A force-acoustic decoupling composite structure for a closed speaker enclosure

Abstract

The application discloses a force-sound decoupling composite structure for a closed loudspeaker box, and solves two problems of structural ringing of a full-metal box and reaction force of an internal sound field. A sandwich gradient structure of a viscoelastic damping layer, a rigid constraint isolation layer and a porous fiber sound absorption layer is sequentially constructed on the metal box wall, and a sound impedance gradient decreasing sequence is deliberately designed, so that the structural vibration and the sound field problem are physically separated and efficiently processed. The design significantly reduces system distortion and transient tailing, and restores the effective electro-acoustic conversion efficiency consumed by internal friction due to the structural defects of the box. The application also discloses a construction method of the composite structure, and strictly controls the layer sequence to ensure the realization of the force-sound decoupling effect.

Description

Technical Field

[0001] This invention relates to the field of electroacoustic conversion technology, specifically to a composite acoustic treatment structure for a closed loudspeaker enclosure, which belongs to the International Patent Classification (IPC) H04R 1 / 28. Background Technology

[0002] The speaker enclosure is constructed entirely of metal to suppress macroscopic bending deformation of the enclosure walls with extremely high rigidity. However, the inherent low internal damping characteristics of metal materials bring serious problems in two dimensions: In the solid vibration domain, the metal enclosure walls are subjected to the reaction force of the speaker unit, which easily excites high-Q narrowband mechanical resonance, commonly known as "metal ringing," resulting in sound coloration; In the acoustic domain, the inner metal walls almost totally reflect sound waves, forming a violent standing wave field within the closed enclosure, which creates a continuous and non-uniform sound pressure reaction force on the back of the diaphragm, dynamically hindering diaphragm movement, leading to energy dissipation and deterioration of transient response. In existing technologies, some high-end brands' speaker cabinet solutions mainly rely on a single homogeneous damping layer inside the aluminum plate to suppress structural vibration, focusing on the treatment of the solid vibration domain; another solution sprays damping adhesive on the inner wall of the wooden cabinet, leaving the structural damping and acoustic boundary issues to be solved by a single homogeneous layer; the solution of simply filling the cabinet with sound-absorbing cotton can absorb some sound energy, but it cannot suppress the structural resonance of the metal cabinet wall, and excessive filling will reduce the system's acoustic compliance, resulting in a decrease in low-frequency efficiency. The common limitation of the above solutions lies in a long-standing technological bias: the assumption that the vibration of the enclosure wall structure and the reaction force of the internal sound field are the same physical problem, which can be solved simultaneously by a single homogeneous damping layer or sound-absorbing material. This bias has led the industry to consistently fail to optimize "solid vibration suppression" and "internal sound field management" as two independent subsystems, thus failing to simultaneously address the problems of metal ringing and standing wave reaction forces. This invention is precisely aimed at addressing this technological bias. Summary of the Invention

[0003] The technical problem to be solved by this invention is to overcome the technical bias in the prior art that conflates "structural vibration suppression" with "sound field management", and to provide a force-acoustic decoupling composite structure for use in enclosed loudspeaker enclosures, which can simultaneously suppress high Q-value mechanical resonance of metal enclosure walls, purify the standing wave sound field inside the enclosure, and restore the effective electroacoustic conversion efficiency that is lost due to enclosure defects. To address the aforementioned technical problems, the present invention employs a functional gradient structure for "force-acoustic decoupling" applied to enclosed enclosures. This structure constructs a deliberately mismatched sequence of acoustic impedances from the metal enclosure walls to the interior space: metal enclosure walls (extremely high impedance) → viscoelastic damping layer (low impedance, shear dissipation) → rigid constraint and substrate isolation layer (extremely high impedance, physical isolation) → porous fiber damping layer (medium impedance, sound energy absorption) → interior air (low impedance). This deliberate impedance mismatch ensures that vibrational energy and acoustic energy dissipate along different physical paths, without interference or modulation. The structure consists of three functional layers: First layer: Viscoelastic damping layer, closely attached to the metal box wall, its dynamic shear modulus is lower than that of the box wall material, and it converts the bending vibration energy of the box wall into heat energy through shear deformation, which is specifically responsible for suppressing metal ringing. The second layer is a rigid constraint and base isolation layer, fixed on top of the damping layer. Its acoustic impedance is significantly higher than that of the viscoelastic damping layer and the air inside the chamber. It performs a dual function: forming a constraint damping structure with the first layer, and simultaneously constructing a high-impedance rigid boundary to physically isolate the soft damping layer from the high sound pressure level sound field inside the chamber, thus blocking the direct impact and nonlinear modulation of the sound pressure inside the chamber on the viscoelastic damping layer. The third layer is a porous fiber damping layer with specific flow resistance, fixed on the rigid layer. Its acoustic impedance is lower than that of the rigid constraint and the substrate isolation layer. It is responsible for absorbing early reflected sound energy in the mid-to-high frequency range, increasing the system acoustic impedance to reduce the Q value, shortening the transient decay time, and thus significantly weakening the non-uniform sound pressure reaction force acting on the back of the diaphragm. The presence of the rigid constraint and the base isolation layer physically isolates the viscoelastic damping layer from the porous fiber damping layer, allowing them to act independently on the structural vibration domain and the acoustic field domain, respectively, thus achieving a gradient functional separation of "structural vibration dissipation - force-acoustic decoupling isolation - acoustic energy absorption". The beneficial effects of this invention are as follows: The high-Q mechanical resonance of the metal box wall is effectively suppressed by the constrained damping structure, reducing the structural borne distortion; A rigid isolation layer is used to block the nonlinear modulation of the sound pressure inside the chamber on the soft damping layer, thus avoiding secondary distortion; The sound field inside the chamber is purified by porous fiber layers, which reduces mid-to-high frequency standing wave interference and shortens transient decay time. The aforementioned synergistic effect enables the speaker diaphragm to operate under a more uniform and linear acoustic load, effectively restoring the effective electroacoustic conversion efficiency that was lost due to the original cabinet defects. Under the same semi-anechoic chamber conditions (IEC 60268-5 standard), a comparison of the same enclosure before and after modification shows: Total Harmonic Distortion (THD) decreased from approximately 2.8% to approximately 1.2% (a reduction of approximately 57%), the cumulative spectral attenuation (CSD) waterfall chart shows an average reduction of approximately 40% in energy decay time in the 200Hz-2kHz frequency band, and axial sensitivity improved by approximately 3dB. This synergistic improvement in the above three indicators was not observed in the comparative examples using only a single damping layer or only a porous sound-absorbing layer, proving that the synergistic effect of the three-layer gradient structure cannot be expected by simple superposition and represents an unexpected technical effect. Attached Figure Description

[0004] Figure 1 This is a cross-sectional structural diagram of an embodiment of the present invention, showing the force-acoustic decoupling gradient sequence from the metal box wall to the internal space of the box.

[0005] Figure 2 This is a schematic diagram comparing the frequency response of total harmonic distortion (THD), showing the differences in distortion curves between Comparative Example 1, Comparative Example 2, and Example 1.

[0006] Figure 3 This is a waterfall plot comparison diagram of cumulative spectral attenuation (CSD), showing the transient attenuation differences of Comparative Example 1, Comparative Example 2 and Example 1 (comparative diagram of cumulative spectral attenuation (CSD) energy attenuation).

[0007] Figure 4 This is a schematic diagram of the impedance gradient principle of the core concept of "force-sound decoupling" in this invention, showing the impedance magnitudes and energy transfer paths of the five physical domains. Detailed Implementation

[0008] To make the objectives, technical solutions, and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the accompanying drawings. Comparative Example 1 (Single viscoelastic damping layer) The inner wall of the sealed subwoofer enclosure, CNC machined from aerospace-grade aluminum alloy, is fitted with only a 5mm thick butyl rubber damping layer, lacking a rigid constraint layer and a porous fiber layer. Tested in a semi-anechoic chamber according to IEC 60268-5 standards: THD in the 100Hz-1kHz frequency band decreased from 2.8% to 2.1%, showing limited improvement; the CSD waterfall plot showed no significant reduction in energy decay time in the 200Hz-2kHz frequency band; and axial sensitivity remained unchanged. This demonstrates that a single damping layer can only partially suppress structural vibrations, failing to address the reaction force of standing waves on the diaphragm within the enclosure, and cannot block the nonlinear modulation of the damping layer by the sound pressure within the enclosure. Comparative Example 2 (Single porous fiber filling) A 15mm high-density wool felt was pasted onto the inner wall of an identical enclosure, without a viscoelastic damping layer or a rigid constraint layer. Test results showed a slight reduction in mid-to-high frequency standing waves, but ringing vibrations in the metal enclosure wall still resulted in a THD as high as 2.5%. Furthermore, excessive filling reduced the effective volume of the enclosure, decreased low-frequency acoustic compliance, and increased the low-frequency response. This demonstrates that a single sound-absorbing layer cannot suppress structural vibrations, and does so at the expense of low-frequency performance. Example

[0009] This embodiment applies to a closed subwoofer enclosure CNC machined from aerospace-grade aluminum alloy. The metal enclosure substrate described in this invention can also be made of other high-rigidity metal materials or alloys, formed by casting, stamping, or welding. The specific steps are as follows: 1. Clean the inner surface of the metal box wall by removing oil and dust, ensuring that the bonding surface is dry and free of oxide layer.

[0010] 2. Apply a 5mm thick butyl rubber-based damping adhesive to the entire surface to form a viscoelastic damping layer. This layer converts the bending vibration energy of the box wall into heat energy through shear deformation.

[0011] 3. A 10mm thick birch plywood layer is tightly bonded to the damping layer using structural adhesive, forming a rigid constraint and substrate isolation layer. This layer, together with the damping layer, constitutes a constrained damping structure, while also serving as a high-impedance rigid boundary to isolate the direct impact of sound pressure within the enclosure on the damping layer, and providing a rigid mounting substrate for the upper porous fiber layer.

[0012] 4. Adhere and fix 15mm thick high-density wool felt onto birch plywood to form a porous fiber damping layer. The flow resistance of the high-density wool felt ranges from 1000 to 50000 N·s / m. 4 The preferred range is 5000-15000 N·s / m 4 This preferred range can achieve peak sound absorption coefficient in the mid-to-high frequency band (500Hz-5kHz). The 15mm thickness comes from the inventor's comparative selection of six specifications: 5mm, 8mm, 10mm, 12mm, 15mm, and 20mm. The selection criteria are the lowest overall THD and the best sensitivity improvement, representing the optimal value for both distortion suppression and efficiency recovery. Objective verification of improved sound quality: In a semi-anechoic chamber, comparative measurements were taken on the same enclosure before and after modification: Total Harmonic Distortion (THD): At an output of 85 dB SPL / 1 m, the THD in the 100 Hz-1 kHz band is reduced from approximately 2.8% to approximately 1.2%.

[0013] Waterfall diagram (CSD): The energy decay time in the 200Hz-2kHz frequency band is shortened by an average of about 40%, and the original standing wave energy tail is significantly suppressed.

[0014] Impulse response: The pre-ringing and trailing energy were significantly reduced after the modification. Observation of effective acoustic radiation efficiency recovery: In the comparative experiment, the system's axial sensitivity improved by approximately 3 dB compared to before the modification. Combined with the aforementioned significant improvements in THD and CSD, it is confirmed that this structure effectively recovered the acoustic radiation efficiency lost due to the defects in the original enclosure structure. This invention is particularly suitable for sealed enclosures. For enclosures with phase inversion holes, the arrangement of the third porous fiber damping layer should avoid the phase inversion hole opening area to prevent airflow blockage. Example 2 (Material Variation) As a variation of Example 1, this example replaces the rigid constraint and base isolation layer with a 12mm thick high-density bamboo fiberboard, and replaces the porous fiber damping layer with an 18mm thick centrifugal glass fiber cotton (flow resistance approximately 8000-12000 N·s / m). 4 The remaining structure is the same as in Example 1. Experiments show that this modified scheme also effectively suppresses the vibration of the enclosure wall structure and purifies the sound field inside the enclosure, with a THD improvement rate close to that of Example 1. This modification demonstrates that the core of this invention lies in the three-layer gradient sequence of "viscoelastic damping - rigid constraint isolation - porous sound absorption" and the concept of force-sound decoupling, rather than the sole choice of a specific material. Comparison Conclusion Example 1 simultaneously achieved a significant reduction in THD (approximately 57%), a significant reduction in transient attenuation (approximately 40%), and an improvement in sensitivity recovery (approximately 3 dB), while Comparative Example 1 and Comparative Example 2 failed to achieve all three indicators simultaneously. This synergistic improvement effect exceeds the expectations of those skilled in the art regarding the simple superposition of single technologies and represents an unexpected technical effect. Example 3 (Construction Method) This embodiment illustrates the construction method of the composite structure of the present invention, including the following steps: S1: Clean the inner surface of the metal box wall to remove oil, oxide layer and attachments, and ensure that the bonding surface is dry and flat.

[0015] S2: A viscoelastic damping layer is pasted to cover the entire inner surface of the box wall, so that the damping layer and the metal box wall form a tight fit without voids.

[0016] S3: The rigid constraint and the base isolation layer are tightly bonded and fixed to the viscoelastic damping layer with structural adhesive, so that the two form a constraint damping structure; the acoustic impedance of this layer is higher than that of the viscoelastic damping layer, forming a high impedance acoustic boundary.

[0017] S4: The porous fiber damping layer is bonded and fixed on the rigid constraint and the base isolation layer. The acoustic impedance of this layer is lower than that of the rigid constraint and the base isolation layer, and it is used to absorb the early reflection sound energy of the mid-to-high frequency range. The order of S2, S3, and S4 cannot be reversed. If the porous fiber layer is laid first and then the rigid layer is pressed, the viscoelastic damping layer cannot adhere tightly to the metal box wall, and the constraint damping effect is lost. If S3 is omitted and the porous fiber layer is laid directly on the damping layer, the sound pressure inside the box will act directly on the soft damping layer, resulting in nonlinear modulation and secondary distortion.

Claims

1. A force-acoustic decoupling composite structure for a sealed loudspeaker enclosure, fitted to the inner wall of the enclosure, characterized in that, From the inner wall towards the interior space, the acoustic impedance decreases in a gradient, including the following: A viscoelastic damping layer is attached to the inner wall of the box, and its dynamic shear modulus is lower than that of the box wall material. It is used to convert the bending vibration energy of the box wall into heat energy through shear deformation. A rigid constraint and base isolation layer is covered and fixed on the viscoelastic damping layer. Its acoustic impedance is significantly higher than that of the viscoelastic damping layer and the air inside the chamber. Together with the viscoelastic damping layer, it forms a constraint damping structure and simultaneously forms a high-impedance acoustic boundary to block the direct impact of the sound pressure inside the chamber on the viscoelastic damping layer. A porous fiber damping layer, fixed on the rigid constraint and substrate isolation layer, has a strength of 1000-50000 N·s / m. 4 A preset flow resistance within a certain range, whose acoustic impedance is lower than that of the rigid constraint and the substrate isolation layer, is used to absorb early mid-to-high frequency reflected acoustic energy. The presence of the rigid constraint and the base isolation layer physically isolates the viscoelastic damping layer from the porous fiber damping layer, allowing them to act independently on the structural vibration domain and the acoustic field domain, respectively, thus achieving a gradient functional separation of "structural vibration dissipation - force-acoustic decoupling isolation - acoustic energy absorption".

2. The structure according to claim 1, characterized in that, The viscoelastic damping layer is a butyl rubber base layer, the rigid constraint and substrate isolation layer is a wood multilayer board or bamboo fiberboard, and the porous fiber damping layer is a high-density wool felt or glass fiber cotton.

3. The structure according to claim 1 or 2, characterized in that, The viscoelastic damping layer has a thickness of 3mm-8mm, the rigid constraint and substrate isolation layer has a thickness of 8mm-15mm, and the porous fiber damping layer has a thickness of 10mm-20mm.

4. The structure according to claim 3, characterized in that, The viscoelastic damping layer is 5 mm thick, the rigid constraint and substrate isolation layer is 10 mm thick, and the porous fiber damping layer is 15 mm thick.

5. The structure according to claim 1, characterized in that, The flow resistance of the porous fiber damping layer is preferably 5000-15000 N·s / m. 4 .

6. A loudspeaker system or subwoofer, characterized in that, Its enclosure uses the force-acoustic decoupling composite structure as described in any one of claims 1-5.

7. A method for constructing a force-acoustic decoupling composite structure for a closed loudspeaker enclosure, characterized in that, Includes the following steps: S1: Clean the inner surface of the metal box wall; S2: A viscoelastic damping layer is pasted to completely cover the inner surface of the box wall; S3: The rigid constraint and the base isolation layer are tightly bonded and fixed to the viscoelastic damping layer with structural adhesive, so that the two form a constraint damping structure. S4: Adhere and fix the porous fiber damping layer onto the rigid constraint and substrate isolation layer; The order of S2, S3, and S4 cannot be reversed, and the acoustic impedance of the rigid constraint and the base isolation layer is higher than that of the viscoelastic damping layer and the porous fiber damping layer, so that the three layers form a sequence of decreasing acoustic impedance gradient.

8. The method according to claim 7, characterized in that, The viscoelastic damping layer has a thickness of 3mm-8mm, the rigid constraint and substrate isolation layer has a thickness of 8mm-15mm, and the porous fiber damping layer has a thickness of 10mm-20mm.

9. The method according to claim 7, characterized in that, The structural adhesive mentioned in S3 is an epoxy structural adhesive or a polyurethane structural adhesive, and its elastic modulus after curing is higher than that of the viscoelastic damping layer.

10. The method according to claim 7, characterized in that, The cleaning process described in S1 includes degreasing, dust removal, and removal of the oxide layer to ensure that the bonding surface is dry and free of oxide layer.

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