Sound absorption devices

Abstract

Broadly speaking, the present invention presents a sound-absorbing device configured to absorb sound within a desired frequency range. In this device, a plurality of tuned resonant elements protrude from a panel sound absorber, such as a restraint plate. In a first aspect, the sound-absorbing device is configured to absorb sound within a desired frequency range. The device comprises a plate configured to be mounted adjacent to a sound-reflecting surface to absorb sound reflected from the sound-reflecting surface, and a plurality of resonant elements. The plurality of resonant elements include a base extending from the plate and a free end, so that each resonant element extends from the plate at an angle to the plate.

Description

[Technical Field]

[0001] The present invention relates to a sound-absorbing device, an assembly of sound-absorbing devices, and a method of absorbing sound. [Background Art]

[0002] Noise is a universal and growing threat to human health. In England alone, road traffic noise is estimated to cost around £9 billion per year. Sound-absorbing devices are required in many applications, including the built environment (including residential and industrial buildings), consumer household appliances, automotive, aerospace, and defense fields.

[0003] While absorbers are already widely used for noise reduction, existing solutions are too bulky to be practical for many applications. [Summary of the Invention]

[0004] Broadly, the present invention provides a sound-absorbing device configured to absorb sound within a desired frequency range. In this sound-absorbing device, a plurality of resonant elements protrude from a panel sound absorber such as a constraining plate.

[0005] In a first aspect, the present invention provides a sound-absorbing device configured to absorb sound within a desired frequency range. The device comprises a plate configured to be mounted adjacent to a sound-reflective surface for absorbing sound reflected from the sound-reflective surface, and a plurality of resonant elements, each resonant element having a base extending from the plate and a free end, each resonant element extending from the plate at an angle relative to the plate.

[0006] This arrangement enables sound absorption over a wider frequency range than known resonant devices. Furthermore, this can be achieved using a relatively thin plate, which is important in applications where the size and / or mass of the device are important design considerations.

[0007] By positioning the resonant element so that it extends from the plate at a certain angle to the plate, with a free end, the resonant element can vibrate freely at one or more resonant frequencies, thereby attenuating sound at those frequencies. This angle is non-zero. It may be acute or obtuse. That is, this angle is preferably not right.

[0008] In some prior art, the resonator-like feature is positioned coplanar with the plate, rather than at an angle to the plate as in the present invention. The inventors of the present invention have shown that the configuration described in the claims overcomes the disadvantages associated with coplanar arrangement while simultaneously providing unexpected advantages. In particular, the arrangement described in the claims, in which the resonant element is positioned at an angle to the plate, offers a good compromise between performance and manufacturability.

[0009] The plate is preferably a thin plate that can be excited at one or more resonant frequencies in one or more vibration ranges within a desired frequency range. The plate is preferably constrained at its periphery. The plate is preferably a plate without holes.

[0010] The device includes a rear hollow section provided between the plate and the sound-reflecting surface when in use. Thus, the plate and the hollow section together form a panel-type sound absorber, in which the rear hollow section functions as a spring and the plate functions as dispersed mass. In a preferred embodiment, the rear hollow section is an enclosed hollow section, and the device includes means for enclosing the hollow section when in use. The sound-reflecting surface may be a building wall, a vehicle structure such as an automobile or aerospace vehicle, or any other surface where sound absorption is required. Similarly, the plate may constitute a structural element of a building or vehicle, such as a car body panel or an aircraft aerodynamic panel. Resonant elements may be placed on the inner surface of such a panel so as not to be visible during normal use.

[0011] In some embodiments, the device includes a sound-reflecting surface positioned adjacent to the plate, thereby forming a rear hollow between the plate and the sound-reflecting surface. For example, the sound-reflecting surface and the plate are mounted on a frame such that a rear hollow is defined between them.

[0012] The desired frequency range of this device can be up to three octaves, for example, between one and three octaves. This allows the device to demonstrate high utility in applications such as building environments where a wide frequency band is expected.

[0013] Plates and resonant elements can together constitute a metamaterial plate. That is, the combination of these features results in an engineered material exhibiting emergent properties not typically found in natural materials. The properties of the metamaterial arise not from the properties of its fundamental engineering elements, but from the shape and / or composition of the metamaterial.

[0014] The plate preferably has one or more (i.e., tuned) plate resonant frequencies, the back hollow has one (i.e., tuned) hollow-plate resonant frequency, and each resonant element has one or more (i.e., tuned) element resonant frequencies that are different from one or more plate resonant frequencies and one hollow-plate resonant frequency. The one or more plate resonant frequencies, hollow-plate resonant frequencies, and one or more element resonant frequencies are preferably within a desired frequency range. In this way, the plate resonant frequencies, hollow-plate resonant frequencies, and element resonant frequencies can be distributed within a desired frequency range, maximizing the sound absorption effect across that range.

[0015] One or more plate resonant frequencies preferably include one or more resonant frequencies of the plate. For example, the plate has one or more natural frequencies and is excited to one or more vibrational modes at such natural frequencies. The hollow-plate resonant frequency may be the frequency at which the back hollow acts as a spring and vibrates the plate. The combination of these resonant frequencies yields an absorption spectrum with multiple peaks corresponding to the resonant frequencies.

[0016] One or more of the multiple resonant elements may have one or more first resonant frequencies, and one or more of the multiple resonant elements may have one or more second resonant frequencies different from the first resonant frequencies. In other words, each resonant element may have a different set of resonant frequencies than the other resonant elements. In this way, the resonant elements(s) together can provide sound absorption effects over a wide bandwidth where sound absorption is needed, for example, between multiple peaks of plate resonant frequencies.

[0017] One or more plate resonance frequencies, hollow-plate resonance frequencies, and one or more element resonance frequencies are preferably distributed across the entire desired frequency range. This arrangement allows for a good level of sound absorption across the entire desired frequency range.

[0018] In a preferred embodiment, the resonant frequencies of one or more resonant elements are distributed over approximately one-third of an octave above and below the resonant frequencies of one or more plate resonant frequencies and / or hollow-plate resonant frequencies. This has been found to yield particularly good results.

[0019] The resonant elements are preferably generally plate-shaped; that is, each element is generally planar, and its thickness is small compared to other dimensions. Each resonant element may have a substantially uniform thickness between its base and free end. Such elements exhibit excellent performance while also being relatively easy to manufacture. In other embodiments, each resonant element may have a non-uniform thickness. For example, the resonant frequency of each resonant element can be adjusted by changing its thickness.

[0020] In a preferred embodiment, each of the resonant elements has a shape that widens from the base towards the free end. This shape may taper from a narrow base to a wider region. Such a shape simplifies the function of the scales found on moth wings and has been shown to exhibit particularly excellent performance. The inventors found that having a relatively narrow base is beneficial because it allows a wide region to be excited from all directions. The stem, i.e., the narrow base, is where bending occurs, and its dimensions determine the stiffness, while the wider portion behaves like a mass. By narrowing the base, the resonant element can vibrate perpendicular to the direction of the element (this is not possible at appropriate frequencies if the base is wide). Furthermore, a narrow base means that the mounting position on the plate can be set to a position where the plate rotates (rather than translation out of the plate plane).

[0021] At least a portion of the resonant element may extend from the first surface of the plate so as to extend toward the sound-reflecting surface during use. That is, the resonant element extends into the space between the plate and the sound-reflecting surface, and the element is not visible during normal use. Thus, the front surface of the device can have a smooth finish and provide an aesthetically pleasing appearance that can be decorated as preferred by the end user. In some embodiments, further portions of the resonant element may extend from the second surface of the plate opposite to the first surface.

[0022] The resonant elements can be arranged on the plate in an orthogonal grid, a radial grid, or an irregular pattern. The specific arrangement pattern is selected based on the vibrational modes of the plate. That is, the resonant elements are preferably arranged on the plate in the active regions of the plate's vibrational modes. The active region includes the area of ​​the plate where, when the plate is excited at one or more plate resonant frequencies, the resonant elements are excited at their own resonator resonant frequencies. Thus, the location of the active region depends on the mode of the resonant element. For example, if a resonant element is tuned to resonate in a bending mode (out-of-plane direction relative to the element's direction), the active region of the plate will be where the plate is translating out-of-plane (antinode antinodes). Similarly, if a resonant element is vibrating perpendicular to its direction (a waving motion), the active region of the plate will be where the plate is rotating (on the nodal lines). If these two modes have the same frequency, then a large portion of the plate will be the active region.

[0023] Each resonant element may extend from the plate at an angle of 10 degrees or more, and / or 80 degrees or less.

[0024] The plates are configured to be mounted adjacent to the sound-reflecting surface, so that the device depth can be less than 1 / 35, most preferably less than 1 / 40, of the longest wavelength in the desired frequency range. This enables devices with less depth (or thinness) than other known devices. This is particularly effective for longer wavelengths (and lower frequencies) encountered in built environments, which are not adequately addressed by existing devices. For example, the upper limit of the desired frequency range can be 250 Hz, or optionally 200 Hz.

[0025] The device may comprise a frame that constrains the plate at one or more peripheral edges of the plate, for example the entire peripheral edge of the plate, or one or more individual peripheral positions. The frame can suppress movement of the peripheral edge. The frame may also have a peripheral wall extending toward the sound-reflecting wall in use, and define an enclosed back hollow portion between the plate and the sound-reflecting surface. In this way, the frame can serve multiple roles: constraining the plate, mounting the device to a sound-reflecting surface (or other structure), and defining the enclosed back hollow portion.

[0026] In some embodiments, the plate has a plurality of layers including first and second outer layers sandwiched by an elastic layer. This arrangement facilitates manufacturing of the plate. The elastic layer can serve to bond the first and second outer layers together. The elastic layer can mechanically couple the first and second outer layers. The elastic layer (e.g., an elastic adhesive) improves the structural damping of the plate and enables adjustment thereof. The loss factor can be adjusted depending on the thickness of the elastic layer to optimize the amplitude of the absorption peak.

[0027] Each resonant element may have a notch formed integrally with the plate, each notch corresponding to a complementary recess or opening in shape relative to each other within the plate. This arrangement enables a particularly efficient manufacturing process. The notch may, for example, be stamped or laser cut from the material of the plate (or the material of the outer layer of the plate), and then bent relative to the plate to form the resonant element.

[0028] In a second aspect of the present invention, there is provided an assembly comprising a plurality of devices according to the first aspect. In some embodiments, each device has a different desired frequency range. In this way, sound can be absorbed over a wider frequency range.

[0029] For example, at least one device among the plurality of devices has a plate configured to be attached to a sound reflecting surface at a different distance and / or angle from the plate of at least one other device among the plurality of devices, whereby sound in different desired frequency ranges can be absorbed.

[0030] Alternatively or additionally, at least one device among the plurality of devices has a plate having a different thickness, shape, and / or material property from the plate of at least one other device among the plurality of devices, whereby sound is absorbed in different desired frequency ranges.

[0031] Alternatively or additionally, at least one device among the plurality of devices comprises a resonance element having a different thickness, shape, and / or material property from the resonance element of at least one other device among the plurality of devices, whereby sound is absorbed in different desired frequency ranges.

[0032] A third aspect of the present invention provides a method of absorbing sound within a desired frequency range. The method comprises mounting a device according to the first aspect or an assembly according to the second aspect adjacent to a sound reflecting surface, thereby defining a back hollow between the plate and the sound reflecting surface.

[0033] The step of mounting the device may comprise defining an enclosed back hollow enclosed between the plate and the sound reflecting surface. The sealed back hollow may be substantially airtight, or may be configured to prevent leakage of gas within the hollow. The back hollow may be defined by the frame of the first aspect.

[0034] The installation step of this device may include positioning the resonant element toward the sound-reflecting surface. That is, the resonant element extends into the space between the plate and the sound-reflecting surface, and the element may be hidden during normal use. Thus, the front surface of the device is smooth and can provide an aesthetically pleasing finish that can be decorated, for example, according to the end user's preference. In some embodiments, further portions of the resonant element may extend from a second surface of the plate opposite to the first surface.

[0035] The installation step of the device may include positioning the plate adjacent to the sound-reflecting surface such that the longest wavelength in the desired frequency range is less than 1 / 40th of the original wavelength. This provides a sound absorption method that requires a device with less depth (or thinness) than other known devices. It is particularly effective for longer wavelengths found in building environments that existing devices do not adequately address.

[0036] The device mounting step may include mounting a plurality of first-face devices adjacent to a sound-reflecting surface, where the plate of at least one of the plurality of devices is mounted to the sound-reflecting surface at a different distance and / or angle than the plate of at least one other of the plurality of devices. In this way, sound can be absorbed over a wider frequency range.

[0037] A fourth aspect of the present invention provides a method for manufacturing a device that has the first aspect, comprising the following steps: preparing a first layer for forming a plate; forming a plurality of notches in the first layer; and bending each notch along a bending line to form a plurality of resonant elements.

[0038] This method may further include laminating the first layer and the second layer. In a preferred embodiment, an elastomer layer is laminated between the first layer and the second layer.

[0039] Another aspect of the present invention provides a method for manufacturing a device according to the first aspect, comprising the steps of forming plates and resonant elements using additive manufacturing techniques.

[0040] In the descriptions and claims of this specification, the words “equipped with,” “having,” and “including,” as well as their variations (e.g., “wrapping” and the third-person singular of “wrapping”), mean “not limited to these,” and do not exclude other components, numbers, or processes. Furthermore, unless the context requires otherwise, the singular form includes the plural form. In particular, where the indefinite article is used, unless the context requires otherwise, this specification is understood to assume not only the singular form but also the plural form.

[0041] Preferred features of each aspect of the present invention may be similar to those described in relation to any other aspect. Within the scope of this application, the various aspects, embodiments, examples, and alternatives described in the preceding paragraphs, claims, and / or the following description and drawings, in particular their individual features, are expressly intended to be adopted independently or in any combination. That is, any embodiment and / or any feature of an embodiment can be combined in any way and / or in any combination, provided that such features do not conflict with each other.

[0042] One or more embodiments of the present invention will be described below for illustrative purposes only, with reference to the accompanying drawings. [Brief explanation of the drawing]

[0043] [Figure 1] This is an orthographic projection showing the front view of a sound-absorbing device according to an embodiment of the present invention. [Figure 2] This is an orthographic projection showing the front surface of a panel including an array of sound-absorbing devices according to an embodiment of the present invention. [Figure 3] This is an orthographic projection showing the back of the panel in Figure 2. [Figure 4] This is a sketch showing a cross-section of a sound-absorbing device according to an embodiment of the present invention attached to a sound-reflecting surface. [Figure 5] This is an orthographic projection showing the back of a sound-absorbing device according to an embodiment of the present invention. [Figure 6] Figure 5 is an exploded view of the device. [Figure 7] This graph compares the sound absorption spectrum of a sound-absorbing device according to an embodiment of the present invention with that of a rigid plate (panel sound absorber) and a non-rigid plate. The horizontal line indicates the improvement in average sound absorption coefficient achieved by the present invention. [Figure 8] The sound absorption spectra of two embodiments of the present invention are shown, compared to a similar device that does not have a resonant element. [Figure 9] Detailed diagrams of the resonant elements and plates manufactured by the cutting and lamination process are shown. [Figure 10] Figure 9 shows the resonant element and plate, illustrating the state before the resonant element is bent at an angle relative to the plate. [Figure 11] This shows an exploded view of the layers of a device according to an embodiment of the present invention, manufactured by a lamination process. [Figure 12] This shows a possible regular grid arrangement of resonant elements on the plate. [Figure 13] Several possible shapes of the resonant element are shown. [Figure 14] This is a detailed diagram showing an arrangement in which resonant elements can be provided on two surfaces of the plate. [Figure 15] This panel according to an embodiment of the present invention is shown, configured such that each unit cell or sound-absorbing device is mounted at a different distance from the sound-reflecting surface. [Figure 16] The sound absorption spectrum for the thickness ratio (t / λ) of a unit cell according to an embodiment of the present invention is shown. [Figure 17] This shows a unit cell according to an embodiment of the present invention in which resonant elements are arranged in a radial grid. [Figure 18] The panel according to an embodiment of the present invention is shown, which is placed in the corner of a room or other space where sound reduction is required. [Figure 19]The panel according to another embodiment of the present invention is shown, which is placed in a corner or other space of a room where sound reduction is required. [Modes for carrying out the invention]

[0044] <Sound absorption device> The illustrated embodiment shows a sound-absorbing panel 100 used in the architectural field, particularly to cover one or more walls in a room and provide a sound-absorbing wall cover. Readers in the art will understand that the features and concepts illustrated herein are also applicable to other applications, such as providing sound-absorbing devices in automotive and aircraft structures.

[0045] Each panel 100 consists of a plurality of (nine in the illustrated embodiment) sound-absorbing unit cells 10 arranged in a row. Each unit cell 10 includes a plate 20 mounted so as to be offset from a sound-reflecting surface 200 (such as a wall) at a predetermined distance and angle when in use. This arrangement forms a back hollow 300 between the sound-reflecting surface 200 and the plate 20. The back hollow 300 and the plate 20 together form a panel sound absorber, with the back hollow 300 acting as a spring and the plate acting as a dispersed mass. The plate 20 can be positioned parallel or oblique to the sound-reflecting surface 200 at a predetermined distance so that the back hollow 300 has a predetermined depth and / or volume. The hollow 300 usually contains air, but may also contain one or more other compressible gases through which sound waves can propagate. In some embodiments, the hollow 300 may also contain other materials such as thermal insulation, or other components such as systems or structural parts.

[0046] The structural characteristics of plate 20 are adjusted so that the resonant frequency (vibration mode or natural frequency) of plate 20 is distributed approximately adjacent to the sound absorption peak at the hollow-plate resonant frequency of Conversely, thin plates or low-density materials tend to have a smaller distributed mass, which increases the frequency at which the hollow-plate resonance occurs.

[0047] The frame 30 extends along the entire periphery of the plate 20, constraining displacement at the peripheral boundary of the plate 20. In other embodiments, only a portion of the periphery of the plate 20 may be constrained. The unit cell 10 is mounted adjacent to the sound-reflecting surface 200 via the frame 30. In the illustrated embodiment, the depth of the frame 30 is equal to the offset distance between the plate 20 and the sound-reflecting surface 200. That is, the frame 30 provides four walls that surround and confine the back hollow 300. In other embodiments, the frame 30 simply serves to constrain the peripheral edge of the plate 20, and the hollow-back 300 may or may not be surrounded by one or more other structural elements. In yet another embodiment, the frame 30 is mounted to the sound-reflecting surface 200, and the unit cell 10 (or panel 100) may be mounted offset from the walls or other structural elements.

[0048] An array of resonant elements 22 extends from the first surface of the plate 20. Each resonant element 22 consists of a generally planar plate-like member having a base 24 attached to the plate 20 and a free end 26. Each resonant element 22 extends from the plate 20 at an angle, typically an acute angle in the range of 10° to 80°. An appropriate angle is selected to ensure the unhindered movement or vibration of the resonant elements 22. In some embodiments, as shown in Figure 14, an additional array of resonant elements extends from a second surface opposite to the first surface.

[0049] Each of the resonant elements 22 is tuned to one or more elemental resonant frequencies. Typically, the one or more elemental resonant frequencies are within a range defined by one or more plate resonant frequencies and / or hollow-plate resonant frequencies. This maximizes the frequency range over which the unit cell 10 can absorb incident sound.

[0050] In a preferred embodiment, one or more elements 22 are tuned to one or more first frequency sets, one or more elements 22 are tuned to one or more second different frequency sets, one or more elements 22 are tuned to one or more third yet different frequency sets, and so on. In this way, the resonant elements 22 as a whole can be tuned to frequencies that span a desired frequency range over which sound should be absorbed.

[0051] The resonant frequency of the resonant element can be controlled by controlling the shape and / or dimensions of the resonant element 22. Numerical modeling can be used to determine the appropriate shape and / or dimensions of the resonant element 22 in order to achieve a specific resonant frequency. Particularly preferred shapes of the resonant element 22 are shown in Figures 9, 10, and 13(d). The base 24 is relatively narrow, and the shape gradually widens towards the free end 26, becoming a relatively wide portion. The base 24 and the free end 26 have parallel edges, and the intermediate portion tapers from the base 24 to the free end 26. This taper angle is determined by the length and width of the base 24 and free end 26 portions.

[0052] The width, length, and shape of each of the resonant elements 22 are selected according to the target resonant frequency. Alternative shapes are shown in Figures 13(a) to (i). While the illustrated shapes are symmetrical, it should be noted that asymmetrical shapes may be suitable for some applications. The inventors found that a shape mimicking the scales of a moth's wing is particularly suitable. This shape is typically narrow at the base 24 and widens towards the free end 26. More generally, the width of the base 24 portion ranges from at least the thickness of the plate 20 to twice the width of the free end 26 portion. In the illustrated embodiment, the resonant element 22 has a uniform thickness along its entire length. In a preferred embodiment, the thickness of each resonant element 22 is at least the thickness of the plate 20, or at least the thickness of the metamaterial layers 20-1, 20-2 of the plate 20.

[0053] The positions of the resonant elements 22 on the panel 20 are selected based on the vibration modes of the panel. The resonant elements 22 are arranged on a regular or irregular grid to allow sufficient space between adjacent resonant elements 22, enabling their unhindered motion or vibration. For example, the resonant elements 22 are arranged on a regular orthogonal grid, a radial grid, or an irregular grid. Examples of regular grid arrangements are shown in Figures 5, 6, and 12. Figure 17 shows a unit cell 10 with a circular plate 20 and resonant elements 22 arranged in a radial grid. The grid type selection is made based on the vibration modes of the panel 20. The position of each resonant element 22 is selected to be located in the most active region of the plate mode where greater strain energy is accumulated. This improves the achievable sound absorption and provides additional frequency peaks in the region where sound absorption is reduced.

[0054] The number of resonant elements 22 provided on each plate 20 of each unit cell 10 is determined by the size of the plate 20, the predetermined dimensions of the resonant elements 22, and the predetermined spacing between the resonant elements 22.

[0055] The shape of each unit cell 10 is determined by the shape of the metamaterial plate 20. In the illustrated embodiment, the plate 20 is square, but it can be any shape suitable for the application. For example, the plate 20 can be a regular polygon or an irregular polygon. Specific examples include rectangular, circular, hexagonal, or elliptical plates.

[0056] The panel 100 is assembled from multiple unit cells 10 having the same plate shape, or from multiple unit cells 10 combining various predetermined plate shapes that generate a grid-like or periodic arrangement. Adjacent unit cells 10 are either tightly connected or separated by a predetermined distance.

[0057] In some embodiments, each unit cell 10 may be configured to tune to frequencies spanning a different frequency range than one or more other unit cells 10 of the panel 100. For example, each unit cell 10 of the panel 100 may be designed to absorb sound over a predetermined frequency range spanning at least one octave. By mixing several unit cells 10 having different plate 20 configurations (e.g., different plate thicknesses) and / or different resonant element 22 configurations (e.g., different element shapes), and / or different hollow section 300 configurations (e.g., different hollow section 300 depths or volumes), the panel 100 can absorb frequencies over an even wider frequency range.

[0058] Figure 15 shows an example of a panel 100 configuration in which each unit cell 10 is positioned at a different location relative to the sound reflection surface, thereby resulting in different depths and volumes of the back hollow portion 300 of each unit cell 10, and therefore different hollow portion-panel resonance frequencies. The plate 20 and resonant element 22 of each unit cell 10 may be tuned to frequencies adjacent to or spanning the respective hollow portion-plate resonance frequencies, as described above.

[0059] Figures 18 and 19 show possible configurations for mounting the panel 100 of the present invention in a room or other space where sound attenuation is required.

[0060] In Figure 18, panel 100 is attached to the edges of three sides of a room surrounded by three orthogonal planar walls defined by three orthogonal sides 700, 702, and 704. The plate 20 of panel 100 is triangular in shape, with each of its three sides touching a planar wall defined by two of the mutually orthogonal sides 700, 702, and 704. Each of the three walls functions as a sound-reflecting surface, and the space defined between the walls and panel 100 provides a sealed back hollow 300.

[0061] In Figure 19, panel 100 is mounted between two orthogonal walls 710 and 712. The plate 20 of panel 100 is rectangular, and its side edges are in contact with either wall 710 or 712. The two walls 710 and 712 each function as sound reflecting surfaces, and the space formed between the walls and panel 100 provides a rear hollow section 300. The rear hollow section 300 is not a closed space because its top and bottom are open, allowing air (or other compressible gas) to freely enter and exit the hollow section 300.

[0062] The panel 100 according to an embodiment of the present invention absorbs sound that would otherwise be reflected from the surface 200, and can cover a wide range of frequencies due to the combined resonance from the metamaterial plate 20, each individual resonant element 22, and the rear hollow section 300. The panel 100 dissipates energy from incident sound waves through the mutual resonance of the plate 20, the rear hollow section 300, and the resonant elements 22. As a result, the panel 100 absorbs sound that would otherwise be reflected from the sound reflection surface 200.

[0063] Figure 7 illustrates how the present invention achieves sound absorption over a wide frequency band.

[0064] The solid line 410 shows the sound absorption spectrum of a unit cell 10 constituting the sound absorption device according to an embodiment of the present invention.

[0065] The dotted line 420 indicates a single sound absorption peak at the hollow-plate resonance frequency in the plate combined with the rear hollow section 300 (together forming a sound-absorbing panel), but does not consider the effect of the plate's own distortion due to its resonance frequency (i.e., vibration mode) (i.e., it is assumed that the plate is rigid). This hollow-plate resonance frequency provides sound absorption over a relatively narrow frequency band.

[0066] The solid gray line 430 shows the additional absorption peak of the plate 20 in the unit cell 10, excluding the effects of the resonant element 22 and the hollow section 300. The plate 20 is constrained at the peripheral boundary, as in the illustrated embodiment, and is deformable by its resonant frequency (i.e., vibration mode). That is, a finite plate with a constrained boundary condition has a natural resonant frequency determined by its mechanical properties such as mass and bending stiffness. When the resonant frequency of such a plate is adjacent to the hollow section-plate resonant frequency, an additional absorption peak may occur as shown. The width and amplitude of the absorption peak are determined by the damping characteristics (i.e., viscoelastic behavior) of the plate. Thus, the gray line 430 shows an example of an absorption peak caused by the vibration mode of the plate, illustrating how such a peak broadens the frequency bandwidth.

[0067] However, although the overall frequency range is expanded by combining the hollow-plate resonance frequency (caused by the back hollow) and the plate resonance frequency (caused by the vibration modes of the plate), there are still frequencies with relatively low absorption coefficients. These are precisely the frequencies that the resonant element 22 of the present invention targets.

[0068] The shaded region 440 indicates the frequency band in which the resonant element 22 has a resonant frequency (i.e., vibration mode). That is, the resonant element 22 is configured to vibrate and dissipate energy in the target frequency range 440 between the first hollow-plate resonant frequency peak 432 and the second hollow-plate resonant frequency peak 434. This is achieved by tuning each resonant element so that each has one or more different elemental resonant frequencies, and the elemental resonant frequencies collectively cover the target frequency range. This is highlighted by the unit cell absorption spectrum 410. In this way, the average absorption coefficient is improved, and it becomes possible to reduce or eliminate frequencies in the target frequency range 440 that result in a relatively low absorption coefficient.

[0069] Figure 8 further illustrates how the present invention achieves broadband sound absorption.

[0070] The solid line 510 shows the sound absorption spectrum of a unit cell 10 equipped with a sound absorption device according to an embodiment of the present invention. In this configuration, each of the resonant elements 22 is tuned to one or more different elemental resonant frequencies, and they combine to cover the target frequency band 540.

[0071] In contrast, the gray line 520 shows the sound absorption spectrum (absorption spectrum, spectrum) in the same unit cell 10, but here the resonant elements 22 are each tuned to 100 Hz. It can be seen that when the resonant elements 22 are tuned to a dispersed frequency range, better sound absorption performance is obtained across the entire target frequency band 540 than when each element 22 is tuned to the same frequency.

[0072] The dotted line 530 shows the performance of a unit cell 10 that does not have any resonant elements 22, and is for comparison purposes.

[0073] <Experimental Results> The performance of the three prototypes was evaluated by measuring the absorption coefficient spectrum, which defines the proportion of acoustic energy absorbed from the incident sound field as a frequency function. Generally, low frequencies (long wavelengths) are absorbed by thick, heavy sound-absorbing panels, and known products vary in thickness. Therefore, the thickness-to-wavelength ratio (t / λ) was used to compare the absorption efficiency of low frequencies. The thickness (t) is the depth of the unit cell 10 from the sound reflection surface 200 to the outer surface of the plate 20.

[0074] Figure 16 shows the absorption spectra of four prototypes (Panels 1 to 4) with different thicknesses of unit cell 10 (thicknesses: 20 mm, 50 mm, 57 mm, and 95 mm). Line 610 represents the spectrum of Panel 1, line 620 represents the spectrum of Panel 2, line 630 represents the spectrum of Panel 3, and line 640 represents the spectrum of Panel 4. The table below provides details on the characteristics of each prototype. [Table 1]

[0075] Panel 3 and Panel 4 were identical except for the depth of the hollow section 300 at the back. In other words, although the thickness (t) of Panel 3 and Panel 4 were different, their other characteristics were identical.

[0076] Panels 1 and 2 were experimentally measured using the two-microphone transfer function method (ASTM E1050). The frequency response of the embedded resonator was verified using a laser Doppler vibrometer. The spectra of panels 3 and 4 were generated using computer models. Experiments have demonstrated that the prototype method employed yields very good agreement between experimental and computational results.

[0077] The latest prototype iterations (Panels 3 and 4) demonstrate that the present invention can achieve a thickness-to-wavelength ratio of 1 / 40 to 1 / 100 at an absorption rate of 40% or more, which is close to the established maximum performance of moth wings.

[0078] <Manufacturing method> The unit cells 10 of panel 100 can be manufactured by various suitable methods.

[0079] One suitable method involves manufacturing the plate 20 and the resonant element 22 as a single component using additive manufacturing technology. Suitable materials include thermosetting resins, thermoplastic materials, or metals.

[0080] Alternatively, the plate 20 and resonant elements 22 are formed by a cutout and lamination process. These consist of at least three layers, as shown in Figures 9, 10, and 11. The first outer layer 20-1 and the second outer layer 20-2 are laminated to both sides of an internal elastic adhesive layer 20-3, which bonds the outer layers together. The first outer layer 20-1 is die-cut or laser-cut along the cutout contour line 20-4 shown in Figure 10, and then bent along the bend line 20-5 to form each resonant element 22. In the illustrated embodiment, the resulting cutout 20-6 is closed (i.e., blocked or otherwise covered) by the elastic adhesive layer 20-3 and / or the second outer layer 20-2, thereby forming a recess.

[0081] Therefore, the first outer layer 20-1 forms a metamaterial layer. In some embodiments, the second outer layer 20-2 is also provided with resonant elements 22 formed by the same cutting and bending method, and this layer 20-1 is also a metamaterial layer. The metamaterial layer or layers 20-1, 20-2 may be plastic, metal, paper, or corrugated cardboard. The second outer layer 20-2, which is not a metamaterial layer, may be plastic, elastomer, metal, paper, cardboard, or plasticized fabric.

[0082] In related embodiments, the notch 20-6 is instead left open or closed by other means (i.e., blocked or otherwise covered). For example, in embodiments in which the adhesive layer 20-3 and / or the second outer layer 20-2 are omitted.

Claims

1. A device configured to absorb sound within a desired frequency range, A plate mounted adjacent to a sound-reflecting surface and configured to absorb sound reflected from the sound-reflecting surface, wherein the space between the plate and the sound-reflecting surface during use defines a hollow portion at the back, A sound-absorbing device comprising a base attached to the plate and a free end, and a plurality of resonant elements extending from the plate at an angle to the plate.

2. The device according to claim 1, further comprising means for surrounding the rear hollow portion between the plate and the sound reflecting surface when in use.

3. The device according to claim 1 or 2, wherein the plate has one or more plate resonant frequencies, the back hollow portion defined between the plate and the sound reflecting surface during use has a hollow portion-plate resonant frequency, each resonant element has one or more element resonant frequencies, the element resonant frequencies being different from the one or more plate resonant frequencies and the hollow portion-plate resonant frequencies, and the one or more plate resonant frequencies, the hollow portion-plate resonant frequencies, and the one or more element resonant frequencies are within the desired frequency range.

4. The device according to claim 3, wherein the hollow-plate resonant frequency is substantially located at the midpoint of the desired frequency range, and optionally, the first plate resonant frequency is lower than the hollow-plate resonant frequency and the second plate resonant frequency is higher than the hollow-plate resonant frequency, and further optionally, the first element resonant frequency of the one or more element resonant frequencies is lower than the hollow-plate resonant frequency and the second element resonant frequency of the one or more element resonant frequencies is higher than the hollow-plate resonant frequency.

5. The device according to claim 3 or 4, wherein one or more of the plurality of resonant elements have one or more first resonant frequencies, and one or more of the plurality of resonant elements have one or more second resonant frequencies different from the first resonant frequencies.

6. The device according to any one of claims 3 to 5, wherein the one or more plate resonant frequencies, the hollow portion-plate resonant frequencies, and the one or more element resonant frequencies are distributed across the entire desired frequency range.

7. The device according to any one of claims 3 to 6, wherein the one or more element resonant frequencies are distributed over approximately one-third of an octave above and / or below one or each of the one or more plate resonant frequencies and / or the hollow-plate resonant frequencies.

8. The device according to any one of claims 1 to 7, wherein the resonant element is generally plate-shaped.

9. The device according to any one of claims 1 to 8, wherein each resonant element has a substantially uniform thickness between the base and the free end.

10. The device according to any one of claims 1 to 9, wherein each of the plurality of resonant elements has a shape that extends from the base toward the free end.

11. The device according to claim 10, wherein the shape tapers from a narrow base to a wider area.

12. The device according to any one of claims 1 to 11, wherein at least a portion of the plurality of resonant elements extend from the first surface of the plate and thereby extend toward the sound-reflecting surface when in use.

13. The device according to claim 12, wherein a further portion of the plurality of resonant elements extends from a second surface opposite to the first surface.

14. The device according to any one of claims 1 to 13, wherein the plurality of resonant elements are arranged on the plate in an orthogonal grid, a radial grid, or an irregular pattern.

15. The device according to any one of claims 1 to 14, wherein each of the plurality of resonant elements is arranged on the plate in an effective region of the vibration mode of the plate.

16. The device according to any one of claims 1 to 15, wherein each of the plurality of resonant elements extends from the plate at an angle of 10 degrees or more and / or 80 degrees or less.

17. The device according to any one of claims 1 to 16, wherein the plate is configured to be mounted adjacent to the sound reflecting surface, so that the device has a depth of 1 / 40 or less of the longest wavelength in the desired frequency range.

18. The device according to any one of claims 1 to 17, wherein the desired frequency range includes up to three octaves.

19. The device according to any one of claims 1 to 18, further comprising a frame that restrains one or more peripheral edges of the plate.

20. The device according to claim 17, wherein the frame includes a peripheral wall that extends to the sound-reflecting surface during use and defines the back hollow portion enclosed between the plate and the sound-reflecting surface.

21. The device according to any one of claims 1 to 20, wherein the plate has a plurality of layers, including first and second outer layers sandwiched between elastomer layers.

22. The device according to any one of claims 1 to 21, wherein each resonant element has a notch integrally formed with the plate, and each notch corresponds to a recess or opening of a mutually complementary shape within the plate.

23. The device according to any one of claims 1 to 22, further comprising a sound-reflecting surface arranged adjacent to the plate, thereby defining the back hollow portion between the plate and the sound-reflecting surface.

24. An assembly comprising a plurality of devices according to any one of claims 1 to 23.

25. The assembly according to claim 24, wherein each device has a different desired frequency range.

26. The assembly according to claim 24 or 25, wherein at least one of the plurality of devices has a plate configured to be attached to the sound reflecting surface at a different distance and / or angle from the plate of at least one other of the plurality of devices, thereby configured to absorb sound of different desired frequency ranges.

27. The assembly according to any one of claims 24 to 26, wherein at least one of the plurality of devices has a plate having different thickness, shape, and / or material properties from the plate of at least one other of the plurality of devices, and is configured to absorb sound of different desired frequency ranges.

28. The assembly according to any one of claims 24 to 27, wherein at least one of the plurality of devices has a resonant element having a different thickness, shape, and / or material properties than the resonant element of at least one other of the plurality of devices, and is configured to absorb sounds of different desired frequency ranges.

29. A method for absorbing sound within a desired frequency range, comprising the step of defining the back hollow portion between the plate and the sound reflective surface by mounting the device according to any one of claims 1 to 22 adjacent to the sound reflective surface.

30. The method according to claim 29, wherein the attachment of the device includes forming a rear hollow portion surrounded between the plate and the sound reflecting surface.

31. The method according to claim 29 or 30, wherein the mounting of the device includes positioning the resonant element toward the sound reflecting surface.

32. The method according to any one of claims 29 to 31, wherein the mounting of the device includes arranging the plate adjacent to the sound reflecting surface such that the device has a depth of no more than 1 / 40 of the longest wavelength in the desired frequency range.

33. The method according to any one of claims 29 to 32, comprising mounting a plurality of devices according to any one of claims 1 to 23 adjacent to a sound reflecting surface, wherein the plate of at least one of the plurality of devices is mounted to the sound reflecting surface at a different distance and / or angle than the plate of at least one other of the plurality of devices.

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