Acoustic structure

Abstract

An acoustic structure according to the present invention comprises at least two cavities 1, a connection part 2 that connects adjacent cavities 1, and a volume alteration part 3 that increases or decreases the volume of at least one of the cavities 1. An increase or decrease in volume by the volume alteration part 3 changes the sound absorption characteristics of the acoustic structure. At least a portion of the acoustic structure is formed from a material that includes a metamaterial.

Description

Acoustic structure

[0001] The disclosed technology relates to a technology for absorbing sound.

[0002] Due to the spread of telework and the like, the need to eliminate external noise has been increasing. Conventionally, fiber-based sound-absorbing materials such as glass wool have been used for such noise elimination. With these fiber-based sound-absorbing materials, a thickness of about 80 to 100 mm was required to achieve sufficient sound-absorbing performance from low frequencies.

[0003] As a means to solve such problems, the application of acoustic metamaterials has been gradually spreading in recent years. An acoustic metamaterial generally refers to a structure in which Helmholtz resonators having cylindrical inertances and cavities with sizes smaller than the target wavelength, which is the wavelength of the target sound wave, are arranged at intervals shorter than the target wavelength.

[0004] Takashi Yamamoto, "Research Background and Trends of Acoustic Metamaterials", Journal of the Acoustical Society of Japan, Vol. 79, No. 8, 2023, pp. 397 Asahi Fiber Glass Co., Ltd., "Trends in New Technologies", [online], [searched on December 5, 2024], Internet <URL: https: / / www.afgc.co.jp / knowledge / 2017 / 04 / 04 / 8>

[0005] By using an acoustic metamaterial, a thinner sound-absorbing structure can be configured compared to glass wool and the like. However, since a resonator is used, the sound-absorbing characteristics may be limited near the resonance frequency.

[0006] The disclosed technology aims to provide an acoustic structure with changeable sound-absorbing characteristics.

[0007] One aspect of the disclosed technology is an acoustic structure including at least two or more cavities, a connection part connecting adjacent cavities to each other, and a volume change part that expands or contracts the volume of at least one cavity. When the volume change part expands or contracts the volume, the sound-absorbing characteristics of the acoustic structure change, and at least a part thereof is composed of a material containing a metamaterial.

[0008] According to the disclosed technology, the sound-absorbing characteristics can be changed.

[0009] Figure 1 shows an example of an acoustic structure according to the first embodiment. Figure 2 shows an example of an acoustic structure according to the second embodiment. Figure 3 shows an example of an acoustic structure according to the third embodiment.

[0010] Embodiments of the disclosed technology will be described below with reference to the drawings. Note that components having the same function are numbered identically in the drawings, and redundant explanations are omitted.

[0011] [First Embodiment] The acoustic structure of the first embodiment, as illustrated in Figure 1, comprises at least two or more cavities 1, a connecting portion 2 that connects adjacent cavities 1 to each other, and a volume changing portion 3 that expands or reduces the volume of at least one cavity 1.

[0012] As illustrated in Figure 1, the acoustic structure of the first embodiment includes a structure in which Helmholtz resonators, each having a cylindrical inertance or cavity 1 smaller than the wavelength of the target sound wave, are arranged at intervals shorter than the target wavelength. For this reason, at least a part of the acoustic structure of the first embodiment can be said to be composed of a material including metamaterial (more specifically, acoustic metamaterial).

[0013] In the example shown in Figure 1, the sound waves to be absorbed pass through the waveguide P. Arrow A in Figure 1 indicates the direction of sound wave propagation. In the example shown in Figure 1, six cavities 1 are arranged on the waveguide P. In this example, the connecting portion 2 is a member that separates adjacent cavities 1.

[0014] Cavity 1 is, for example, cylindrical. The cavity may also have other shapes such as a cube, a rectangular prism, a prismatic body, or an ellipsoid. Cavity 1 is connected to the waveguide P via a cylindrical neck 11 that is narrower than cavity 1.

[0015] In this example, a volume changing unit 3 is provided on the bottom surface of each cavity 1. In this example, the volume changing unit 3 is an actuator that can move the bottom surface of the cavity 1 up and down. By moving the bottom surface of the cavity 1 up and down, the volume changing unit 3 can change the volume of the cavity 1, thereby changing the resonant frequency. This makes it possible to absorb sound at a desired frequency.

[0016] Furthermore, the volume changing section 3 may be able to expand or contract the volume by moving all or part of the inner surface of the cavity 1 other than the bottom surface. When the volume changing section 3 expands or contracts the volume, the sound absorption characteristics of the acoustic structure change. Furthermore, each of the multiple volume changing sections 3 may be able to arbitrarily change the sharpness of the overall sound absorption frequency characteristics of the acoustic structure by changing the volume of the cavity 1.

[0017] For example, if the height of the neck portion 11 is 10 mm, the diameter of the neck portion 11 is 3 mm, the height of the cavity 1 is 10 mm, and the diameter of the cavity 1 is 10 mm, the resonant frequency with end correction will be 1467 Hz. In this case, if the height of the cavity 1 is 9 mm, the resonant frequency with end correction will be 1546 Hz. Thus, for example, changing the height of the cavity 1 by 1 mm will change the resonant frequency by 79 Hz.

[0018] In this way, by changing the resonant frequency, the center value of the sound absorption frequency can be changed. This makes it possible to arbitrarily change the conventionally fixed sound absorption characteristics. Therefore, even if the frequency of the sound to be absorbed changes, the optimal sound absorption characteristics can be maintained by dynamically or quasi-statically adjusting the characteristics of the sound absorber. This mechanism makes it possible to flexibly respond with a single sound absorber, even in situations where it was previously necessary to replace or add sound absorbers to accommodate different frequencies.

[0019] The volume changing section 3 may also be a vent installed on the waveguide P. Examples of vents include openings and ventilation structures. Alternatively, the volume changing section 3 may be a screw that can change the height of the bottom surface of the cavity 1. In this way, the volume changing section 3 may be able to change the volume of the cavity 1 quasi-statically.

[0020] Acoustic structures may be used in situations other than when sound waves travel through waveguide P. These situations include cases where sound waves strike the acoustic structure from the side (orthogonal direction).

[0021] [Second Embodiment] The acoustic structure of the second embodiment will be described below. The following description will focus on the parts that differ from the acoustic structure of the first embodiment. Parts that are the same as those of the acoustic structure of the first embodiment will not be explained again.

[0022] In the acoustic structure of the second embodiment, the volume-changing section 3 is a derivative. In the example in Figure 2, the bottom surface of the cavity 1 is the volume-changing section 3, which is made of a derivative. Examples of derivatives are materials whose volume changes depending on the applied voltage, such as piezoelectric materials, electrothermal expansion materials, ionic polymer metal composites, and PZT (lead zirconate titanate).

[0023] Since the volume of a dielectric material changes depending on the applied voltage, by using a dielectric material for the volume changing section 3, it becomes possible to control the volume of the cavity 1 with voltage.

[0024] The volume changing section 3 may have an amplification mechanism that amplifies minute fluctuations in the dielectric. Examples of amplification mechanisms include a leverage mechanism that uses a mechanical lever to convert small displacements into large displacements, and a multi-layer structure in which multiple layers of dielectric material are stacked to cumulatively increase the displacement.

[0025] While volume variations due to dielectrics are generally small, typically ranging from 1% to 10%, incorporating an expansion mechanism can significantly increase these variations.

[0026] The volume-changing section 3 may be made of a magnetostrictive material. A magnetostrictive material is a material whose volume changes when a magnetic field is applied.

[0027] Since the volume of a magnetostrictive material changes when a magnetic field is applied, using a magnetostrictive material for the volume changing section 3 makes it possible to control the volume of the cavity 1 with a magnetic field. Even when the volume changing section 3 is made of a magnetostrictive material, the volume changing section 3 may also have an amplification mechanism to amplify minute fluctuations in the magnetostrictive material.

[0028] In the example shown in Figure 2, the bottom surface of the cavity 1 is a volume-changing section 3 made of a derivative. However, all or part of the inner surface of the cavity 1 may be made of a derivative or magnetostrictive material.

[0029] [Third Embodiment] The acoustic structure of the third embodiment will be described below. The following description will focus on the parts that differ from the acoustic structure of the first embodiment. Parts that are the same as those of the acoustic structure of the first embodiment will not be explained again.

[0030] The acoustic structure of the third embodiment, as illustrated in Figure 3, includes a temperature changing unit 4 that changes the temperature of at least one cavity 1, instead of a volume changing unit 3.

[0031] In this example, a temperature changing unit 4 is provided on the bottom surface of each cavity 1. The temperature changing unit 4 is, for example, a Peltier element. The temperature difference between the two sides of a Peltier element can be controlled by applying a voltage. Therefore, the temperature of one cavity can be changed by applying a voltage to the Peltier element.

[0032] The acoustic compliance of cavity 1 is affected by its temperature. Therefore, the temperature change unit 4 can control the resonance frequency by changing the temperature of cavity 1. This allows for control of the sound absorption characteristics.

[0033] Here, assuming an external room temperature of 15°C, the speed of sound is approximately 340 m / s. However, if the temperature of cavity 1 is 30°C, the speed of sound that determines the resonant frequency of the resonator becomes approximately 350 m / s.

[0034] Similar to the first embodiment, when the height of the neck portion 11 is 10 mm, the diameter of the neck portion 11 is 3 mm, the height of the cavity 1 is 10 mm, and the diameter of the cavity 1 is 10 mm, the resonant frequency with end correction is 1467 Hz at 15°C. In this case, when the temperature of the cavity 1 is raised to 30°C, the resonant frequency with end correction becomes 1506 Hz. Thus, changing the temperature of the cavity 1 by 15°C can change the resonant frequency by 39 Hz.

[0035] The temperature changing section 4 may be made of a material that changes temperature, such as a magnetocaloric material whose temperature changes in response to a change in a magnetic field, or an electrocaloric material whose temperature changes in response to a change in an electric field.

[0036] [Variations] The specific configuration of the embodiments of the disclosed technology is not limited to the configuration described above. The specific configuration of the embodiments of the disclosed technology can be modified as appropriate, without departing from the spirit of the embodiments of the disclosed technology.

[0037] For example, the acoustic structures of the first and second embodiments may include a temperature changing unit 4, as described in the third embodiment, in addition to the volume changing unit 3. By making it possible to change not only the volume of the cavity 1 but also the temperature of the cavity 1, the control of sound absorption characteristics can be made more flexible. Furthermore, the redundancy and reliability of the control of sound absorption characteristics are improved.

[0038] It goes without saying that this invention can be modified as appropriate without departing from the spirit of the invention.

[0039] All documents, patent applications, and technical standards described herein are incorporated by reference to the same extent as if each individual document, patent application, and technical standard were specifically and individually described as being incorporated by reference.

Claims

1. An acoustic structure comprising at least two or more cavities, a connecting portion connecting adjacent cavities to each other, and a volume changing portion for expanding or contracting the volume of at least one cavity, wherein when the volume changing portion expands or contracts the volume, the sound absorption properties of the acoustic structure change, and at least a portion of the acoustic structure is made of a material including a metamaterial.

2. An acoustic structure according to claim 1, wherein the volume changing portion is capable of expanding or contracting the volume by moving all or part of the inner surface of the cavity.

3. An acoustic structure according to claim 1, wherein the volume changing portion is a derivative.

4. An acoustic structure according to claim 1, wherein the volume changing portion is made of a magnetostrictive material.

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