20), the deviation Δ (shaded area) from the ideal blackbody behavior can be quantified and used as a measure, defined as:

[0161] Figure 10b shows the relationship between the deviation Δ and the thickness d of the acoustic absorber, as verified by numerical simulation. As can be seen from Figure 10b, perfect blackbody absorption (deviation Δ approaches 0) can be achieved when d approaches zero. The only practical limitation is the required critical unstable potential energy, which has a divergence of 1 / d.

[0162] Verification has shown that acoustic radiation behavior also follows a similar trend towards blackbody characteristics, which can be characterized as follows:

[0163] This further confirms the dual absorption-radiation characteristics of the acoustic unit disclosed herein.

[0164] The above mainly describes in detail how the critical state of zero static bulk modulus is achieved by incorporating the instability of the present disclosure into the component.

[0165] The following is supplementary information regarding this acoustic unit or system:

[0166] 1. System stability and frequency response characteristics of acoustic systems

[0167] Although acoustic units or systems are ideally designed to achieve zero static stiffness under critical conditions, it is important to note that in practice, they still need to maintain a small but certain degree of stability. This is to prevent instability or random instability from causing the system to collapse under external disturbances.

[0168] (1) System stability

[0169] In some embodiments, the most effective way to ensure system stability is to keep the unstable energy very close to, but always slightly below, a critical value. This approach maintains excellent performance while preventing instabilities that could lead to uncontrolled displacement or system damage.

[0170] Other stabilization mechanisms may include, but are not limited to:

[0171] ● Higher-order terms in the potential energy function provide increased drag at larger displacements;

[0172] ● Acoustic damping of fluid media provides energy dissipation;

[0173] ●The special design of the transition section in the composite panel introduces stiffness related to the amplitude of motion;

[0174] ●Physical constraints that limit the maximum displacement range.

[0175] (2) Frequency response characteristics

[0176] The frequency response characteristics of the technical solution disclosed herein have the following features:

[0177] ●Ultra-low resonant frequency under critical conditions;

[0178] ● Due to the zero modulus condition, the response bandwidth is relatively wide;

[0179] ● In absorption scenarios, there is a smooth transition from high absorption to high reflection as the frequency increases;

[0180] ● In radiation scenarios, there is a smooth transition from high to low radiation efficiency as the frequency increases.

[0181] As shown in Figures 7 and 8, the absorption and radiation spectra exhibit a very smooth broadband response, especially when η approaches 1. It is noteworthy that in the acoustic absorption scenario, unabsorbed energy is reflected back, while in the acoustic radiation scenario, unradiated energy is completely lost in the output.

[0182] 2. Sound dissipation elements and their integration with other acoustic elements

[0183] It should be understood that this disclosure can utilize various acoustic dissipation mechanisms and be integrated with other acoustic technologies to further improve performance or meet specific application requirements.

[0184] (1) Sound dissipation mechanism. Although this paper focuses on the microperforated plate (MPP) as the main sound dissipation mechanism, the system can be combined with several other dissipation sources, including but not limited to:

[0185] ● The intrinsic viscosity of the solid material in the composite plate (e.g., the transition section and / or the center section, especially the transition section).

[0186] ● A perforation is made in the reinforced central portion to utilize gas-solid friction.

[0187] ● Acoustic damping materials within the acoustic cavity

[0188] ● Controlled fluid viscosity in the gap between the composite plate and the indirectly supported shell

[0189] It will be understood that these additional acoustic dissipation sources contribute an equivalent real acoustic impedance to the entire acoustic unit or system. in and μ iklm Let be the viscosity tensor of the system. Therefore, they can be described within the same theoretical framework, often as an equivalent addition to the MPP impedance.

[0190] (2) Microperforated plate (MPP) integration. MPPs can be part of a composite plate or as independent components within an acoustic cavity. For MPPs, the air motion equation for a tube whose length is much shorter than the wavelength is:

[0191] In the formula, Δp is the acoustic pressure difference inside the pipe, and r is the radial coordinate. Considering the viscosity effect of the pipe surface (v = 0), When the particle velocity v(r) is (at time), the solution is:

[0192] When the aperture and aperture spacing are comparable to the wavelength, the acoustic impedance of the MPP is:

[0193] Among them, items This is Ingard's correction for airflow near the tube ends. The real zeroth-order term indicates constant dissipation at all frequencies. Smaller It can achieve low dispersion, thereby maximizing the contrast between the zeroth-order and higher-order terms.

[0194] (3) Implementation of a multilayer acoustic structure. In some embodiments, multiple acoustic units or acoustic devices with zero static bulk modulus may be arranged in series or in parallel to create an acoustic filter with a specific frequency response. In some embodiments, these arrangements may be optimized for specific applications, such as noise reduction for specific sound wave frequencies.

[0195] In some embodiments, the acoustic units or acoustic devices described above may also be arranged in a layered structure. In some embodiments, each acoustic device may be configured to be independently adjusted for different frequency ranges of sound waves. For example, as can be seen from Figures 7 and 8, absorption or sound radiation of different sound wave frequencies can be achieved by adjusting different η values.

[0196] It will be understood that, in practice, the acoustic performance (absorption and radiation) of the disclosed solution is better at low frequencies than at high frequencies, while conventional solutions (acoustic porous materials, broadband acoustic metamaterials, or conventional loudspeakers) have better high-frequency performance than low-frequency performance. Therefore, the two can be used complementaryly in series or parallel in practice. For example, in some embodiments, the acoustic device may include a high-frequency sound-absorbing structure and a low-frequency sound-absorbing structure, wherein the low-frequency sound-absorbing structure may be the acoustic unit or acoustic device used as a sound-absorbing unit as described above, while the high-frequency sound-absorbing structure may be a conventional sound-absorbing material.

[0197] 3. Control System

[0198] For the above-mentioned electromagnetic or mechanical implementation solutions, a control system can be used to maintain the critical state under changing environmental conditions, or to dynamically adjust the acoustic response.

[0199] (1) Feedback Control. A feedback control system monitors the system's displacement (e.g., the displacement of a composite plate) or acoustic response and adjusts the coil current or the pre-compression state of the elastic beam to maintain a critical state. This can be achieved through various sensing technologies, including but not limited to:

[0200] ●Optical displacement sensor

[0201] ●Magnetic field sensor

[0202] ● Sound pressure sensor

[0203] ●Acoustic impedance monitoring

[0204] (2) Adaptive tuning. Adaptive tuning algorithms can optimize the system response based on real-time analysis of the acoustic environment. This method enables the system to adapt to constantly changing conditions, such as temperature changes or changes in the incident sound field.

[0205] (3) Activation on demand. For electromagnetic implementations, it is advantageous to introduce current only when the system is in use and to disconnect the current when the system is not in use. Similarly, for mechanical implementations, stress can be applied only during operation. This approach can significantly extend the service life of materials and equipment by reducing fatigue, power consumption, and component wear.

[0206] 4. Related Applications

[0207] This invention, by utilizing its unique broadband absorption and radiation capabilities, can have a wide range of applications in multiple fields.

[0208] (1) Architectural Acoustics. In architectural applications, this disclosure can be implemented as modular panels for treating walls, ceilings, and other surfaces. These panels can provide unprecedented low-frequency absorption in a compact form, addressing a long-standing challenge in the field of interior acoustics.

[0209] (2) Noise Control Using Airflow. A particularly valuable application is in scenarios where noise control is required in the presence of airflow, such as HVAC systems, engine intake / exhaust systems, wind tunnels, and ventilation ducts. This invention offers significant advantages over conventional solutions because it provides a rigid, smooth, and non-porous surface (when the MPP is behind the plate), minimizing interference with airflow patterns while maintaining superior acoustic performance. This addresses a major limitation of conventional porous materials and cavity resonators used in mufflers or acoustic pads, which typically generate turbulence or flow resistance.

[0210] (3) Audio devices. In audio applications, this invention can be used in loudspeakers and microphones to improve radiation and sensitivity over a wider frequency range. The zero-modulus condition enables more efficient coupling between the transducer and the acoustic medium.

[0211] (4) Underwater acoustics. For underwater applications, this invention is applicable to liquid media. The principle remains the same, but modifications may be required in implementation to account for the higher density and speed of sound in water.

[0212] (5) Medical Applications. In medical applications, this invention can enhance the performance of ultrasound devices by improving the coupling between the transducer and tissue. This can improve imaging quality and treatment effectiveness.

[0213] 5. Manufacturing considerations.

[0214] The actual implementation of this invention requires specific manufacturing considerations to ensure its normal operation and reliability.

[0215] (1) Precision requirements. The critical state requires a precise balance between the positive and negative stiffness components. Manufacturing tolerances must be carefully controlled to ensure that this balance is achieved through adjustment mechanisms.

[0216] (2) Material Selection. Material selection must consider not only its mechanical properties, but also its long-term stability, resistance to environmental factors, and compatibility with other system components. Besides PMMA and PDMS used in this disclosure, other suitable materials include:

[0217] For rigid components, including but not limited to:

[0218] ●Lightweight metals (aluminum, titanium alloys)

[0219] ● Carbon fiber or glass fiber composite materials

[0220] ●High-performance engineering plastics (polycarbonate, polyamide, ABS)

[0221] ● Ceramic composite materials for extreme environments

[0222] For flexible components, including but not limited to:

[0223] ●Various silicone formulas with different hardness grades

[0224] ●Thermoplastic elastomers (TPE)

[0225] ●Polyurethane elastomers

[0226] ●Natural or synthetic rubber compounds

[0227] ●Specialized elastomer composite materials

[0228] The specific choice should be based on application requirements, including temperature range, chemical or UV exposure, expected lifespan, and cost constraints.

[0229] (3) Assembly Procedure. The assembly procedure must ensure the correct alignment and initial adjustment of all components. This may include specialized fixtures, fasteners, and calibration equipment. For the Halbach array, precise alignment of the magnets in a specific orientation is crucial for achieving the desired quadrupole field.

[0230] (4) Quality Control. Quality control procedures must verify each manual to ensure that each manufacturing unit meets critical conditions and maintains the required acoustic performance. This may include automated testing systems that characterize mechanical and acoustic properties.

[0231] 6. Variations and Modifications

[0232] This disclosure may include various variations and modifications to adapt the core technology to specific applications or operating conditions.

[0233] (1) Scalable Implementation. The system can be scaled up to accommodate different frequency ranges. Larger implementations can target very low frequencies, while micro-versions can target higher frequencies.

[0234] (2) Multi-degree-of-freedom systems. Extensions of the basic concept include realizing zero-modulus multi-degree-of-freedom systems in multiple directions or modes, thereby enhancing control over three-dimensional sound fields.

[0235] (3) Variations of active control. While the core invention can operate passively once adjusted to a critical state, variants of active control can dynamically adjust the critical state to optimize performance under changing conditions or introduce specific acoustic characteristics. These active systems can activate unstable introduction components on demand, introducing current or mechanical stress only during operation to extend device life and reduce power consumption.

[0236] (4) Integrated sensor array. For advanced applications, this disclosure allows for the integration of sensor arrays to monitor the sound field and provide data for analysis or control. This enables adaptive operation and integration with a wider range of acoustic management systems.

[0237] The embodiments of various acoustic units of this disclosure have been described in detail above. It will be understood that, in addition to the various acoustic units described above, this disclosure may also relate to various acoustic devices that include the acoustic units described above, as well as various methods related to the acoustic units described above.

[0238] For example, an acoustic device, acoustic apparatus, noise reduction device, or sound radiation device may include one or more of the acoustic units described above. Specifically, the one or more acoustic units are arranged in series and / or parallel. In particular, in some embodiments, the instability-introducing member described above may be coupled to a composite plate and arranged to provide negative stiffness to the composite plate during vibration to counteract the positive stiffness of both the acoustic cavity and the composite plate, thereby achieving a critical state of the acoustic unit, wherein the critical state indicates that the static bulk modulus of the acoustic unit approaches or equals zero. In some embodiments, the instability-introducing member described above may be coupled to a composite plate and arranged to provide negative stiffness to the composite plate during vibration to at least partially counteract the positive stiffness of both the acoustic cavity and the composite plate, thereby reducing the static bulk modulus of the acoustic unit. In some embodiments, the instability-introducing member described above may be coupled to the composite plate and arranged to provide negative stiffness to the composite plate during vibration to counteract the positive stiffness of both the acoustic cavity and the composite plate, thereby achieving a critical state of the acoustic unit. In the critical state, the fundamental frequency of the acoustic unit is less than or equal to 50 Hz.

[0239] For example, a sound absorber for an airflow environment includes: an acoustic cavity having a volume of compressible fluid medium enclosed by a composite plate with a smooth outer surface; an instability-introducing member coupled to the composite plate, the instability-introducing member being configured to provide negative stiffness to the composite plate, thereby reducing the effective bulk modulus of the acoustic cavity, the composite plate, and the instability-introducing member as a whole; and a sound dissipation component coupled to the acoustic cavity or the composite plate, wherein the smooth outer surface of the composite plate reduces interference with airflow, and the reduced effective bulk modulus enables broadband acoustic absorption.

[0240] For example, a system for actively controlling acoustic properties includes: an acoustic unit or device as described above; at least one sensor configured to monitor at least one of the acoustic response, displacement, and impedance of the acoustic device; and a controller configured to adjust the current through the coil according to input from the at least one sensor to maintain desired acoustic properties.

[0241] For example, a method for fabricating an acoustic unit includes: providing an acoustic cavity having a compressible fluid medium within it; providing a vibrating composite plate to close an opening on at least one side of the acoustic cavity; providing an instability introduction member and coupling it to the vibrating composite plate such that the instability introduction member is adapted to provide a negative stiffness to the composite plate during vibration, the negative stiffness being able to counteract the positive stiffness of both the acoustic cavity and the vibrating composite plate, thereby achieving a critical state of the acoustic unit, wherein the critical state indicates that the static bulk modulus of the acoustic unit approaches or equals zero.

[0242] For example, a method for achieving acoustic absorption includes: providing an acoustic cavity having a compressible fluid medium within it; providing a vibrating composite plate to close an opening on at least one side of the acoustic cavity; providing a sound dissipation component for dissipating sound, the sound dissipation component being positioned within or outside the acoustic cavity, adjacent to or integrated with the composite plate; providing an instability introduction member and coupling it to the vibrating composite plate such that the instability introduction member is adapted to provide a negative stiffness to the composite plate during vibration of the composite plate, the negative stiffness being able to counteract the positive stiffness of both the acoustic cavity and the vibrating composite plate, thereby achieving a critical state of the acoustic unit, wherein the critical state indicates that the static bulk modulus of the acoustic unit approaches or equals zero; and causing the composite plate to receive sound from the outside.

[0243] For example, a method for achieving acoustic radiation includes: providing an acoustic cavity having a compressible fluid medium within it; providing a vibrating composite plate to close an opening on at least one side of the acoustic cavity; providing an instability introduction member and coupling it to the vibrating composite plate such that the instability introduction member is adapted to provide a negative stiffness to the composite plate during vibration, the negative stiffness being able to counteract the positive stiffness of both the acoustic cavity and the vibrating composite plate, thereby achieving a critical state of the acoustic unit, wherein the critical state indicates that the static bulk modulus of the acoustic unit approaches or equals zero; and applying an external driving force to the composite plate via an acoustic driving device to generate the acoustic radiation.

[0244] For example, a method for adjusting acoustic properties may include: providing an acoustic unit according to the foregoing description; and adjusting the acoustic properties of the acoustic unit by adjusting parameters of the instability-introducing component.

[0245] For example, a method for achieving broadband sound absorption may include: providing an acoustic cavity enclosed by a composite plate, the acoustic cavity having a compressible fluid medium of a certain volume; coupling an instability-introducing member to the composite plate, the instability-introducing member being configured to provide negative stiffness to the composite plate; and adjusting the negative stiffness to achieve a critical state in which the static bulk modulus of the acoustic cavity, the composite plate, and the instability-introducing member as a whole approaches or equals zero, thereby achieving broadband sound absorption that increases with wavelength.

[0246] A method for achieving broadband acoustic radiation may include: providing an acoustic cavity enclosed by a composite plate, the acoustic cavity containing a volume of compressible fluid medium; coupling an instability-introducing member to the composite plate, the instability-introducing member being configured to provide negative stiffness to the composite plate; adjusting the negative stiffness to achieve a critical state in which the static bulk modulus of the acoustic cavity, the composite plate, and the instability-introducing member as a whole approaches or equals zero; and applying an external driving force to the composite plate via an acoustic driving device to generate acoustic radiation with an efficiency close to 1 over a wide frequency range.

[0247] For example, a method of manufacturing an acoustic device with controlled instability includes: forming an acoustic cavity having a volume of compressible fluid medium; manufacturing a composite plate having a central reinforcing portion, a flexible transition zone, and an external sealing ring, such that the composite plate closes one side of the acoustic cavity; coupling an instability-introducing member to the central reinforcing portion of the composite plate; and adjusting the instability-introducing member to provide negative stiffness to the composite plate so that the acoustic device reaches a critical state in which the static bulk modulus of the acoustic cavity, the composite plate, and the instability-introducing member as a whole is close to or equal to zero.

[0248] The various embodiments of this disclosure have been described in detail above. It will be understood that this disclosure provides a variety of performance advantages for acoustic applications, specifically:

[0249] ● Suitable for absorption applications. When configured as an absorber and tuned to a critical zero static bulk modulus, the system achieves near-perfect absorption over an unprecedented bandwidth, particularly in the low-frequency range, traditionally challenging for compact absorbers. Absorption performance remains consistent over a wide range of incident angles and is minimally affected by environmental conditions.

[0250] ●Suitable for radiation applications. When configured as a radiator and maintained at a critical state of zero static bulk modulus, this system can efficiently convert input energy into acoustic radiation over a wide frequency range. Radiation efficiency is particularly enhanced at low frequencies, whereas conventional radiators typically have poor impedance matching with the surrounding medium.

[0251] ●System Integration. This invention can be integrated with microperforated plates (MPPs) or other acoustic elements to further enhance performance characteristics. These integrated systems combine the broadband capabilities of a zero-modulus cavity with the additional absorption mechanisms of additional elements, thereby achieving acoustic performance that surpasses that of any single method.

[0252] In summary, this disclosure represents a fundamental advancement in acoustics by circumventing traditional causal constraints through a novel method and apparatus for achieving zero static modulus. By precisely balancing instabilities introduced into the stability components of the structure and the system itself under critical conditions where negative stiffness exactly cancels out positive stiffness, the system achieves an ideal soft boundary state, thereby enabling unprecedented broadband absorption and radiation capabilities in compact devices and opening up new possibilities for a wide range of acoustic applications.

[0253] In particular, compared with traditional acoustic technologies, the technical solution disclosed herein has several revolutionary advantages:

[0254] ● Ultra-wideband absorption capability, approaching the theoretically ideal "acoustic blackbody";

[0255] ● Compact design delivers unprecedented low-frequency performance;

[0256] ● Depending on the configuration, it can be used as both an absorber and a radiator;

[0257] ● It operates passively and can absorb energy without the need for an active control system or external power source;

[0258] ● Adjustable characteristics, which can be adjusted according to specific applications to achieve the critical state of zero static bulk modulus condition;

[0259] ● Modular design, expandable for different applications.

[0260] These advantages enable the technical solution disclosed herein to be widely applied in numerous fields, including but not limited to:

[0261] ●Architectural acoustics used to create an acoustically neutral environment;

[0262] ● Noise control in transportation, industrial and urban environments;

[0263] ● High-fidelity audio devices (e.g., speakers) with higher performance transducers;

[0264] ● Acoustic sensing and imaging systems with higher sensitivity;

[0265] ● Used for underwater acoustic applications involving absorption and transmission;

[0266] ●Medical devices that utilize sound energy;

[0267] While the invention has been detailed and described in the accompanying drawings and foregoing description, these descriptions and descriptions should be considered illustrative or exemplary rather than restrictive; the invention is not limited to the disclosed embodiments. Other variations of the disclosed embodiments will be understood and practiced by those skilled in the art in practicing the claimed invention through study of the drawings, disclosure, and appended claims.

[0268] In the claims, the word "comprising" does not exclude other elements, and the indefinite articles "a" or "an" do not exclude a plurality. A single element or other unit may fulfill the function of multiple items set forth in the claims. The mere fact that certain features are recited only in dissimilar embodiments or dependent claims does not imply that combinations of these features cannot be used advantageously. Without departing from the spirit and scope of this application, the scope of protection of this application covers any possible combination of the various features recited in the various embodiments or dependent claims.

[0269] Furthermore, although the methods of this disclosure have been described simply, it should be understood that the steps of the methods described above are merely examples. Moreover, the methods described in a specific order in the specification do not require or imply that these operations must be performed in that specific order, or that all the operations shown must be performed to achieve the desired result; on the contrary, the order of execution of the described steps may be changed. Additionally or alternatively, certain steps may be omitted, multiple steps may be combined into one step, and / or one step may be broken down into multiple steps.

[0270] Furthermore, various embodiments of this disclosure can also be described using the following terms:

[0271] 1. An acoustic unit, comprising:

[0272] An acoustic cavity containing a compressible fluid medium;

[0273] The composite plate is arranged to vibrately close at least one opening of the acoustic cavity;

[0274] An instability-introducing component, coupled to the composite plate, is arranged to provide negative stiffness to the composite plate during vibration to counteract the positive stiffness of both the acoustic cavity and the composite plate, thereby achieving a critical state of the acoustic unit, wherein the critical state indicates that the static bulk modulus of the acoustic unit approaches or equals zero.

[0275] 2. The acoustic unit according to Clause 1, wherein, in the critical state, the fundamental frequency of the acoustic unit is less than or equal to 50 Hz.

[0276] 3. The acoustic unit according to clause 1 or 2, wherein the composite panel comprises:

[0277] A vibratory yet rigid central portion, to which the instability-introducing component is coupled;

[0278] A flexible transition portion surrounds the central portion; and

[0279] An attachment portion, located around the transition portion, is arranged to fix one end of the transition portion opposite to the central portion to a shell structure defining the acoustic cavity.

[0280] 4. The acoustic unit according to Clause 3, wherein the central portion is provided with a reinforcing rib on at least one side.

[0281] 5. The acoustic unit according to Clause 4, wherein the reinforcing ribs are in a honeycomb structure.

[0282] 6. The acoustic unit according to any one of clauses 3 to 5, wherein the central part is a planar structure or a dome structure.

[0283] 7. The acoustic unit according to Clause 6, wherein the central portion covers more than 90% of the area of ​​the opening of the acoustic cavity.

[0284] 8. The acoustic unit according to any one of clauses 3 to 7, wherein the flexible transition portion is a pleated portion, the pleated portion being adapted to reduce the force from the attachment portion when the central portion moves in a piston-like manner.

[0285] 9. The acoustic unit according to any one of the preceding clauses, wherein the acoustic cavity is a cylindrical cavity.

[0286] 10. The acoustic unit according to any one of the preceding clauses, wherein the instability-introducing component is selected from one or more of electrical, magnetic, electromagnetic or mechanical structures.

[0287] 11. The acoustic unit according to any one of the preceding clauses, wherein the instability-introducing component comprises an electromagnetic structure, the electromagnetic structure comprising:

[0288] A current-carrying coil, which is integrated near the edge of the composite board and is adapted to receive current; and

[0289] A magnet structure is arranged around the coil, wherein the magnitude of the current is adapted to be tuned such that the current through the current-carrying coil interacts with the magnetic field generated by the magnet structure, producing a negative stiffness force on the composite plate that resists the restoring force of the acoustic cavity and the composite plate caused by the current-carrying coil deviating from its equilibrium position.

[0290] 12. The acoustic unit according to Clause 11, wherein the magnet structure generates a magnetic field distribution of one of a quadrupole field, a hexapole field, or an octapole field, and the current-carrying coil is located at the center of symmetry of the magnetic field distribution in the equilibrium position.

[0291] 13. The acoustic unit according to Clause 11 or 12, wherein the magnet structure includes four magnetic poles for generating the quadrupole field, wherein any two adjacent magnetic poles have opposite polarities, and the current-carrying coil is positioned at the center of the quadrupole field.

[0292] 14. The acoustic unit according to Clause 13, wherein the four magnetic poles are located on the outside of the cavity, and the four magnetic poles include a first pair of magnetic poles and a second pair of magnetic poles, the first pair of magnetic poles having opposite polarities and being provided by a pair of annular magnets disposed on both sides of the composite plate, the second pair of magnetic poles being located on the side of the first pair of magnetic poles closer to the acoustic cavity and being provided by a pair of iron rings disposed on both sides of the composite plate, wherein the transverse interface of each iron ring is L-shaped, and one end of each iron ring is connected to a magnetic pole of the corresponding annular magnet adjacent to the iron ring in the pair of annular magnets.

[0293] 15. The acoustic unit according to Clause 11 or 12, wherein the magnet structure is composed of magnets of a Halbach array for generating a quadrupole field.

[0294] 16. The acoustic unit according to any one of clauses 11 to 13 further comprises: a current control device adapted to maintain the magnitude of the current at or slightly below a critical current level to achieve a critical state of the acoustic unit.

[0295] 17. The acoustic unit according to any one of the preceding clauses, wherein the instability-introducing component comprises a mechanical structure, the mechanical structure including:

[0296] Multiple elastic beams, each with one end converging to form a convergence section, are attached to the center of the composite plate. The other end of each elastic beam is attached to a sidewall structure defining the acoustic cavity. The multiple elastic beams are arranged radially.

[0297] Each elastic beam is installed in a pre-compressed state and is adapted to generate a negative stiffness force on the composite plate to resist the restoring force of the acoustic cavity and the composite plate caused by the composite plate deviating from its equilibrium position during vibration.

[0298] 18. The acoustic unit according to Clause 17, wherein the forces exerted by the plurality of elastic beams on the composite panel are balanced in the planar direction of the composite panel.

[0299] 19. The acoustic unit as described in Clause 17, wherein the plurality of elastic beams comprises four, six, or eight elastic beams.

[0300] 20. The acoustic unit according to any one of Clauses 17 to 19, wherein the cross-section of each elastic beam is of a T-shaped, arcuate, or H-shaped structure for suppressing lateral buckling modes.

[0301] 21. The acoustic unit according to any one of clauses 17 to 19 further includes a pre-compression mechanism disposed near the other end of each elastic beam and adapted to tune the degree of pre-compression of each elastic beam when the elastic beam is mounted to the acoustic unit.

[0302] 22. The acoustic unit according to Clause 21, wherein the pre-compression mechanism is selected from a helical adjustment mechanism, a lever system or a thermal actuator.

[0303] 23. The acoustic unit according to any one of the preceding clauses, wherein the acoustic unit is a sound-absorbing unit, the acoustic unit further comprising: a sound dissipation component for dissipating sound, the sound dissipation component being positioned within or outside the acoustic cavity, adjacent to or integrated with the composite panel.

[0304] 24. The acoustic unit as described in Clause 23, wherein the acoustic dissipation component is a micro-perforated plate (MPP) or an acoustic damping material.

[0305] 25. The acoustic unit according to clause 23 or 24, wherein the acoustic dissipation component is the microperforated plate (MPP), the microperforated plate being positioned adjacent to the composite plate and arranged substantially parallel to the composite plate within or outside the acoustic cavity.

[0306] 26. The acoustic unit according to Clause 23, wherein the composite plate has intrinsic viscosity and the composite plate itself serves as the sound dissipation component.

[0307] 27. The acoustic unit according to Clause 26, wherein the central portion of the composite plate is formed of a micro-perforated plate, the micro-perforated plate serving as a sound dissipation component.

[0308] 28. The acoustic unit according to any one of the preceding clauses, wherein the acoustic unit is a sound-absorbing unit, and in the critical state, the sound absorption rate of the sound-absorbing unit increases with increasing wavelength.

[0309] 29. The acoustic unit according to Clause 28 above, wherein, in the critical state, the sound-absorbing unit exhibits a sound absorption rate of 90% for a sound wavelength greater than 100 times the unit thickness d, wherein the unit thickness d is defined as the thickness from the upper surface of the composite plate to the surface of the acoustic cavity facing the composite plate.

[0310] 30. The acoustic unit according to any one of the preceding clauses 1 to 22, wherein the acoustic unit is a sound radiating unit, wherein,

[0311] The acoustic radiation unit also includes an acoustic driving device configured to drive the composite plate to vibrate in order to generate sound.

[0312] In the critical state, the acoustic radiation efficiency of the acoustic radiation unit approaches 1 as the wavelength of the generated sound increases.

[0313] 31. The acoustic unit according to Clause 30, wherein the acoustic driving device is a voice coil or piezoelectric component coupled to the composite plate.

[0314] 32. The acoustic unit according to any one of the preceding clauses further comprises: at least one sensor, said at least one sensor being configured to monitor one or more of acoustic response, composite plate displacement, and acoustic impedance.

[0315] 33. The acoustic unit according to the preceding clause 32 further includes: a controller configured to adjust the operating parameters of the instability-introducing member based on the output of the at least one sensor to achieve a critical state of the acoustic unit.

[0316] 34. A noise reduction device, comprising:

[0317] Two or more acoustic units according to any one of clauses 1 to 29, wherein the two or more acoustic units are arranged in series and / or in parallel.

[0318] 35. A sound radiation device, comprising:

[0319] One or more acoustic units as described in any one of Clauses 1 to 22, 30 to 33, wherein the two or more acoustic units are arranged in series and / or in parallel.

[0320] 36. The acoustic device according to clause 34 or 35, wherein the plurality of acoustic units in the noise reduction device or the plurality of acoustic units in the sound radiation device are arranged in series and / or in parallel.

[0321] 37. A method for fabricating an acoustic unit, comprising:

[0322] An acoustic cavity is provided, wherein a compressible fluid medium is contained within the acoustic cavity;

[0323] A vibrating composite plate is provided to close an opening on at least one side of the acoustic cavity;

[0324] An instability-introducing member is provided and coupled to the vibrating composite plate such that the instability-introducing member is adapted to provide negative stiffness to the composite plate during vibration, the negative stiffness being able to counteract the positive stiffness of both the acoustic cavity and the vibrating composite plate, thereby achieving a critical state by the acoustic unit, wherein the critical state indicates that the static bulk modulus of the acoustic unit approaches or equals zero.

[0325] 38. A method for achieving acoustic absorption, comprising:

[0326] An acoustic cavity is provided, wherein a compressible fluid medium is contained within the acoustic cavity;

[0327] A vibrating composite plate is provided to close an opening on at least one side of the acoustic cavity;

[0328] A sound dissipation component is provided for dissipating sound, the sound dissipation component being positioned within or outside the acoustic cavity, adjacent to or integrated with the composite plate;

[0329] An instability-introducing member is provided and coupled to the vibrating composite plate, such that the instability-introducing member is adapted to provide negative stiffness to the composite plate during vibration, the negative stiffness being able to counteract the positive stiffness of both the acoustic cavity and the vibrating composite plate, thereby achieving a critical state of the entire acoustic system, wherein the critical state indicates that the static bulk modulus of the acoustic unit approaches or equals zero, and the entire acoustic system includes the acoustic cavity, the composite plate, the instability-introducing member, and the sound dissipation component;

[0330] This allows the composite board to receive sound from the outside.

[0331] 39. The method according to clause 38, wherein the instability-introducing component includes an electromagnetic structure, the electromagnetic structure comprising:

[0332] A current-carrying coil, which is integrated near the edge of the composite board and is adapted to receive current; and

[0333] A magnet structure, arranged around the coil, is adapted to generate a negative stiffness force on the composite plate under the interaction of the current and the magnetic field of the magnet structure, resisting the restoring force of the acoustic cavity and the composite plate caused by the current-carrying coil deviating from its equilibrium position.

[0334] 40. The method according to clause 38 or 39 further comprises: adjusting the current to ensure that the resulting negative stiffness is equal to or slightly lower than the positive stiffness of both the acoustic cavity and the vibrating composite plate.

[0335] 41. The method according to clause 38 or 39 further comprises: adjusting the current to slightly below a critical current to ensure that the resulting negative stiffness is equal to or slightly below the positive stiffness capable of offsetting both the acoustic cavity and the vibrating composite plate.

[0336] 42. The method according to Clause 41 further comprises: determining that the current is equal to or slightly below a critical current based on the acoustic response monitored for the acoustic unit.

[0337] 43. The method according to any one of the preceding clauses 38 to 42, wherein the acoustic absorption rate increases with increasing wavelength.

[0338] 44. The method according to any one of the preceding clauses 38 to 42, wherein, in the critical state, the acoustic absorption of the acoustic unit exhibits an absorption rate of 90% for wavelengths greater than 100 times the unit thickness d, wherein the unit thickness d is defined as the thickness from the upper surface of the composite plate to the surface of the acoustic cavity facing the composite plate.

[0339] 45. A method for achieving sound radiation, comprising:

[0340] An acoustic cavity is provided, wherein a compressible fluid medium is contained within the acoustic cavity;

[0341] A vibrating composite plate is provided to close an opening on at least one side of the acoustic cavity;

[0342] An instability-introducing member is provided and coupled to the vibrating composite plate, such that the instability-introducing member is adapted to provide negative stiffness to the composite plate during vibration, the negative stiffness offsetting the positive stiffness of both the acoustic cavity and the vibrating composite plate, thereby achieving a critical state for the entire acoustic system, wherein the critical state indicates that the static bulk modulus of the acoustic unit approaches or equals zero, the entire acoustic system including the acoustic cavity, the composite plate, the instability-introducing member, and the sound dissipation component; and

[0343] An external driving force is applied to the composite plate via an acoustic driving device to generate the acoustic radiation.

[0344] 46. ​​The method according to Clause 45, wherein the acoustic driving device is a voice coil or piezoelectric component coupled to the composite plate.

[0345] 47. The method according to Clause 46, wherein, in the application of an external driving force to the composite plate via the acoustic driving device to generate acoustic radiation and in the critical state, the efficiency of the acoustic radiation approaches 1 with increasing wavelength.

[0346] 48. A method for adjusting acoustic properties, comprising:

[0347] Provide an acoustic unit in accordance with any one of Clauses 1 to 30;

[0348] The critical state of the acoustic unit is achieved by adjusting the parameters of the instability-introducing component in the acoustic unit.

[0349] 49. An acoustic device, comprising:

[0350] An acoustic cavity is a compressible fluid medium with a certain volume.

[0351] A composite plate, enclosing at least one surface of the acoustic cavity, the composite plate being configured to vibrate in response to changes in sound pressure; and

[0352] An instability-introducing member is coupled to the composite plate, the instability-introducing member being configured to provide negative stiffness to the composite plate, the negative stiffness being capable of reducing the effective bulk modulus of the acoustic cavity, wherein the negative stiffness can be adjusted to achieve a critical state in which the static bulk modulus of the acoustic device approaches or equals zero.

[0353] 50. An acoustic device, comprising:

[0354] An acoustic cavity is a compressible fluid medium with a certain volume.

[0355] A composite plate that encloses at least one surface of the acoustic cavity, the composite plate being configured to vibrate in response to changes in sound pressure.

[0356] A magnetic structure for generating a multipolar magnetic field, the magnetic structure being fixedly positioned with the acoustic cavity; and

[0357] A current-carrying coil is attached to the composite plate and located within the multipole magnetic field, wherein the interaction between the current through the coil and the magnetic field generates a negative stiffness force that resists the restoring force of the acoustic cavity and the composite plate caused by the composite plate deviating from its equilibrium position, thereby reducing the effective bulk modulus of the acoustic device.

[0358] 51. An acoustic device, comprising:

[0359] An acoustic cavity is a compressible fluid medium with a certain volume.

[0360] A composite plate, enclosing at least one surface of the acoustic cavity, the composite plate being configured to vibrate in response to changes in sound pressure; and

[0361] Multiple elastic beams arranged in a radial pattern are installed in a pre-compressed state and coupled to the composite plate. The elastic beams are configured to generate a negative stiffness force when operating in a post-buckling state, resisting the restoring force of the acoustic cavity and the composite plate caused by the composite plate deviating from its equilibrium position, thereby reducing the effective bulk modulus of the acoustic device.

[0362] 52. A method for achieving broadband sound absorption, comprising:

[0363] An acoustic cavity enclosed by a composite plate is provided, the acoustic cavity having a certain volume of compressible fluid medium;

[0364] An instability-introducing member is coupled to the composite plate, the instability-introducing member being configured to provide negative stiffness to the composite plate; and

[0365] Adjusting the negative stiffness to achieve a critical state, in which the static bulk modulus of the acoustic cavity, the composite plate, and the instability-introducing component as a whole approaches or equals zero, thereby achieving broadband sound absorption that increases with wavelength.

[0366] 53. A method for achieving broadband sound radiation, comprising:

[0367] An acoustic cavity enclosed by a composite plate is provided, wherein the acoustic cavity contains a certain volume of compressible fluid medium;

[0368] An instability-introducing member is coupled to the composite plate, the instability-introducing member being configured to provide negative stiffness to the composite plate; and

[0369] The negative stiffness is adjusted to achieve a critical state, in which the static bulk modulus of the acoustic cavity, the composite plate, and the instability-inducing components as a whole approaches or equals zero; and

[0370] An external driving force is applied to the composite plate via an acoustic driving device to generate acoustic radiation with an efficiency close to 1 over a wide frequency range.

[0371] 54. A sound absorber for use in an airflow environment, comprising:

[0372] An acoustic cavity is enclosed by a composite plate with a smooth outer surface, and the acoustic cavity contains a certain volume of compressible fluid medium.

[0373] An instability-introducing member coupled to a composite plate, the instability-introducing member being configured to provide negative stiffness to the composite plate, thereby reducing the overall effective bulk modulus of the acoustic cavity, the composite plate, and the instability-introducing member; and

[0374] A sound dissipation component is coupled to the acoustic cavity or the composite plate, wherein the smooth outer surface of the composite plate reduces interference with airflow, and the reduced effective bulk modulus enables broadband acoustic absorption.

[0375] 55. A system for actively controlling acoustic properties, comprising:

[0376] Acoustic device as described in Clause 50 above;

[0377] At least one sensor configured to monitor at least one of the acoustic response, displacement, and impedance of the acoustic device; and

[0378] A controller configured to adjust the current through the coil based on input from at least one sensor to maintain desired acoustic characteristics.

[0379] 56. A system for actively controlling acoustic properties, comprising:

[0380] Acoustic apparatus as described in Clause 51;

[0381] At least one sensor is configured to monitor at least one of the acoustic response, displacement, and mechanical strain of the acoustic device; and

[0382] The controller is configured to adjust the degree of compression of the elastic beam based on input from the at least one sensor in order to maintain the desired acoustic properties.

[0383] 57. A multilayer acoustic device comprising a plurality of acoustic devices according to any one of claims 49 to 51, the acoustic devices being arranged in a layered structure, wherein each acoustic device is configured to be independently adjusted for a different frequency range of sound waves.

[0384] 58. An acoustic device, comprising:

[0385] High-frequency sound absorption structure; and

[0386] A low-frequency sound absorption structure, the low-frequency sound absorption structure comprising an acoustic unit according to any one of clauses 1 to 33, an acoustic device according to any one of clauses 49 to 51, a sound absorber according to clause 54, a system according to clause 55 or 56, or a multi-layer acoustic device according to clause 58.

[0387] 59. A method for manufacturing an acoustic device with controlled instability, comprising:

[0388] An acoustic cavity is formed, wherein the acoustic cavity contains a certain volume of compressible fluid medium;

[0389] A composite plate is manufactured having a central reinforcing portion, a flexible transition zone, and an external sealing ring, such that the composite plate closes one side of the acoustic cavity;

[0390] Instability is introduced into the component and coupled to the central reinforcement of the composite plate; and

[0391] The instability-introducing component is adjusted to provide negative stiffness to the composite plate, so that the acoustic device reaches a critical state, in which the static bulk modulus of the acoustic cavity, the composite plate, and the instability-introducing component as a whole is close to or equal to zero.

[0392] Furthermore, any reference numerals in the claims should not be construed as limiting the scope of the invention.

Claims

1. An acoustic unit, comprising: An acoustic cavity containing a compressible fluid medium; The composite plate is arranged to vibrately close at least one opening of the acoustic cavity; An instability-introducing component, coupled to the composite plate, is arranged to provide negative stiffness to the composite plate during vibration to counteract the positive stiffness of both the acoustic cavity and the composite plate, thereby achieving a critical state of the acoustic unit, wherein the critical state indicates that the static bulk modulus of the acoustic unit approaches or equals zero.

2. The acoustic unit according to claim 1, wherein, In the critical state, the fundamental frequency of the acoustic unit is less than or equal to 50 Hz.

3. The acoustic unit according to claim 1 or 2, wherein the composite plate comprises: A vibratory yet rigid central portion, to which the instability-introducing component is coupled; A flexible transition section surrounds the central portion; as well as An attachment portion, located around the transition portion, is arranged to fix one end of the transition portion opposite to the central portion to a shell structure defining the acoustic cavity.

4. The acoustic unit according to claim 3, wherein the central portion is provided with a reinforcing rib on at least one side.

5. The acoustic unit according to any one of claims 3 to 4, wherein the central portion covers more than 90% of the area of ​​the opening of the acoustic cavity.

6. The acoustic unit according to any one of the preceding claims, wherein the instability-introducing component comprises an electromagnetic structure, the electromagnetic structure comprising: A current-carrying coil is integrated near the edge of the composite plate and is adapted to receive current; as well as A magnet structure is arranged around the coil, wherein the magnitude of the current is adapted to be tuned such that the current through the current-carrying coil interacts with the magnetic field generated by the magnet structure, producing a negative stiffness force on the composite plate that resists the restoring force of the acoustic cavity and the composite plate caused by the current-carrying coil deviating from its equilibrium position.

7. The acoustic unit according to claim 6, wherein the magnet structure generates a magnetic field distribution of one of a quadrupole field, a hexapole field, or an octapole field, and the current-carrying coil is located at the center point of symmetry of the magnetic field distribution when in the equilibrium position.

8. The acoustic unit according to claim 6 or 7, wherein the magnet structure includes four magnetic poles for generating the quadrupole field, wherein any two adjacent magnetic poles have opposite polarities, and the current-carrying coil is positioned at the center of the quadrupole field.

9. The acoustic unit according to claim 6 or 7, wherein the magnet structure is composed of magnets of a Halbach array for generating a quadrupole field.

10. The acoustic unit according to any one of the preceding claims further comprises: A current control device adapted to maintain the magnitude of the current at or slightly below a critical current level to achieve the critical state of the acoustic unit.

11. The acoustic unit according to any one of the preceding claims, wherein the instability-introducing component comprises a mechanical structure, the mechanical structure comprising: Multiple elastic beams, each with one end converging to form a convergence section, are attached to the center of the composite plate. The other end of each elastic beam is attached to a sidewall structure defining the acoustic cavity. The multiple elastic beams are arranged radially. Each elastic beam is installed in a pre-compressed state and is adapted to generate a negative stiffness force on the composite plate to resist the restoring force of the acoustic cavity and the composite plate caused by the composite plate deviating from its equilibrium position during vibration.

12. The acoustic unit of claim 11, wherein the forces exerted by the plurality of elastic beams on the composite plate are balanced in the planar direction of the composite plate.

13. The acoustic unit according to claim 11 or 12 further includes a pre-compression mechanism disposed near the other end of each elastic beam and adapted to tune the pre-compression degree of each elastic beam when the elastic beam is mounted to the acoustic unit.

14. The acoustic unit according to any one of the preceding claims, wherein the acoustic unit is a sound-absorbing unit, and the acoustic unit further comprises: A sound dissipation component, which is used to dissipate sound and is positioned inside or outside the acoustic cavity, adjacent to or integrated with the composite plate.

15. The acoustic unit of claim 14, wherein the acoustic dissipation component is the microperforated plate (MPP), the microperforated plate being positioned adjacent to the composite plate and arranged substantially parallel to the composite plate within or outside the acoustic cavity.

16. The acoustic unit according to claim 14, wherein the central portion of the composite plate is formed by a micro-perforated plate, the micro-perforated plate serving as a sound dissipation component.

17. The acoustic unit according to any one of the preceding claims, wherein the acoustic unit is a sound-absorbing unit, and in the critical state, the sound absorption rate of the sound-absorbing unit increases with increasing wavelength.

18. The acoustic unit according to claim 17, wherein in the critical state, the sound-absorbing unit exhibits a sound absorption rate of 90% for a sound wavelength greater than 100 times the unit thickness d, wherein the unit thickness d is defined as the thickness from the upper surface of the composite plate to the surface of the acoustic cavity facing the composite plate.

19. The acoustic unit according to any one of claims 1 to 13, wherein the acoustic unit is a sound radiating unit, wherein, The acoustic radiation unit also includes an acoustic driving device configured to drive the composite plate to vibrate in order to generate sound. In the critical state, the acoustic radiation efficiency of the acoustic radiation unit approaches 1 as the wavelength of the generated sound increases.

20. The acoustic unit according to claim 19, wherein the acoustic driving device is a voice coil or a piezoelectric component, the voice coil or piezoelectric component being coupled to the composite plate.

21. The acoustic unit according to any one of the preceding claims further comprises: At least one sensor, the at least one sensor being configured to monitor one or more of acoustic response, composite plate displacement, and acoustic impedance.

22. A noise reduction device, comprising: One or more acoustic units according to any one of claims 1 to 18, 21.

23. The noise reduction device according to claim 22, wherein the noise reduction device comprises two or more acoustic units according to any one of claims 1 to 18, 21, and the two or more acoustic units are arranged in series and / or in parallel.

24. A sound radiation device, comprising: One or more acoustic units according to any one of claims 1 to 13, 19 to 21.

25. The acoustic radiation device according to claim 24, wherein the acoustic radiation device comprises two or more acoustic units according to any one of claims 1 to 13, 19 to 21, and the two or more acoustic units are arranged in series and / or in parallel.

26. A method for fabricating an acoustic unit, comprising: An acoustic cavity is provided, wherein a compressible fluid medium is contained within the acoustic cavity; A vibrating composite plate is provided to close an opening on at least one side of the acoustic cavity; An instability-introducing member is provided and coupled to the vibrating composite plate such that the instability-introducing member is adapted to provide negative stiffness to the composite plate during vibration, the negative stiffness being able to counteract the positive stiffness of both the acoustic cavity and the vibrating composite plate, thereby achieving a critical state of the acoustic unit, wherein the critical state indicates that the static bulk modulus of the acoustic unit approaches or equals zero.

27. An acoustic device, comprising: An acoustic cavity is a compressible fluid medium with a certain volume. A composite plate that encloses at least one surface of the acoustic cavity, the composite plate being configured to vibrate in response to changes in sound pressure. A magnetic structure for generating a multipolar magnetic field, wherein the magnetic structure is fixedly positioned with the acoustic cavity; as well as A current-carrying coil is attached to the composite plate and located within the multipole magnetic field, wherein the interaction between the current through the coil and the magnetic field generates a negative stiffness force that resists the restoring force of the acoustic cavity and the composite plate caused by the composite plate deviating from its equilibrium position, thereby reducing the effective bulk modulus of the acoustic device.

28. An acoustic device, comprising: An acoustic cavity is a compressible fluid medium with a certain volume. A composite plate that encloses at least one surface of the acoustic cavity, the composite plate being configured to vibrate in response to changes in sound pressure. as well as Multiple elastic beams arranged in a radial pattern are installed in a pre-compressed state and coupled to the composite plate. The elastic beams are configured to generate a negative stiffness force when operating in a post-buckling state, resisting the restoring force of the acoustic cavity and the composite plate caused by the composite plate deviating from its equilibrium position, thereby reducing the effective bulk modulus of the acoustic device.

29. A method for achieving broadband sound absorption, comprising: An acoustic cavity enclosed by a composite plate is provided, the acoustic cavity having a certain volume of compressible fluid medium; An instability-introducing member is coupled to the composite plate, the instability-introducing member being configured to provide negative stiffness to the composite plate; as well as Adjusting the negative stiffness to achieve a critical state, in which the static bulk modulus of the acoustic cavity, the composite plate, and the instability-introducing component as a whole approaches or equals zero, thereby achieving broadband sound absorption that increases with wavelength.

30. A method for achieving broadband sound radiation, comprising: An acoustic cavity enclosed by a composite plate is provided, wherein the acoustic cavity contains a certain volume of compressible fluid medium; An instability-introducing member is coupled to the composite plate, the instability-introducing member being configured to provide negative stiffness to the composite plate; as well as The negative stiffness is adjusted to achieve a critical state, in which the static bulk modulus of the acoustic cavity, the composite plate, and the instability-introducing component as a whole approaches or equals zero. as well as An external driving force is applied to the composite plate via an acoustic driving device to generate acoustic radiation with an efficiency close to 1 over a wide frequency range.

31. A sound absorber for use in an airflow environment, comprising: An acoustic cavity is enclosed by a composite plate with a smooth outer surface, and the acoustic cavity contains a certain volume of compressible fluid medium. An instability-introducing member coupled to a composite plate, the instability-introducing member being configured to provide negative stiffness to the composite plate, thereby reducing the overall effective bulk modulus of the acoustic cavity, the composite plate, and the instability-introducing member; as well as A sound dissipation component is coupled to the acoustic cavity or the composite plate, wherein the smooth outer surface of the composite plate reduces interference with airflow, and the reduced effective bulk modulus enables broadband acoustic absorption.

32. An acoustic device, comprising: High-frequency sound absorption structure; and A low-frequency sound absorption structure, the low-frequency sound absorption structure comprising an acoustic unit according to any one of claims 1 to 18, an acoustic device according to claim 27 or 28, or a sound absorber according to claim 31.

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