JP5973102B1 - Propagation constant acquisition method, sound absorption coefficient calculation method, sound absorption coefficient evaluation device - Google Patents

Abstract

A method for obtaining a propagation constant, a method for calculating a sound absorption coefficient, and a sound absorption coefficient evaluation apparatus capable of obtaining a propagation constant and a sound absorption coefficient easily and accurately are provided. A propagation constant is obtained by irradiating a sound absorbing material 2 disposed on the other end side with a sound wave from a sound wave generating source 4 disposed on one end side in a measurement tube 3 extending along an axis Ac. And a propagation constant obtaining method for obtaining a propagation constant of the sound absorbing material 2 by using a plurality of microphones 5 that are spaced apart from each other in the axis Ac direction so as to contact the sound absorbing material 2. A step of calculating a transfer function of the sound wave between the five, a step of calculating a propagation constant of the sound absorbing material 2 by the equation (1) based on the transfer function and the value of the separation dimension between the plurality of microphones 5; ,including. [Expression 1] Note that L represents the distance between the microphones, and H12 and H13 represent transfer functions, respectively. [Selection] Figure 1

Description

  The present invention relates to a method for obtaining a propagation constant, a method for calculating a sound absorption coefficient of a sound absorbing material, and a sound absorption coefficient evaluation apparatus.

  For example, a sound absorbing material is generally provided on a wall surface of a building that houses a noise generation source, such as a gas turbine building, in order to reduce leakage of noise to the outside. As such a sound absorbing material, a material made of a porous material such as glass wool or urethane is particularly preferably used.

  Here, in order to obtain a good noise reduction effect, not only the dimensions (thickness etc.) of the sound absorbing material are appropriately determined according to the assumed noise level, but also the type of sound absorbing material having an appropriate performance. It is important to select. As an index for evaluating the performance of such a sound absorbing material, the sound absorption rate is particularly dominant.

As a method for measuring the sound absorption coefficient of a sound absorbing material, a two-microphone method and a method based on Biot theory described in Non-Patent Document 1 below are widely known.
In the two-microphone method, the sound absorbing material to be evaluated is placed on one end side of the measuring tube, and the sound source is placed on the other end side of the measuring tube, so that the sound absorbing material and the two microphones provided in the middle of the sound source are used. The attenuation rate between the incident sound wave and the reflected sound wave is obtained. It is said that the sound absorption coefficient of the sound absorbing material can be obtained based on this attenuation coefficient.

  In the method described in Non-Patent Document 1, the porous sound-absorbing material is a model including two elements: an elastic porous material (solid part) and air (fluid part) flowing through voids in the elastic porous material. Defined. The Biot theory is rooted in the elastic wave propagation theory derived on the basis of energy attenuation due to the interaction between the elastic porous material and air.

Mori, “Application of Biot Theory (Elastic Porous Vibration Propagation Theory) to Automotive Soundproofing Materials” NICHIAS TECHNOLOGY REPORT 2013 No. 2 361, Nichias Corporation, February 2013, p. 1-4

  However, in the above-described two-microphone method, the measurement results obtained differ depending on the thickness of the sound absorbing material. Therefore, when there are a plurality of types and sizes of the sound absorbing material, it is necessary to perform measurement for each type. For this reason, the request | requirement of the measuring method which can measure a sound absorption rate without being influenced by a dimension, a shape, etc. has increased.

  On the other hand, when calculating the sound absorption coefficient based on the above-mentioned Biot theory, many measurement parameters are particularly required. Examples of measurement parameters include the effective density of the fluid part, the effective bulk modulus of the fluid part, the Young's modulus of the solid part, the Poisson's ratio of the solid part, and the porosity of the solid part and the fluid part. Among these parameters, it is known that the effective density and effective bulk modulus of the fluid part are particularly difficult to directly measure. That is, the difficulty in obtaining these parameters has been a barrier to accurate measurement of the sound absorption coefficient.

  Furthermore, the Johnson-Champoux-Allard model by Allard et al. Has been proposed as a model that extends the Biot theory. While the number of parameters (maze degree of fluid part, viscosity characteristic length, thermal characteristic length) required by this method is small, a dedicated device using ultrasonic waves or helium gas is required for the measurement. In that respect, the measurement is still difficult.

  The present invention has been made to solve the above-described problems, and an object of the present invention is to provide a sound absorption coefficient measurement method and a sound absorption coefficient evaluation apparatus that can easily and accurately measure the sound absorption coefficient.

According to the first aspect of the present invention, a method for obtaining a propagation constant is obtained from a sound wave generation source disposed on one end side in a measurement tube extending along an axis to a sound absorbing material disposed on the other end side. A method for obtaining a propagation constant for obtaining a propagation constant of the sound absorbing material by using a plurality of microphones that are spaced apart from each other in an axial direction so as to irradiate a sound wave and come into contact with the sound absorbing material. The propagation constant of the sound-absorbing material is calculated by the equation (1) based on the step of calculating the transfer function of the sound wave between the microphones, the value of the transfer function, and the distance between the plurality of microphones. Steps. Note that L represents the distance between the microphones, and H ₁₂ and H ₁₃ represent transfer functions, respectively.

  According to this method, the propagation constant of the sound absorbing material can be easily calculated by obtaining the transfer function of the sound wave between the microphones. Furthermore, the transfer function of the sound wave in the measurement tube can be obtained based on an equation describing the pressure fluctuation of the air in the measurement tube without actually measuring the physical properties of the sound absorbing material. The expression describing this pressure fluctuation is obtained based only on the sound wave input component to the microphone. That is, according to this method, the propagation constant of the sound wave inside the sound absorbing material can be obtained easily and accurately only by using the measurement tube, the sound wave generation source, and the microphone.

According to the second aspect of the present invention, the calculation method of the sound absorption coefficient of the sound absorbing material is based on the propagation constant obtained based on the propagation constant obtaining method according to the first aspect. The step of calculating the acoustic impedance of the sound-absorbing material and the step of calculating the sound-absorbing rate of the sound-absorbing material according to the equation (3) based on the acoustic impedance.
Z is acoustic impedance, ρ is the density of air, c is the speed of sound, σ is the porosity of the sound absorbing material, λ is the propagation constant, k is the wave number of the sound wave in the region where the sound absorbing material is not present in the measurement tube, L ₀ represents the dimension of the sound absorbing material in the axial direction, and α represents the sound absorption coefficient.

  According to this method, the acoustic impedance of the sound absorbing material can be obtained from the equation (2) based on the propagation constant inside the sound absorbing material obtained by the method according to the first aspect. Furthermore, since the acoustic impedance has been obtained, the sound absorption coefficient of the sound absorbing material can be calculated as in equation (3) based on the air density ρ and the sound velocity c.

  According to the third aspect of the present invention, the sound absorption coefficient evaluation apparatus is provided on the tubular measurement tube centered on the axis and on one side in the axial direction of the measurement tube, and the sound wave is directed toward the other side in the axial direction. , A sound absorbing material provided on the other side in the axial direction of the measurement tube, a plurality of microphones that are in contact with the sound absorbing material and arranged at intervals in the axial direction, and the plurality of microphones An arithmetic device that calculates the propagation constant of the sound absorbing material by executing the propagation constant obtaining method according to claim 1 based on the sound wave detected by the microphone.

  According to this configuration, the propagation constant of the sound wave inside the sound absorbing material can be easily and accurately obtained by using only the measurement tube, the sound wave generation source, and the microphone.

According to a fourth aspect of the present invention, in the sound absorption coefficient evaluation device, the computing device executes the calculation method of the sound absorption coefficient of the sound absorbing material according to the second embodiment, the sound absorption of the sound absorbing material The rate may be calculated.

  According to this configuration, the acoustic impedance of the sound absorbing material is calculated based on the propagation constant obtained by the arithmetic device according to the third aspect. Based on the acoustic impedance, the air density ρ, and the sound velocity c, the sound absorption coefficient of the sound absorbing material can be easily calculated.

  According to a fifth aspect of the present invention, in the sound absorption coefficient evaluation apparatus, the sound absorbing material has a plurality of types of sound absorbing elements adjacent in the axial direction, and a plurality of microphones are provided for each of the sound absorbing elements. The arithmetic unit may calculate the propagation constant as the sound absorbing material by integrating the transmission matrices including the propagation constant calculated for each of the sound absorbing elements.

  According to this configuration, the propagation constant can be easily calculated for a sound absorbing material having a plurality of different types of sound absorbing elements.

  ADVANTAGE OF THE INVENTION According to this invention, the acquisition method of the propagation constant which can obtain a propagation constant and a sound absorption factor easily and with high precision, the calculation method of a sound absorption factor, and a sound absorption coefficient evaluation apparatus can be provided.

It is a schematic diagram which shows the structure of the sound absorption rate evaluation apparatus which concerns on 1st embodiment of this invention. It is the figure which modeled the sound absorption coefficient evaluation apparatus which concerns on 1st embodiment of this invention. It is process drawing which shows each step in the acquisition method of the propagation constant which concerns on 1st embodiment of this invention, and the calculation method of a sound absorption coefficient. It is a schematic diagram which shows the structure of the sound absorption coefficient evaluation apparatus which concerns on 2nd embodiment of this invention. It is process drawing which shows each step in the acquisition method of the propagation constant which concerns on 2nd embodiment of this invention, and the calculation method of a sound absorption coefficient.

[First embodiment]
A first embodiment of the present invention will be described with reference to the drawings. The sound absorption coefficient evaluation apparatus 1 according to the present embodiment is an apparatus for measuring the sound absorption coefficient of a sound absorbing material 2 as a specimen. Specifically, as shown in FIG. 1, the sound absorption coefficient evaluation device 1 includes a measurement tube 3, a sound wave generation source 4, a sound absorbing material 2, a plurality of microphones 5, and a calculation device 6. .

  The measurement tube 3 extends along the axis Ac and has a circular tube shape centered on the axis Ac. The inner diameter of the measuring tube 3 is the same over the axis Ac direction. That is, the measurement tube 3 is formed in a straight tube shape.

  The sound wave generation source 4 is provided on one end side of the measurement tube 3 in the axis Ac direction, and generates a sound wave in the measurement tube 3 toward the other end side in the axis Ac direction. A sound absorbing material 2 is provided on the other end side in the axis Ac direction in the measurement tube 3 so as to face the sound wave generation source 4. The sound absorbing material 2 is disposed so as to fill the entire surface of the measuring tube 3 on the other end side in the axis Ac direction.

  The sound absorbing material 2 is provided with a plurality (three) of microphones 5 arranged at equal intervals in the axis Ac direction. More specifically, these microphones 5 are provided in contact with the sound absorbing material 2. That is, sound waves that pass through the inside of the sound absorbing material 2 are captured by these microphones 5.

  The calculation device 6 generates a mathematical expression related to pressure fluctuation based on the waveform of the sound wave captured by the microphone 5 and then performs a calculation described later, thereby calculating the propagation constant and the sound absorption rate of the sound absorbing material 2. .

  Below, the calculation which the arithmetic unit 6 performs is demonstrated. As shown schematically in FIG. 3, the arithmetic device 6 first calculates a transfer function between the microphones based on the waveforms respectively input to the plurality of microphones 5 (step S <b> 1). Next, the propagation constant of the sound absorbing material is calculated from the transfer function (step S2). Subsequently, the arithmetic device 6 calculates an acoustic impedance (step S3). Finally, a sound absorption rate is calculated based on this acoustic impedance (step S4).

First, the process of deriving the transfer function will be described.
As shown in FIG. 2, when a sound wave is irradiated from the sound wave generation source 4, the pressure fluctuation of air existing inside the measurement tube 3 occurs. The pressure fluctuation p (x, t) includes an incident wave p ₊ (x, t) from one side in the axis Ac direction to the other side (that is, the sound absorbing material 2 from the sound wave generation source 4) and the other side in the axis Ac direction. There is a reflected wave p ₋ (x, t) from one side to the other side (that is, from the sound absorbing material 2 to the sound wave generating source 4) (see the following formula (1)). These incident waves and reflected waves propagate while being attenuated inside the sound absorbing material 2. In addition, said x represents the position of the axial line Ac direction in the measurement tube 3, and t represents time.

The following (2) Formula and (3) Formula are obtained by performing Fourier transform about time t with respect to said (1) Formula. Here, ω represents an angular frequency, and i represents an imaginary unit. In the expressions (1) and (3), the terms on the right side correspond to each other. That is, the first term on the right side in Equation (3) represents an incident wave, and the second term on the right side represents a reflected wave.

  Here, since the sound wave propagates while being attenuated inside the sound absorbing material 2, the above-mentioned λ corresponding to the wave number of the sound wave is generally a complex number. Hereinafter, λ is referred to as a complex wave number, but more generally, λ may be referred to as a sound wave propagation constant. The real part of the complex wave number λ is related to the propagation speed of the sound wave, and the imaginary part is related to the attenuation characteristic of the sound wave. Further, both a (ω) and b (ω) in the equation (3) correspond to the amplitude of the sound wave.

Next, the following equation (4) is obtained by performing second order partial differentiation on the above equation (3).

  Furthermore, from the relationship between the above equation (4) and the following equation (5), the following equation (6) is obtained. More specifically, equation (6) can be obtained by factoring equation (4) with equation (5). Here, the equation (5) is obtained based on the fluid continuity equation, and means the law of conservation of mass regarding sound waves. Even if the sound absorbing material 2 has an unknown characteristic, it is considered that at least the law of conservation of mass is established when considering the behavior of the sound wave, and therefore the equation (5) is established in the measurement tube 3.

  This equation (5) describes the relationship between air pressure fluctuations caused by sound waves and particle velocity fluctuations. When the expression (6) is applied to a region where the sound absorbing material 2 does not exist, it can be theoretically assumed that λ = k. In this case, equation (6) expresses a momentum conservation law regarding sound waves.

U ^ (x, ω) can be obtained by considering the air as an aggregate of particles and Fourier-transforming the particle velocity variation with respect to time. ρ represents the density of the fluid (air) filling the inside of the measurement tube 3, c represents the speed of sound, and k represents the wave number k (= ω / c) of the sound wave propagating through the region where the sound absorbing material 2 does not exist.

The following expression (7) is obtained based on the expression (6) derived as described above and the above expression (3). That is, by introducing the relationship represented by the above formula (5), the formula (7) can be derived from the formula (3) in an inverse solution manner. In other words, the expression (7) can be derived from the expression (3) representing the input to the microphone 5 without performing any measurement of the physical property parameter regarding the sound absorbing material 2.

Next, a method for calculating and evaluating the sound absorption rate of the sound absorbing material 2 will be described based on the above equation (7). Here, a model as shown in FIG. 2 is assumed. Specifically, an end face (upstream end face 7) on one side in the axis Ac direction and an end face (downstream end face 8) on the other side in the axis Ac direction are defined in the sound absorbing material 2, and the upstream end face 7 and the downstream side are defined. Let L ₀ be the distance to the end face 8.
Here, by eliminating the coefficients a (ω) and b (ω) from the equation (3) representing the pressure fluctuation and the equation (7) representing the particle velocity fluctuation, respectively, the upstream end face 7 and the downstream side A relational expression (transmission matrix) of pressure fluctuation and particle velocity fluctuation at the end face 8 is derived (the following expression (8)).

From the relationship shown in the equation (8), the acoustic impedance Zu at the upstream end face 7 of the sound absorbing material 2 is expressed as the following equation (9).

As shown in the above (9), in order to determine the acoustic impedance Z _U at the upstream side end face 7, it requires an acoustic impedance Z _D of the downstream end face 8 of the sound absorbing material 2, and the complex wave number λ of the sound absorbing material 2 is It becomes.
Here, if the rigidity of the downstream end face 8 is sufficiently high, the acoustic impedance Z _D is infinite. Thus, the acoustic impedance Z _U at the upstream side end face 7 is described as follows (10).

つまり、（１０）式と（１１）式とにより、音響インピーダンスＺは下記（１２）式のように記述することができる。

Note that the acoustic impedance Z _U described above, represents the acoustic impedance when evaluating the noise absorbing member 2 from the inside. On the other hand, when evaluating the sound absorption rate from the outside of the sound absorbing material 2, it is necessary to consider the porosity σ of the sound absorbing material 2 (the volume ratio occupied by the fluid inside the sound absorbing material 2). By using this porosity σ, the acoustic impedance Z when evaluated from the outside of the sound-absorbing material 2 is described as the following equation (11).

That is, the acoustic impedance Z can be described as the following equation (12) by the equations (10) and (11).

  Next, a method for obtaining the complex wave number λ of the sound wave in the sound absorbing material 2 will be described. Here, as shown in FIG. 1, a sound absorption coefficient evaluation apparatus 1 is used in which a plurality (three) of microphones 5 are spaced apart from each other at equal intervals on the sound absorbing material 2. In the following description, of the three microphones 5, the microphone 5 located at the center is referred to as a first microphone 51, and the microphone 5 located on one side (upstream side) in the axis line Ac direction from the first microphone 51. This is called the second microphone 52. Furthermore, the microphone 5 located on the other side (downstream side) in the axis Ac direction from the first microphone 51 is referred to as a third microphone 53.

  These microphones 5 are arranged at equal intervals L (separation dimension L). Further, with the position of the first microphone 51 as a reference, the position in the axis Ac direction of the second microphone 52 is set to −L, and the position in the axis Ac direction of the third microphone 53 is set to + L.

Since the pressure fluctuation in the sound absorbing material 2 is described as the above-mentioned formula (3), the pressure fluctuation obtained by the three microphones 5 (first microphone 51, second microphone 52, and third microphone 53). p ^ 1, p ^ 2, and p ^ 3 can be respectively expressed as the following equation (13).

Furthermore, the transfer function of the second microphone 52 when the first microphone 51 is used as a reference is expressed as H ₁₂ (= p2 / p1), and the transfer function of the third microphone 53 when the first microphone 51 is used as a reference is H. ₁₃ (= p3 / p1). In this case, the relationship between these transfer functions H ₁₂ and H ₁₃ is described as the following equation (14).

From this equation (14), the complex wave number λ (propagation constant) is obtained as the following equation (15).

As described above, the complex wave number λ of the sound absorbing material 2 in the above equation (9) and the acoustic impedance Z _D at the downstream end face 8 of the sound absorbing material 2 were obtained. Here, the relationship between the acoustic impedance of the substance and the sound absorption coefficient is generally given by the following equation (16). That is, the sound absorption coefficient α of the sound absorbing material 2 can be obtained based on the value of the acoustic impedance Z.

  As described above, according to the method and apparatus according to the present embodiment, the propagation constant of the sound absorbing material 2 can be easily calculated by obtaining the transfer function of the sound wave between the microphones 5. Furthermore, the transfer function of the sound wave in the measurement tube 3 can be obtained based on an equation describing the pressure fluctuation of the air in the measurement tube 3 without actually measuring the physical properties of the sound absorbing material 2 or the like. The expression describing the pressure fluctuation is obtained based only on the sound wave input component to the microphone 5. That is, according to this method, the propagation constant of the sound wave inside the sound absorbing material 2 can be obtained easily and accurately only by using the measurement tube 3, the sound wave generation source 4, and the microphone 5.

  Furthermore, according to the method according to the present embodiment, the acoustic impedance of the sound absorbing material 2 can be obtained from the above equation (2) based on the propagation constant obtained based on the relationship between the sound wave transfer functions. Furthermore, since the acoustic impedance is obtained, the sound absorption coefficient of the sound absorbing material 2 can be easily calculated using the equation (3) based on the air density ρ and the sound velocity c.

  On the other hand, in the conventionally proposed two-microphone method, the measurement results obtained differ depending on the thickness of the sound absorbing material 2, so when there are multiple types of dimensions and shapes of the sound absorbing material 2, measurement is performed for each type. There is a need. In addition, in the calculation of the sound absorption rate based on the Biot theory, many measurement parameters are particularly required. Examples of measurement parameters include the effective density of the fluid part, the effective bulk modulus of the fluid part, the Young's modulus of the solid part, the Poisson's ratio of the solid part, and the porosity of the solid part and the fluid part. Among these parameters, it is known that the effective density and effective bulk modulus of the fluid part are particularly difficult to directly measure.

  However, according to the method and apparatus according to the present embodiment, the propagation constant and the sound absorption coefficient can be obtained easily and accurately without measuring the plurality of parameters as described above.

[Second Embodiment]
Next, a second embodiment of the present invention will be described with reference to FIG. As shown in the figure, in the sound absorption coefficient evaluation device 9 according to the present embodiment, the sound absorbing material 10 is formed by a plurality of sound absorbing elements 11 adjacent in the axis Ac direction. The plurality of sound absorbing elements 11 have different types (characteristics). A plurality (three each) of microphones 5 are provided for each of the plurality of sound absorbing elements 11.

  Under such a configuration, the steps shown in FIG. 5 are executed in the propagation constant acquisition method and the sound absorption coefficient calculation method according to the present embodiment. First, the transfer matrix in each sound absorbing element 11 is derived (step S21). The transmission matrices of these sound absorbing elements 11 are integrated with each other as in the above equation (17) (step S22). Next, the propagation constant of the sound absorbing material 10 is calculated from the transfer function (step S23). Subsequently, the arithmetic device 12 calculates an acoustic impedance (step S24). Finally, the sound absorption rate is calculated based on this acoustic impedance (step S25).

More specifically, the calculation device 12 performs the following calculation. First, the transfer matrix (corresponding to the above equation (8)) corresponding to each sound absorbing element 11 is derived by the method described in the first embodiment. Thereafter, in the arithmetic unit 12, the transfer matrix is integrated for each of the plurality of sound absorbing elements 11 (the following equation (17)).

  That is, the transmission matrix in each sound absorbing element 11 is individually evaluated, and these transmission matrices are integrated with each other, and then the calculation corresponding to the above equation (15) is performed to obtain the propagation constant of the sound absorbing material 10 as a whole. be able to. Based on this propagation constant, the sound absorption coefficient of the sound absorbing material 10 as a whole is derived from the above equation (16).

  According to this configuration, the propagation constant can be easily calculated for the sound absorbing material 10 having a plurality of different types of sound absorbing elements 11. In addition, the sound absorption coefficient of the sound absorbing material 10 as a whole can be calculated easily and accurately based on this propagation constant.

The embodiments of the present invention have been described above. However, various modifications can be made to each of the above configurations without departing from the gist of the present invention.
For example, in each of the above embodiments, an example in which three microphones 5 are provided for one sound absorbing material (or one sound absorbing element) has been described. However, the number of microphones 5 is not limited to the above, and may be four or five or more. As the number of microphones 5 is increased, the accuracy of the acquired propagation constant and the value of the sound absorption rate based on the propagation constant can be increased.

DESCRIPTION OF SYMBOLS 1 ... Sound absorption rate evaluation apparatus 2 ... Sound absorption material 3 ... Measurement tube 4 ... Sound wave generation source 5 ... Microphone 6 ... Calculation apparatus 7 ... Upstream side end surface 8 ... Downstream side end surface 9 ... Sound absorption rate evaluation device 10 ... Sound absorption material 11 ... Sound absorption element DESCRIPTION OF SYMBOLS 12 ... Operation apparatus 51 ... 1st microphone 52 ... 2nd microphone 53 ... 3rd microphone Ac ... Axis

Claims (5)

A sound wave source disposed on one end side of the measurement tube extending along the axis irradiates the sound absorbing material disposed on the other end side with a sound wave, and is separated from each other in the axial direction so as to contact the sound absorbing material. Using a plurality of microphones arranged as described above, a propagation constant obtaining method for obtaining a propagation constant of the sound absorbing material,
Calculating a sound wave transfer function between the plurality of microphones;
Calculating a propagation constant of the sound-absorbing material according to the equation (1) based on the transfer function and the value of the spacing between the plurality of microphones;
To obtain a propagation constant containing.
Note that L represents the distance between the microphones, and H12 and H13 represent transfer functions, respectively.

Based on the propagation constant obtained based on the propagation constant obtaining method according to claim 1, the step of calculating the acoustic impedance of the sound absorbing material according to the equation (2);
A step of calculating a sound absorption rate of the sound absorbing material according to the equation (3) based on the acoustic impedance;
Of calculating sound absorption coefficient of sound absorbing material including
Here, Z is acoustic impedance, ρ is the density of air, c is the speed of sound, σ is the porosity of the sound absorbing material, λ is the propagation constant, k is the wave number of the sound wave in the region where the sound absorbing material is not present in the measurement tube, L0 Represents the dimension of the sound absorbing material in the axial direction, and α represents the sound absorption coefficient.

A tubular measuring tube centered on the axis;
A sound wave source that is provided on one side in the axial direction of the measurement tube and that emits sound waves toward the other side in the axial direction;
A sound absorbing material provided on the other side in the axial direction of the measurement tube;
A plurality of microphones that are in contact with the sound absorbing material and are arranged at intervals in the axial direction;
A sound absorption coefficient evaluation apparatus comprising: an arithmetic unit that calculates a propagation constant of the sound absorbing material by executing the propagation constant acquisition method according to claim 1 based on sound waves detected by the plurality of microphones. . The arithmetic unit executes the calculation method of the sound absorption coefficient of the sound absorbing material according to claim 2, sound absorption coefficient evaluation apparatus according to claim 3 for calculating the sound absorption coefficient of the sound absorbing material. The sound absorbing material has a plurality of types of sound absorbing elements adjacent in the axial direction,
A plurality of the microphones are provided for each of the sound absorbing elements,
5. The sound absorption coefficient evaluation device according to claim 3, wherein the arithmetic device calculates the propagation constant as the sound absorbing material by mutually integrating a transmission matrix including the propagation constant calculated for each of the sound absorbing elements. .

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