JP2009175469A - Sonic velocity control member - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a sonic velocity control member capable of securing stably quality as an industrial product, by explicating a correlation between a physical property of a porous member and an acoustic characteristic thereof. <P>SOLUTION: A heat capacity C and a through-flow resistance R satisfy Expression (1) and Expression (2), in the sonic velocity control member. Expression (1): C>1.2×10<SP>3</SP>(C=D×Cm) and Expression (2): R>F/42.9, where D represents a bulk density of a structure, C represents a specific heat of the structure, and F represents a frequency of a desired sound wave reduced in a sonic velocity. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

The present invention relates to a sound speed control member for reducing sound speed.

Porous materials such as foamed urethane resin, glass wool, zeolite, and activated carbon can be used as sound-absorbing materials for speaker systems equipped in audio equipment such as TV sets, car stereos, and home theaters, or for sound-proofing materials such as audio rooms and sound barriers. It's being used.

For example, a speaker system has been proposed in which a low resilience foamed urethane resin is filled in a speaker box to lower the lowest resonance frequency of the speaker system. This is because the sound pressure energy radiated from the back surface of the diaphragm of the speaker unit attached to the speaker system is rubbed with the air contained in the urethane resin, that is, the air flow friction, and the urethane in contact with the air. This utilizes the phenomenon of conversion into thermal energy by both friction with resin, that is, viscous flow friction (Patent Document 1).
In addition, a speaker system has been proposed in which activated carbon having fine holes is loaded in a speaker box to lower the lowest resonance frequency of the speaker system. This is an attempt to increase the volume of the speaker box by utilizing the physical adsorption / desorption phenomenon of air by activated carbon (Patent Document 2).
Patent Publication 2006-352647 Patent Publication 2007-6459

However, since Patent Documents 1 and 2 do not describe the correlation between the physical properties and acoustic characteristics of the porous member, there is a problem that the material design of the porous member cannot be performed. Therefore, a porous member having desired acoustic characteristics has to be found by trial and error, and it has been difficult to stably ensure the quality as an industrial product.
In addition, many of the previous studies on the acoustic characteristics of porous members are related to the propagation of sound waves propagating through the gas inside the porous member and the solid porous member, and there have been few studies on the physical properties of the porous member. (Non-Patent Documents 1 and 2).
MA Biot, “Theory of Propagation of Elastic Waves in a Fluid-Saturated Porous Solid”, The Journal of The Acoustical Society of America Vol.28, Num.2, p168-191 (1956) Hiroshi Nakagawa et al. "Prediction of acoustic characteristics using Biot's theory, part 1", Proceedings of the Acoustical Society of Japan, p885-886 (2002)

SUMMARY OF THE INVENTION An object of the present invention is to provide a sound speed control member capable of elucidating the correlation between the physical properties and acoustic characteristics of a porous member and stably ensuring the quality as an industrial product.

In order to solve the above problems and achieve the object, the sonic speed control member of the present invention has air permeability, and the heat capacity (C) and the air resistance (R) satisfy the following expressions 1 and 2. It is characterized by comprising the structure to be made.
“Formula 1” C> 1.2 × 10 ³ (C = D × Cm)
"Formula 2"R> F / 42.9
However, D is the bulk density of the structure, Cm is the specific heat of the structure, and F is the frequency of the desired sound wave for which the speed of sound is desired to be reduced.
With such a configuration, it is possible to reduce the speed of sound waves propagating through the gas in the structure.

Moreover, the ventilation resistance (R) of the structure is 10 times or more of F / 42.9. By adopting such a configuration, it is possible to lower the speed of sound.
In addition, the ventilation resistance (R) of the structure is 100 times or more than F / 42.9. By adopting such a configuration, the sound speed can be further lowered.

The speaker system of the present invention is characterized in that the above-described sound speed control member is provided in a speaker box. With such a configuration, it becomes possible to lower the minimum resonance frequency of the speaker system. Therefore, it is possible to improve the bass reproducibility of the speaker system.

The soundproof wall of the present invention includes the above-described sound speed control member. With such a configuration, it is possible to absorb low-frequency sound waves.

According to the sound speed control member of the present invention, it is possible to design the material of the sound speed control member only by setting a desired frequency at which the sound speed is desired to be reduced. Therefore, it is possible to stably ensure the quality of industrial products using the sound member. Is obtained.
Moreover, according to the speaker system of the present invention, an effect of lowering the minimum resonance frequency of the speaker system can be obtained.
In addition, according to the soundproof wall of the present invention, an effect of absorbing low-frequency sound waves can be obtained.

Hereinafter, embodiments of the present invention will be described with reference to the drawings of FIGS. FIG. 1 is a schematic cross-sectional view showing a structural example of a sound speed control member according to the present invention. FIG. 2 is a schematic cross-sectional view showing a configuration example of the speaker system according to the present invention. FIG. 3 is a schematic cross-sectional view showing a configuration example of a sealed speaker system according to the present invention. FIG. 4 is a sound absorption characteristic diagram showing the normal incidence sound absorption coefficient of the sound velocity control member according to the present invention. FIG. 5 is a sound absorption characteristic diagram showing the normal incidence sound absorption coefficient of only the air layer of the sound absorption structure. FIG. 6 is a sound absorption characteristic diagram showing the normal incident sound absorption coefficient of another sound speed control member according to the present invention.

(Embodiment 1)
A structural example of the sound speed control member according to the present invention will be described.
[Configuration of Sonic Control Member]
The sonic speed control member of the present invention is composed of a structure having air permeability, a heat capacity (C) larger than 1.2 × 10 ³ , and a physical property having a ventilation resistance (R) larger than F / 42.9. ing. Here, F is the frequency of the desired sound wave for which the speed of sound is desired to be reduced.
The heat capacity (C) was obtained by measuring the bulk density (D) and specific heat (Cm) of the structure by a normal measuring method and multiplying the bulk density (D) and specific heat (Cm) (D × Cm). It is a calculated value.
When the heat capacity (C) is larger than 1.2 × 10 ³ , the gas in the air holes in the structure is quickly transmitted to the structure by heat generation or cooling caused by the vibration of the gas having a frequency of F [Hz] or less. The temperature does not change. Therefore, it is preferable in that the gas vibrates in a constant temperature state according to Newton's theory, and the effect of reducing the sound velocity of the sound wave propagating through the gas in the structure is exhibited.

The ventilation resistance (R) is a calculated value obtained from the following calculation formula using the air permeability (Q) [m / sec] measured by the JIS L1096 air permeability A method for the structure having the unit thickness (D). .
In order for the gas in the vent hole in the structure to vibrate at a constant temperature, it is necessary to absorb the temperature change of the gas in the vent hole caused by the vibration of the gas within a time of 1 / F or less. Here, if the heat conduction equation is solved with the average diameter of the air holes obtained from the size / shape distribution of the air holes being L and the temperature conductivity of the gas being a, the frequency of the desired sound wave to reduce the speed of sound ( The relationship between F) and L and a can be expressed as: Here, π is the circumference ratio.
a (π / L) ² > F [HZ] (1)
Further, the air permeability (Q) can be expressed by the following equation according to Poisage's law. Where μ is the viscosity coefficient of the gas.
Q = 125 [Pa] ÷ D × L ² ÷ 32 ÷ μ (2)
Accordingly, when Q is obtained by substituting Formula 2 into Formula 1, the following formula is obtained. However, the following equation is calculated with gas as air. The air temperature transfer rate (a) was 0.02 × 10 ⁻³ and the air viscosity coefficient (μ) was 1.79 × 10 ⁻⁵ [Pa / sec]. In addition, the unit thickness of the structure was calculated as 1.
1 / Q> F / 42.9 [sec / m] (3)
If the ventilation resistance (R) is defined as 1 / Q, the ventilation resistance (R) can be expressed by the following equation.
R> F / 42.9 [sec / m] (4)

When the ventilation resistance (R) of the structure is 10 times or more, preferably 100 times or more of F / 42.9, the temperature change of the gas in the ventilation holes is absorbed into the structure more quickly. It is preferable in that the speed of sound at a low frequency of 1 kHz or less can be lowered further.

The ventilation resistance (R) of the structure can be controlled by adjusting the structure of the ventilation hole, specifically, the hole length, hole diameter, surface area of the hole wall surface, and the like. That is, the ventilation resistance (R) can be increased by increasing the hole length, decreasing the hole diameter, and increasing the hole surface area.
In order to increase the length of the hole and increase the surface area of the wall surface of the hole, the hole should have a meander structure. Such a structure is preferable in that the thickness of the structure relative to the incident direction of the sound wave can be reduced.

[Structure structure]
As a structure applicable to the present invention, any air-permeable material that satisfies the above-described heat capacity and air resistance can be used. For example, an open-cell foam made of resin, synthetic rubber, metal, ceramic, metal, or a composite material made of ceramic and resin, or a semi-closed-cell foam in which closed cells and open cells are mixed can be applied.

Specific structural materials include elastomers such as thermoplastic polyurethane, thermoplastic polyolefin, and thermoplastic polystyrene, synthetic rubbers such as NBR, SBR, silicon rubber, and ethylene propylene rubber, resins such as polypropylene resin and polyethylene resin, aluminum A foam obtained by foaming a metal such as alumina or a ceramic such as alumina can be used. Further, a composite material in which metal particles such as aluminum, ceramic particles such as porcelain, and particles such as glass particles are dispersed in a binder resin such as a styrene-butadiene copolymer can be applied.

The structure made of the foam or composite material described above is preferable in that the pores can be formed in a meandering structure. Moreover, these structures are preferable in that the heat capacity (C) is larger than 1.2 × 10 ³ and the ventilation resistance (R) can be easily controlled. In order to make the airflow resistance (R) larger than F / 42.9 [sec / m], in the case of a foam, the foaming rate, and in the case of a composite material, the particle size and the mixing ratio of the particles and the binder Can be achieved by optimizing

(Embodiment 2)
FIG. 1 is a schematic cross-sectional view showing a structural example of a sound speed control member according to the present invention. The sonic control member 10 includes the structure 11 described in the first embodiment and a cooling means 12 such as a Peltier element. The cooling means 12 is supported on the four surfaces of the structure 11 by supporting means such as an adhesive and a mounting bracket.
In FIG. 1, the cooling unit 12 is supported on the four surfaces of the structure 11. However, the cooling unit 2 can be used as long as the air permeability of the structure 11 is maintained and the gas in the structure 11 is cooled. The carrying method is not limited to that shown in FIG.

When the structure 11 is cooled by the cooling means 12 as shown in FIG. 1, the heat generated when the sound pressure is increased by the vibration of the gas in the vent hole is quickly absorbed by the cooling means 12, and the temperature of the gas in the vent hole is increased. Does not change, it is possible to effectively reduce the speed of sound in a low frequency range of several tens Hz or less.
In addition to the cooling means 12, a heating means (not shown) for heating the structural body 11 is provided, and heat absorption generated when the sound pressure is lowered is quickly heated by the heating means so that the gas in the vent hole If the temperature is not changed, a more effective sound speed reduction effect can be obtained.
Therefore, the sound speed control member of this embodiment is suitable for a speaker system that emphasizes low-frequency reproducibility, a soundproof system that absorbs sound waves in the low frequency range, and the like.

(Embodiment 3)
FIG. 2 is a schematic cross-sectional view showing a configuration example of the speaker system according to the present invention. The speaker system 20 according to the present embodiment includes a speaker box 21, a speaker unit 22 provided on one surface of the speaker box 21, and the sound speed control member 23 described in the first or second embodiment. .
With such a configuration, the sound velocity of the sound wave radiated from the back surface of the speaker unit 22 is reduced, so that the lowest resonance frequency of the speaker system 20 can be reduced.
The mounting position of the sonic speed control member 23 may be an arbitrary position in the speaker box 21 and is not limited to the position shown in FIG.

The sound speed control member 23 has a function as an acoustic resistance member. In order to express the function of the acoustic resistance member, a sound speed control member 23 may be attached in the vicinity of the speaker unit 22 as shown in FIG. Specifically, the ratio (V1) between the volume V1 of the space surrounded by the back surface of the speaker unit 22 and the sound speed control member 23 and the volume V2 of the space surrounded by the back surface of the sound speed control member 23 and the inner wall of the speaker box 21. The sound speed control member 23 is provided at a position where / V2) is 1/2 or less, preferably 1/5 or less.
If the sound speed control member 23 is arranged at such a position, it becomes possible to control the braking resistance of the speaker system 20, so that flattening of sound pressure near the lowest resonance frequency of the speaker system 20 and lowering of the lowest resonance frequency are caused. It is preferable in that it is realized at the same time.

In the present embodiment, a closed speaker system has been described as an example of the speaker system 20. However, the speaker system 20 can be applied to other generally known speaker systems such as a bass reflex speaker system and a drone cone speaker system. It is.

(Embodiment 4)
FIG. 3 is a schematic cross-sectional view showing a configuration example of a sealed speaker system according to the present invention. The basic configuration of the sealed speaker system of the present embodiment is the same as that of the speaker system 20 shown in FIG. Therefore, the same components as those in FIG. 2 are denoted by the same reference numerals, and detailed description thereof is omitted. Here, only the configuration different from FIG. 2 will be described.

The sealed speaker system 30 shown in FIG. 3 has a configuration in which humidity control means 31 is provided at an arbitrary location in the speaker box 21 shown in FIG. Here, the humidity control means 31 is a humidity control means for controlling the humidity in the speaker box 21 to a predetermined value.

According to experiments by the inventors, a correlation was obtained between the humidity of the atmosphere in which the sound speed control member was left and the sound speed propagating the gas in the sound speed control member. That is, when the sonic speed control member is left in a low humidity atmosphere and when it is left in a high humidity atmosphere, a result is obtained that the effect of lowering the sonic speed is greater in low humidity than in high humidity. Therefore, as a result of detailed examination of the humidity conditions that increase the sound speed reduction effect, the relative speed becomes 50% or less, preferably 30% or less (excluding 0%). It has been found that the effect of reducing the sound speed is remarkably reduced when the ratio is more than%.

The correlation between the humidity of the atmosphere in which the sonic control member is left and the sonic velocity propagating through the gas in the sonic control member was obtained because of the sonic control member caused by water vapor (gas in water) in the air. It is thought that this is because adsorption / desorption to the surface of the air hole occurs. That is, it is presumed that the adsorption of water vapor occurs when the sound pressure of the sound wave propagating in the sound speed control member increases, and the desorption of water vapor occurs when the sound pressure decreases. The reason will be described in detail below.

In the equation of state of air (PV = nRT), when adsorption / desorption of water vapor occurs, the number of moles of water vapor changes, which causes a change in the number of moles n of air. When the change Δn in the number of moles of air is related to Δn = −ν (ΔP / P) by the sound pressure change ΔP and the constant ν, the volumetric modulus κ of water vapor becomes ΓP when the state equation is differentiated. Here, Γ is 1 / (1 + ν / n).

Since the sound velocity of air becomes the square root of the ratio (κ / ρ) of the bulk modulus κ and the air density ρ, when there is adsorption / desorption, the square root of Γ (less than 1) compared to when there is no adsorption / desorption. The sound speed slows down by the minute.
Further, in the vent hole, the action of damping the vibration of air works by the viscosity of the air contacting the wall surface of the vent hole. This is equivalent to an increase in the air density ρ by a factor of 4/3. Therefore, the sound speed is proportional to the square root of the reciprocal (1 / ρ) of the air density ρ. Therefore, under the condition of “Expression 2” (R> F / 42.9) in claim 1, the sound speed of air is 0. 866
Doubled.

Therefore, in the vent hole, the sound velocity drop due to gas vibration in a constant temperature state according to Newton's theory, the sound velocity drop due to adsorption / desorption of water vapor, and the sound velocity drop due to the viscosity of the air contacting the wall of the vent hole are superimposed. It is assumed that this happens.

Any humidity control means 31 applicable to the present invention can be used as long as the relative humidity in the speaker box 21 is maintained at 50% or less, preferably 30% or less.
For example, humidity control materials such as silica gel, activated carbon, shirasu, sodium carbonate water, diatomaceous earth, or a dehumidifier can be applied. Among them, the humidity control material is preferable in that the configuration of the humidity control means 31 is simplified.

[How to find the speed of sound]
Below, the method of calculating | requiring the sound speed of the sound wave which propagates in the ventilation hole of a sound speed control member is demonstrated.
In the present invention, the normal incidence sound absorption coefficient of the sound absorbing structure having an air layer on the back surface of the perforated board was measured, and the sound velocity was obtained by calculation from the peak frequency of the sound absorbing coefficient of the perforated board.

The peak frequency f of the sound absorption coefficient of the perforated board is equivalent to the resonance frequency f [Hz] of the capacitance Cb of the air layer as shown in equation (5).
f = 1 / (2π√MCb) (5)
However, M is the air mass of the hole of a perforated board.
On the other hand, the capacitance Cb is expressed by Equation 6.
Cb = V / (ρc ² S) (6)
Where V is the volume of the air layer, ρ is the density of air, c is the speed of sound, and S is the total area of the holes.
Therefore, by measuring the peak frequency f, the sound velocity of the air layer can be obtained from Equations 5 and 6.

When the sound speed c ′ when the sound speed control member is filled in the air layer is calculated by the above-described method, the sound speed c of the air layer alone is changed to the sound speed c ′. Therefore, the peak frequency f ′ of the sound absorption coefficient of the perforated board when the sonic control member is filled in the air layer is f × (c ′ / c).
Therefore, since the sound velocity c of only the air layer is known, the ratio between the peak frequency f of the sound absorption coefficient of the air layer only and the peak frequency f ′ of the sound absorption coefficient when the sound velocity control member is filled in the air layer is expressed as follows. By obtaining, the sound velocity c ′ (c ′ = c × f ′ / f) in the sound velocity control member is obtained.

(Specific Example 1)
Based on the above-mentioned “how to obtain the sound speed”, the normal incident sound absorption coefficient of the following sound speed control member was measured. The measurement results are shown in FIG. FIG. 5 shows measurement results obtained by measuring the normal incident sound absorption coefficient when the air layer is not filled with the sound velocity control member.
Hole board: board thickness: 0.2mm, hole diameter: diameter 0.2mm, hole opening rate: 2%
Sonic velocity control member: Urethane foam resin “Urethane foam DOX” manufactured by Bridgestone (hereinafter referred to as “DOX” for short)
Heat capacity of “DOX”: about 2 × 10 ³
Ventilation resistance (R) of “DOX”: about 1500 (air permeability: about 6.4 × 10 ⁻⁴ m ² / sec)
Sample volume: φ30mm × 30mm
Measurement environment: 20 ° C, 60% RH

As can be seen from the comparison between FIG. 4 and FIG. 5, the peak wavelength of the sound absorption coefficient of “DOX” is about 70% lower than the peak wavelength of the sound absorption coefficient of the air layer alone. Therefore, if the sound velocity of the air layer in FIG. 5 is about 340 m / sec, the sound velocity of the sound wave propagating through the vent hole of “DOX” is about 284 m / sec.

(Specific Example 2)
Using the following foamed urethane resin as the sound speed control member, the normal incident sound absorption coefficient was measured under the same measurement conditions as in Example 1. The measurement results are shown in FIG.
Urethane foam resin: "Urethane foam CWZZ" (hereinafter referred to as "CWZZ" for short) manufactured by Bridgestone
Heat capacity of “CWZZ”: about 2 × 10 ³
Ventilation resistance (R) of “CWZZ”: about 11000 (air permeability: about 0.88 × 10 ⁻⁴ m ² / sec)

As can be seen from FIG. 6, the peak wavelength of the sound absorption coefficient of “CWZZ” is about 60% lower than the peak wavelength of the sound absorption coefficient of the air layer alone. Therefore, if the sound velocity of the air layer in FIG. 5 is about 340 m / sec, the sound velocity of the sound wave propagating through the air hole “CWZZ” is about 204 m / sec.

(Comparative Example 1)
Using the following foamed urethane resin as the sound speed control member, the normal incident sound absorption coefficient was measured under the same measurement conditions as in Example 1. The measurement results are shown in FIG.
Urethane foam resin: “Urethane foam DOX” manufactured by Bridgestone (hereinafter referred to as “VPX” for short)
Heat capacity: About 2 × 10 ³
Ventilation resistance (R): about 24 (air permeability: about 1.04 cc / sec)

As can be seen from FIG. 7, the peak wavelength of the sound absorption coefficient of “VPX” is only about 10% lower than the peak wavelength of the sound absorption coefficient of the air layer alone. Therefore, it can be seen that “VPX” has almost no sound velocity lowering effect compared to “DOX” and “CWZZ”.

The sound speed control member according to the present invention is useful for sound equipment such as a speaker system, a car stereo, and a television device, and a soundproof wall material for an audio room and a soundproof room.

The cross-sectional schematic which shows the structural example of the sonic speed control member which concerns on this invention. 1 is a schematic cross-sectional view showing a configuration example of a speaker system according to the present invention. 1 is a schematic cross-sectional view showing a configuration example of a sealed speaker system according to the present invention. The sound absorption characteristic figure which shows the normal incidence sound absorption coefficient of the sound speed control member which concerns on this invention. The sound absorption characteristic figure which shows the normal incidence sound absorption coefficient of only the air layer of a sound absorption structure. The sound absorption characteristic figure which shows the normal incidence sound absorption coefficient of the other sound speed control member which concerns on this invention. The sound absorption characteristic figure which shows the normal incidence sound absorption coefficient of the urethane foam resin used in the comparative example 1.

Explanation of symbols

10 Sonic Speed Control Member 11 Structure
12 Cooling means 20 Speaker system
21 Speaker box 23 Sound speed control member
30 Sealed speaker system 31 Humidity control means

Claims (7)

A sound velocity that has air permeability and has a heat capacity (C) and a gas flow resistance (R) satisfying the following formulas 1 and 2 and reduces the speed of sound waves that propagate in the gas in the structure. Control member.
“Formula 1” C> 1.2 × 10 ³ (C = D · Cm)
"Formula 2"R> F / 42.9
However, D is the bulk density of the structure, Cm is the specific heat of the structure, and F is the frequency of the desired sound wave for which the speed of sound is desired to be reduced. The sound speed control member according to claim 1, wherein a ventilation resistance (R) of the structure body is 10 times or more of F / 42.9. The sound velocity control member according to claim 1, wherein the ventilation resistance (R) of the structure is 100 times or more than F / 42.9. The sound speed control member according to claim 1, wherein the structure is provided with cooling means. A speaker system in which the sound velocity control member according to claim 1 is provided in a speaker box. The speaker system according to claim 5, wherein humidity control means is provided in the speaker box. A soundproof wall provided with the sound speed control member according to claim 1.

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