JP6977949B2 - Vertically incident sound absorption coefficient measuring device and vertically incident sound absorption coefficient measuring method - Google Patents

Description

The present invention relates to a vertical incident sound absorption coefficient measuring device and a vertical incident sound absorption coefficient measuring method using the device.

Vertical incident sound absorption coefficient of a sample (the ratio of the power of a sound wave incident vertically to the power absorbed by the sample when the sound wave incident perpendicular to the sample is reflected perpendicularly to the sample) is vertical. As an incident sound absorption coefficient measuring device, as shown in FIG. 19, a device including an acoustic tube (impedement measuring tube) and two microphones arranged at predetermined intervals in the axial direction of the acoustic tube is known. ing. In this type of vertical incident sound absorption coefficient measuring device, the vertical incident sound absorption coefficient is a sound (generated by a signal generator) from a speaker arranged on one end side of the acoustic tube toward a sample held on the other end side of the acoustic tube. issues a random signal), the transfer function between the two microphones that are measured at that time _(. to _{H 12),} fit the transfer function (ISO 10534-2, JIS a 1405-2, ASTM E1050) It can be obtained by. The transfer function H ₁₂ is measured by inputting the output signals from the two microphones to the fast Fourier transform analyzer (FFT analyzer).

That is, according to the transfer function, normal incidence sound pressure reflectance of the sample (and r _0.) Can be represented by the following formula (1), (a alpha.) Normal incidence sound absorption coefficient of the sample, It can be expressed by the following equation (2). In the following equation (1), k is the wave number (= ω / c, ω: angular frequency, c: sound velocity), s is the distance between _two microphones, and z 2 is between the microphone farther from the sample and the sample. The distance.

_{Therefore, the transfer function H 12} output from the high-speed Fourier conversion analyzer is substituted into the above equation (1) to obtain the vertical incident sound pressure reflectance r ₀ , and the vertical incident sound pressure reflectance r ₀ is obtained by the above equation. By substituting into (2), the vertically incident sound absorption coefficient α can be obtained. However, since the transfer function H ₁₂ between two microphones includes the influence of the mismatch between the microphones, it is necessary to correct the mismatch between the two microphones in advance (JIS A 1405-2 7. 5).

By the way, the range of the frequency f that can be measured by the above-mentioned vertical incident sound absorption coefficient measuring device is defined by the following equation (3). In Formula (3), f _l is the lower limit frequency is determined by the accuracy of the signal processing apparatus. Further, _fu is a frequency that does not generate a non-axial plane wave (diagonal traveling wave), and can be expressed by the following equation (4), where D is the inner diameter of the acoustic tube. The following equation (3) and the following equation (4) indicate that the frequency needs to be lower than the cut-on frequency of the (1,0) order acoustic mode of the acoustic tube. Therefore, in order to measure the sound absorption coefficient at a high frequency with the above-mentioned vertically incident sound absorption coefficient measuring device, it is necessary to reduce the inner diameter D of the acoustic tube.

Further, the distance s between the two microphones needs to be sufficiently small with respect to the wavelength, and the standard stipulates that the following equation (5) is satisfied. The following equation (5) means that it is necessary to reduce the distance s between the two microphones in order to measure the sound absorption coefficient at a high frequency with the above-mentioned vertical incident sound absorption coefficient measuring device.

As described above, in order to measure the sound absorption coefficient at a high frequency in the above-mentioned vertical incident sound absorption coefficient measuring device, it is necessary to reduce the inner diameter D of the acoustic tube and the distance s between the two microphones. be. In this regard, in recent years, there has been an increasing need for measuring sound absorption characteristics at higher frequencies in electric vehicles, devices equipped with inverters, and the like. In view of such circumstances, some measuring device manufacturers have developed a vertically incident sound absorption coefficient measuring device capable of measuring up to around 10 kHz by reducing the inner diameter D of the acoustic tube to about 15 mm. ..

In addition, as a method for measuring a sound wave propagating in an acoustic tube using a plurality of microphones, there are also those described in Non-Patent Document 1 and Non-Patent Document 2. In each of these, the transfer function between a plurality of microphones is measured to separate the acoustic modes in the acoustic tube for measurement.

M. Abom (“A” in “Abom” is “A with top ring” in JIS X 0213), “Modal decomposition in ducts based on transfer function function -114, 1989 Todd Schultz, Louis N. et al. Catafesta III and Mark Sheplak, "Modal decomposition measurement for acoustic impedance testing in square ducts", The Journal of

However, in the vertical incident sound absorption coefficient measuring device of FIG. 19, when the inner diameter D of the acoustic tube is reduced, the sample size is inevitably reduced. Therefore, there is a problem that the variation of the sample, the friction generated between the sample and the acoustic tube, and the influence of the method of attaching the sample to the acoustic tube are likely to appear in the measurement result of the vertical incident sound absorption coefficient. In particular, as the frequency of the incident sound wave increases, not only the component ((0,0) order acoustic mode component) that is vertically incident on the sample and reflected vertically on the sample but also the sample is inside the acoustic tube. In the vertical incident sound absorption coefficient measuring device of FIG. There is also a problem that the influence of the traveling component is included in the obtained vertically incident sound absorption coefficient α.

Further, neither Non-Patent Document 1 nor Non-Patent Document 2 describes removing the influence of the diagonally advancing component that causes the (1,0) order acoustic mode or the like. In addition, since the method of Non-Patent Document 2 focuses on strictly measuring the sound field in the acoustic tube, not only the structure is complicated, but also the position of the sound source needs to be changed and measured multiple times. It is hard to say that it is practical.

The present invention has been made to solve the above problems, and it is possible to reduce the influence of the diagonally traveling component on the measurement result (vertically incident sound absorption coefficient), and the inner diameter of the acoustic tube is not particularly reduced. , Provided is a vertically incident sound absorption coefficient measuring device capable of measuring up to a high frequency band. It is also an object of the present invention to provide a vertical incident sound absorption coefficient measuring device capable of evaluating the reliability of the vertical incident sound absorption coefficient obtained by the measurement by measuring the influence of scattering on the sample. Furthermore, it is also an object of the present invention to provide a vertical incident sound absorption coefficient measuring method for measuring a vertical incident sound absorption coefficient of a sample using the vertical incident sound absorption coefficient measuring device of the present invention.

The above issues are
An acoustic tube in which the shape of the cross section perpendicular to the axial direction forms a tubular shape with line symmetry and point symmetry, and the sample is placed inside the tip side in the axial direction.
A speaker mounted on the axial base end side of the acoustic tube,
Multiple microphones installed on the peripheral wall of the acoustic tube,
It is a vertical incident sound absorption coefficient measuring device equipped with
Multiple microphones
At least one set of microphones MA _{. 1} , MA _. The first microphone group consisting of _{2 and}
At least one set of microphones MB arranged point-symmetrically on another cross section B separated by a predetermined distance in the axial direction from the cross section A of the acoustic tube _{. 1} , MB _. A second microphone group consisting of _2,
Consists of
Microphones that make up the first microphone group MA _{. 1} , MA _{. 2} , ... output signals SA _{. 1} , SA _{. 2} , ... sum SA _{. 1} + SA _{. 2} + ... (hereinafter, referred to as "[sigma] s _A".) And a microphone _{M B.} constituting the second Micron Group ₁ , MB _{. 2} , ... output signals SB _{. 1} , SB _{. 2} , ... sum SB _{. 1} + SB _{. From 2} + ... (hereinafter referred to as "ΣS _B "), the transfer function between the first microphone group and the second microphone group is obtained, and the transfer function method is applied to this transfer function to sample. A vertical incident sound absorption coefficient calculation means (usually a program in a computer) for calculating the vertical incident sound absorption coefficient of
At least, the microphones MA _{. 1} , MA _. Output signal of _{2 SA. 1} , SA _. Difference of _{2 SA. 1-} S _A. Reliability evaluation parameter calculation means (usually a program in a computer) for calculating _{2 and}
It is solved by providing a vertically incident sound absorption coefficient measuring device, which is further provided with the above.

This is because the microphones MA _{. 1} , MA _{. 2} of the output signal and the sum [sigma] s _A, the microphone M _B. constituting the second Micron Group ₁ , MB _. By taking the sum [sigma] s _{B 2} of the output signal, it is possible to cancel the influence of the oblique traveling component such as (1,0) following acoustic modes. Therefore, as described above, the transfer function between the first microphone group and the second microphone group from _{the sum ΣS A} of the output signals of _{the first microphone group and the sum ΣS B of the output signals of the second microphone group.} By calculating the vertical incident sound absorption coefficient from this transmission function, it becomes possible to make it difficult for the influence of the diagonally traveling component such as the (1,0) order acoustic mode to appear in the obtained vertical incident sound absorption coefficient. Therefore, it is possible to measure up to a high frequency band without reducing the inner diameter D of the acoustic tube.

In addition, the microphones MA _{. 1} , MA _. Output signal of _{2 SA. 1} , SA _. Difference of _{2 SA. 1-} S _{A. In} No. 2, the diagonally traveling component such as the (1,0) order acoustic mode is doubled, and this difference SA _{. 1-} S _{A. Looking at 2} , it becomes possible to determine whether or not a diagonally traveling component is generated inside the acoustic tube. In other words, the difference SA _{. 1-} S _{A. Reference numeral 2} can be used as a parameter (reliability evaluation parameter) for evaluating the vertical incident sound absorption coefficient calculated by the vertical incident sound absorption coefficient calculating means. Therefore, as described above, the microphones MA _{. 1} , MA _. Output signal of _{2 SA. 1} , SA _. Difference of _{2 SA. 1-} S _{A. By calculating 2} , it is also possible to evaluate the reliability of the vertically incident sound absorption coefficient obtained by the measurement.

In the vertical incident sound absorption coefficient measuring apparatus of the present invention,
The first microphone group, at least
A set of microphones MA _{. 1} , MA _{. 2} and
Microphone MA _{. 1} , MA _. Are line symmetry arranged on section A for _two pair of microphones M _{A. 3} , MA _{. 4} and
Consists of
The second microphone group, at least
A set of microphones arranged point-symmetrically on the cross section _{B. 1} , MB _{. 2} and
Microphone MB _{. 1} , MB _{. A} set of microphones MB arranged line-symmetrically on the cross section B with respect to _{2. 3} , MB _{. 4} and
Consists of
A means for calculating the vertical incident sound absorption coefficient,
Microphones that make up the first microphone group MA _{. 1} , MA _{. 2} , MA _{. 3} , MA _{. 4} , ... Output signals SA _{. 1} , SA _{. 2} , SA _{. 3} , SA _{. 4,} the sum [sigma] s _A of ..., microphones _{M B.} constituting the second microphone group ₁ , MB _{. 2} , MB _{. 3} , MB _{. 4} , ... Output signals SB _{. 1} , SB _{. 2} , SB _{. 3} , SB _{. From} the sum ΣS _B of 4, ..., the transfer function between the first microphone group and the second microphone group is obtained, and the transfer function method is applied to this transfer function to calculate the vertical incident sound absorption coefficient of the sample. It is preferable to use the above.

As described above, two sets (four each) of microphones are arranged on the cross section A and the cross section B of the acoustic tube, and the microphones MA arranged on the cross section _{A. 1} , MA _{. 2} , MA _{. 3} , MA _. Output signal of _{4 SA. 1} , SA _{. 2} , SA _{. 3} , SA _{. 4} and the sum [sigma] s _A, the microphone _{M B.} disposed on section B ₁ , MB _{. 2} , MB _{. 3} , MB _. Output signal _{SB of 4. 1} , SB _{. 2} , SB _{. 3} , SB _. By taking the sum ΣS _{B of 4} , ..., it is possible to cancel not only the diagonal progression component of the (1,0) order acoustic mode but also the influence of the diagonal progression component such as the (2,0) order acoustic mode. It will be possible. Therefore, it is possible to make it difficult for the influence of the oblique traveling component such as the (1,0) order acoustic mode and the (2,0) order acoustic mode to appear in the obtained vertically incident sound absorption coefficient. Therefore, it becomes possible to measure the vertical incident sound absorption coefficient in a higher frequency band.

In the vertical incident sound absorption coefficient measuring device of the present invention, when two sets (four each) of microphones are arranged on the cross section A and the cross section B of the acoustic tube as described above.
Reliability evaluation parameter calculation means,
To the sum ΣS _A, microphone _{M A. 1} , MA _. Output signal of _{2 SA. 1} , SA _. Difference of _{2 SA. 1-} S _{A. 2} ratio _(S A.1 _{-S A.2)} / and [sigma] s _A,
To the sum ΣS _A, microphone _{M A. 3} , MA _. Output signal of _{4 SA. 3} , SA _. Difference of _{4 SA. 3-} S _{A. 4} ratio and _{(S A.3 -S A.4) / ΣS} A,
It is preferable to calculate.

Since the ratio _{(S A.1 -S A.2) / ΣS} A and the ratio _(S A.3 _{-S A.4)} / _ΣS when _A is known, wave component entering perpendicularly to the sample ( To obtain the ratio A ₁₀ / B _{00 of the component (referred to as “sound wave component A 10} ”) obliquely reflected from the sample in the (1,0) order acoustic mode to the “sound wave component B _00”). This is because the reliability of the vertical incident sound absorption coefficient obtained by the measurement can be evaluated with relatively high accuracy by a relatively simple method. In the following, by determining the ratio _{(S A.1 -S A.2) / ΣS} A and the ratio _{(S A.3 -S A.4) / ΣS} A from the ratio _A 10 _{/ B 00,} obtained by measurement The method of evaluating the reliability of the vertical incident sound absorption coefficient is sometimes called a "simple method".

Further, in the vertical incident sound absorption coefficient measuring device of the present invention, when two sets (four each) of microphones are arranged on the cross section A and the cross section B of the acoustic tube as described above.
Reliability evaluation parameter calculation means,
To the sum ΣS _A, microphone _{M A. 1} , MA _. Output signal of _{2 SA. 1} , SA _. Difference of _{2 SA. 1-} S _{A. 2} ratio _(S A.1 _{-S A.2)} / and [sigma] s _A,
To the sum ΣS _A, microphone _{M A. 3} , MA _. Output signal of _{4 SA. 3} , SA _. Difference of _{4 SA. 3-} S _{A. 4} ratio and _{(S A.3 -S A.4) / ΣS} A,
To the sum ΣS _B, microphone _{M B. 1} , MB _. Output signal of _{2 SB. 1} , SB _. Difference between _{2 SB. 1-} S _{B. 2} ratio _(S B.1 _{-S B.2)} / and [sigma] s _B,
To the sum ΣS _B, microphone _{M B. 3} , MB _. Output signal _{SB of 4. 3} , SB _. Difference of _{4 SB. 3-} S _{B. 4} ratio _(S B.3 _{-S B.4)} / and [sigma] s _B,
It is also preferable to calculate.

As mentioned in "simple method" described above, the ratio _{(S A.1 -S A.2) / ΣS} A and the ratio _{(S A.3 -S A.4) / ΣS} A is found when the ratio _{A 10} / When it is possible to determine the B _00, further, the ratio _{(S B.1 -S B.2) / ΣS} B and the ratio _(S B.3 _{-S B.4)} / _ΣS When _B is found, the sample The ratio of _{the component (referred to as “sound wave component B 10} ”) obliquely incident on the sample in the (1,0) order acoustic mode to the sound wave component (referred to as “sound wave component B _{00”) vertically incident on the sample.} It is also possible to obtain B ₁₀ / B _00. Therefore, the reliability of the vertically incident sound absorption coefficient obtained by the measurement can be evaluated with higher accuracy than the simple method. Hereinafter, the ratio _{(S A.1 -S A.2) / ΣS} A and the ratio _{(S A.3 -S A.4) / ΣS} A and the ratio _(S B.1 _{-S B.2)} / _ΣS by obtaining the _B and the ratio _(S B.3 _{-S B.4)} / _ΣS ratio _a 10 _{/ B 00} and the ratio _B 10 _{/ B 00} from _B, evaluate the reliability of the normal incidence sound absorption coefficient obtained by measuring The method of doing this is sometimes called the "detailed method".

In the vertical incident sound absorption coefficient measuring device of the present invention, the reflectance evaluation parameter calculation means is used to reflect the incident power reflectance in the acoustic tube (in the case of the above simplified method, the reflection defined by the equation (45) described later. It is also preferable to calculate the rate R, in the case of the above-mentioned detailed method, the reflectance R defined by the formula (58) described later. The same shall apply hereinafter).

This is because, in reality, scattering occurs on the surface of the sample, so the sound waves reflected diagonally by the sample are repeatedly reflected and stopped at the inner peripheral surface of the acoustic tube or at the end of the speaker side of the acoustic tube. After that, it is considered that the sample is incident again. In this regard, if the reflectance of the incident power in the tube is known, the influence of scattering on the surface of the sample can be evaluated, and the reliability of the vertical incident sound absorption coefficient obtained by the measurement can be evaluated with higher accuracy. Is possible.

In the vertically incident sound absorption coefficient measuring device of the present invention, the microphones MA _{. 1} , MA _{. 2} , ..., and the microphones MB that make up the second micron group _{. 1} , MB _{. It is also preferable to arrange 2} , ... In a state where they do not overlap each other when viewed from the axial direction of the acoustic tube (hereinafter, may be referred to as “non-polymerization arrangement”).

This is because the microphones MA _{. 1} , MA _{. 2} , ..., and the microphones MB that make up the second micron group _{. 1} , MB _{. When 2} , ... Are arranged so as to overlap each other when viewed from the axial direction of the acoustic tube (hereinafter, may be referred to as "polymerization arrangement"), the microphone spacing s (cross section A). Even if an attempt is made to narrow the distance between the cross sections B), the microphones MA _{. 1} , MA _{. 2} , ... The housing and the microphones MB that make up the second microphone group _{. 1} , MB _. The distance s (distance between the cross section A and the cross section B) of the microphones can be narrowed only within the range where the housings of _{2, ... Do not interfere with each other.} That is, as already described, the smaller the interval s, the more the sound absorption coefficient can be measured in the high frequency band, but the polymerization arrangement cannot reduce the interval s. In this respect, if the non-polymerization arrangement is adopted, even if the distance s (distance between the cross section A and the cross section B) of the microphones is narrowed, the microphones MA _{. 1} , MA _{. 2} , ... The housing and the microphones MB that make up the second microphone group _{. 1} , MB _{. Since} it does not interfere with the housing of 2, ..., the microphone spacing s (distance between cross-section A and cross-section B) can be set smaller, and the sound absorption coefficient in a higher frequency band can be measured. Because it will be possible.

As described above, according to the present invention, it is possible to reduce the influence of the diagonally traveling component on the measurement result (vertically incident sound absorption coefficient), and it is possible to measure up to a high frequency band without particularly reducing the inner diameter of the acoustic tube. It becomes possible to provide a vertically incident sound absorption coefficient measuring device made possible. It is also possible to provide a vertical incident sound absorption coefficient measuring device capable of evaluating the reliability of the vertical incident sound absorption coefficient obtained by the measurement. Further, it is also possible to provide a vertical incident sound absorption coefficient measuring method for measuring the vertical incident sound absorption coefficient of a sample by using the vertical incident sound absorption coefficient measuring device of the present invention.

It is a figure explaining the cylindrical sound field inside an acoustic tube. It is a figure explaining the sound pressure distribution in the cross section of each acoustic mode. It is a figure which showed an example of the vertical incident sound absorption coefficient measuring apparatus which concerns on this invention. It is a figure explaining the outline of an experiment. This is a picture of the experiment. It is a graph which showed the measurement result of the vertical incident sound absorption coefficient when the anti-hair felt (20 mm thickness) was used as a sample. It is a graph which showed the measurement result of the vertical incident sound absorption coefficient when the melamine foam (25 mm thickness) was used as a sample. It is a graph which showed the measurement result of the vertical incident sound absorption coefficient when the glass wool (thickness 50mm) was used as a sample. It is a graph which showed the measurement result of the vertical incident sound absorption coefficient when PET felt (12 mm thickness) was used as a sample. It is a photograph of a state in which the back wall surface is provided in an inclined state with respect to the cross section of the acoustic tube by filling the inside of the acoustic tube with oil clay. A graph showing the measurement results of the vertical incident sound absorption coefficient when the back wall surface is tilted 30 degrees with respect to the cross section of the acoustic tube and when the back wall surface is parallel to the cross section of the acoustic tube (0 degree). be. It is a graph which showed the result of having calculated the reflectance of the incident power in a tube by obtaining the reflectance _{R 0 → 1} of the vertical incident (1,0) order acoustic mode by a simple method. It is a graph which showed the result of having calculated the reflectance of the incident power in a tube by obtaining the reflectance _{R 0 → 1} of the vertical incident (1,0) order acoustic mode by a detailed method. It is a photograph of the sound absorbing material (sample) that was the subject of verification in the experiment. It is a graph which showed the measurement result of the vertical incident sound absorption coefficient and the like when the sample of FIG. 14A was used. It is a graph which showed the measurement result of the vertical incident sound absorption coefficient and the like when the sample of FIG. 14B was used. It is a graph which showed the measurement result of the vertical incident sound absorption coefficient and the like when the sample of FIG. 14C was used. It is a figure explaining the arrangement of a microphone. It is a figure which showed an example of the conventional vertical incident sound absorption coefficient measuring apparatus.

The vertical incident sound absorption coefficient measuring device of the present invention will be described more specifically with reference to the drawings. First, the vertical incident sound absorption coefficient in the "8 microphone type vertical incident sound absorption coefficient measuring device" and the "4 microphone type vertical incident sound absorption coefficient measuring device" which are typical embodiments of the vertical incident sound absorption coefficient measuring device of the present invention. The measurement principle of is explained.

In the following, for convenience of explanation, the "microphone _MA.1 ", "microphone _MA.2 ", "microphone _MA.3 " and "microphone _MA.4 " constituting the above-mentioned first microphone group are referred to. each "Mic.1", "Mic.3", denoted as "Mic.2" and "Mic.4", constituting the second microphone group the aforementioned "microphone _{M B.1",} "microphone _{M B .2} "," microphone _{M B.3} "and" microphone _{M B.4} ", respectively, referred to as" Mic.5 "," Mic.7 "," Mic.6 "and" Mic.8 ". These correspondences are shown in Table 1 below.

[1] 8 Microphone-type vertical incident sound absorption coefficient measuring device [1.1] Measurement principle of 8 microphone type vertical incident sound absorption coefficient measuring device First, the measuring principle of 8 microphone type vertical incident sound absorption coefficient measuring device will be described. The 8-microphone type vertical incident sound absorption coefficient measuring device is a type of vertical incident sound absorption coefficient measuring device in which four microphones are arranged on each of a cross section A and a cross section B of an acoustic tube, for a total of eight. Here, consider the cylindrical sound field inside the acoustic tube as shown in FIG. The sound pressure inside the acoustic tube can be expressed by the following equation (6).

However, in the above equation (6), r _T is the distance from the center of the circular cross section of the acoustic tube, θ is the angle from the x-axis, J _m is the first-class Vessel function, and m and n are in the circumferential direction and the radial direction, respectively. Represents the acoustic mode order. Further, C _mn is a standardized constant, and A ^σ _mn , B ^σ _mn , A ^τ _mn , and B ^τ _mn are the amplitudes of sound waves. Further, k _{z (m, n)} represents the wave number in the z direction of the (m, n) order acoustic mode, and if k ₀ = ω / c (c: sound velocity), the following equation (7) is satisfied.

_{However, kr (m, n} ) in the above equation (7) is a wave number in the cross section of the acoustic tube derived from the boundary condition on the inner wall of the acoustic tube, and satisfies the following equation (8).

Satisfies the above formula _{(8), k r (m} , n) when the value of _R and alpha _{m, n,} the value of the cross-section Uchioto pressure distribution and alpha _{m, n} of each acoustic mode, as shown in FIG. 2 Become. The (0,0) order acoustic mode represents a plane wave traveling perpendicular to the cross section inside the acoustic tube. The vertical incident sound absorption coefficient is the sound absorption coefficient for the planar sound wave in this (0,0) order acoustic mode. On the other hand, the higher-order acoustic mode other than the (0,0) order acoustic mode represents a plane wave traveling diagonally inside the acoustic tube. However, as can be seen from the above equation (7), at a low frequency _{where k 0} < _{kr (m, n) , k z (m, n)} is a pure imaginary number, so that an evanescent wave is inside the acoustic tube. It attenuates and does not propagate. Therefore, it can be represented by (m, n) the minimum frequency (cut-on frequency) f m the following acoustic modes can propagate in the interior of the sound _{tube, n} represents the following formula (9).

In an acoustic tube for measuring a normal sound absorption coefficient, measurement is performed in a frequency region in which a higher-order acoustic mode other than the (0,0) order acoustic mode does not propagate. That is, since the acoustic mode having the lowest cut-on frequency is the (1,0) order acoustic mode, the upper limit frequency that can be measured from the above equation (9) is as shown in the following equation (10). The following formula (10) corresponds to the above formula (4).

Here, consider the frequency domain in which the (0,0) order acoustic mode, the (1,0) order acoustic mode, and the (2,0) order acoustic mode propagate. In this case, the sound pressure inside the acoustic tube is expressed by the following formula (11) from the above formula (6).

First, consider the cross section A of FIG. sound pressure at four points r _{1, r} 2, _{r 3} and _{r 4} are represented by the following formula (12) to (15).

_{However, α 10} and α ₂₀ in the above equations (12) to (15) are _{kr (m} ) for the (1,0) order acoustic mode and the (2,0) order acoustic mode satisfying the above equation (8), respectively. _{, N)} Shows the value of R _{, where α 10} is about 1.8413 and α ₂₀ is about 3.0543. Further, z ₁ is the z coordinate of the cross section A. The sum of r ₁ , r ₂ , r ₃ and r ₄ gives the following equation (16).

Next, consider the cross section B. It is assumed that the position of the microphone in the cross section B is offset by _{an angle θ m} from the center with respect to the position of the microphone in the cross section A. The sound pressure inside the acoustic tube can be expressed by the following equations (17) to (20) as before.

However, z ₂ is the z coordinate of the cross section B. The sum of r ₅ , r ₆ , r ₇ and r ₈ gives the following equation (21).

From the above equations (16) and (21), the influence of the (1,0) order acoustic mode and the (2,0) order acoustic mode is affected by adding the signals of the respective microphones in both the cross section A and the cross section B. It can be seen that it is removed and only the vertical traveling component ((0,0) order acoustic mode component) is extracted. p _{s {1}} (The "{1}" in the subscript "s {1}" of "p" is the encircled "1". Below, for the numbers between "{" and "}" , The number is expressed in circles.) The transfer function H _{{1} {2}} expressed by the ratio p _{s {2}} / p _{s {1} to p s {2}} is calculated by the following equation. It becomes as shown in (22).

Here, considering a measuring device as shown in FIG. 3, it is assumed that a sound absorbing material to be measured (sample) is installed on the z = 0 plane, and the sample is perpendicular to a plane sound wave incident perpendicular to the sample. Assuming that the sound pressure reflectance of the reflected flat sound wave is r _{0 → 0} = A ₀₀ / B ₀₀ , the above equation (22) can be expressed by the following equation (23).

From the above equation (23), the reflectance r _{0 → 0} is obtained by the following equation (24).

In the above equation (24), s is the distance between the cross section A and the cross section B, and s = z ₂ −z ₁ . _{If the reflectance r 0 → 0} is obtained from the above equation (24), the sound absorption coefficient α can be obtained by α = 1 | r _{0 → 0} | ² . That is, the 8-microphone type vertical incident sound absorption coefficient measuring device can measure the vertical incident sound absorption coefficient up to the frequency range in which the (1,0) order acoustic mode and the (2,0) order acoustic mode propagate.

[1.2] Configuration of 8-microphone-type vertical incident sound absorption coefficient measuring device In the 8-microphone-type vertical incident sound absorption coefficient measuring device, the cross-sectional shape of the acoustic tube is particularly long as it has line symmetry and point symmetry. Not limited. Examples of such a cross-sectional shape include a perfect circle, an ellipse, and a rectangle (including a square). Of course, if it has line symmetry and point symmetry, a cross-sectional shape other than the above (triangle, pentagon, hexagon, etc., hereinafter referred to as "triangle, etc.") can be adopted, but triangle, etc. In the case of, it is only difficult to prepare an acoustic tube or a sample, and there is no particular merit to be obtained. This also applies to the 4-microphone type vertical incident sound absorption coefficient measuring device described later.

Further, in the 8-microphone type vertical incident sound absorption coefficient measuring device, the first microphone group (microphones Mic. 1 to Mic. 4 in FIG. 3) arranged to face each other in the cross section A is a pair of microphones (microphone Mic) facing each other. Of the set consisting of .1 and microphone Mic.3, and the set consisting of microphone Mic.2 and microphone Mic.4)
As shown in FIG. 18B, one set (for example, a set consisting of microphones Mic.1 and Mic.3) is arranged point-symmetrically, and another set (for example, microphones Mic.2 and Mic.4) is arranged at the line-symmetrical position. It is necessary to arrange a set consisting of).

On the other hand, for example, as shown in FIG. 18A, one set (for example, a set consisting of microphones Mic.1 and Mic.3) is arranged point-symmetrically, and another set (for example, microphone Mic) is arranged at a position orthogonal to the point symmetry. When the set consisting of .2 and Mic.4 is arranged, if the cross-sectional shape is a perfect circle (in the case of a perfect circle, the above-mentioned one set is point-symmetrically arranged, and another one is arranged at the line-symmetrical position. Except for the case where the pair is arranged), the second set of microphones does not have a line-symmetrical position of the first set of microphones, and the above measurement principle cannot be applied.

The same can be said not only for the first microphone group arranged in the cross section A but also for the second microphone group (microphones Mic. 5 to Mic. 8 in FIG. 3) arranged in the cross section B. Therefore, in order to make it possible to apply the above measurement principle in the 8-microphone type vertical incident sound absorption coefficient measuring device, the microphones Mic. 1-Mic. 4 and the microphones Mic. 5-Mic. In No. 8, it is necessary to arrange the first set of microphones point-symmetrically and the second set of microphones line-symmetrically with respect to the first set of microphones among the two sets of microphones arranged opposite to each other in each cross section.

However, if the line of symmetry and the first set of microphones are close to each other, the second set of microphones will be close to the first set of microphones, and the accuracy of the reliability evaluation parameter may deteriorate. In order to improve the accuracy of the reliability evaluation parameters, it is preferable to arrange the microphones in each cross section as evenly as possible. In other words, it is preferable to place the first set of microphones as far away from the line of symmetry as possible. At this point, if the cross-sectional shape is a perfect circle, the symmetry line can be taken in any direction. Therefore, an orthogonal symmetry line is determined, and a position far from the two symmetry lines (that is, 45 degrees with respect to the symmetry line). It is optimal to arrange the first set of microphones in the direction in which the angle is formed) and the second set of microphones at a position line-symmetrical with respect to the first set of microphones.

Further, the microphones Mic. That constitute the first microphone group arranged in the cross section A. 1-Mic. 4 and the microphones Mic. That constitute the second micron group arranged in the cross section B. 5-Mic. As shown in FIG. 3, it is preferable to arrange No. 8 in a state where they do not overlap each other when viewed from the axial direction (z-axis direction) of the acoustic tube (see the angle θ _{m in the figure).}
As a result, the microphone Mic. 1-Mic. 4 and microphone Mic. 5-Mic. Even if the distance s from 8 (the distance between the cross section A and the cross section B) is narrowed, the microphones Mic. 1-Mic. The housing of No. 4 and the microphones Mic. 5-Mic. Since it does not interfere with the housing of 8, the interval s can be set smaller. Therefore, it becomes possible to measure the sound absorption coefficient in a higher frequency band. This also applies to the 4-microphone type vertical incident sound absorption coefficient measuring device described later.

[2] Measurement Principle of 4-Microphone Type Vertical Incident Sound Absorption Rate Measuring Device Next, the measurement principle of the 4-microphone type vertical incident sound absorption coefficient measuring device will be described. The 4-microphone type vertical incident sound absorption coefficient measuring device is a type of vertical incident sound absorption coefficient measuring device in which two microphones are arranged on each of a cross section A and a cross section B of an acoustic tube, for a total of four. With the 8-microphone type vertical incident sound absorption coefficient measuring device described in the previous section, it was possible to measure the vertical incident sound absorption coefficient up to the frequency range where the (1,0) order acoustic mode and the (2,0) order acoustic mode propagate. When considering frequencies below the region where the (1,0) order acoustic mode propagates, it is possible to measure the vertical incident sound absorption coefficient with a 4-microphone type vertical incident sound absorption coefficient measuring device using four microphones. be.

In the 4-microphone type vertical incident sound absorption coefficient measuring device, two microphones are arranged on the cross section A so as to be point-symmetrical with respect to the center of the acoustic tube, and two microphones are also arranged on the cross section B so as to be point-symmetrical. It is arranged so as to face a position that is point-symmetrical with respect to the center of. For example, in the example of FIG. 2, when considering the region where the (1,0) order acoustic mode propagates, the _{sound pressure p (r 1 ) at r 1 and} the sound pressure p (r) at r ₃ of the cross section A are considered. ₃ ) can be expressed by the following formulas (25) and the following formulas (26) from the above formulas (12) and the above formulas (14), respectively.

By adding the above formula (25) and the above formula (26), the following formula (27) can be obtained in the same manner as the above formula (16).

Similarly, for the cross section B, the following equation (28) can be obtained by obtaining the sum of the two microphones.

From the above equations (27) and (28), even with the 4-microphone type vertical incident sound absorption coefficient measuring device, the vertical propagation component can be separated to the frequency range where the (1,0) order acoustic mode propagates, and the vertical incident sound absorption coefficient can be separated. It turns out that it is possible to measure. In the 8-microphone type vertical incident sound absorption coefficient measuring device, when summing in the frequency range, the input of the measuring instrument also requires 8 channels, so there is a disadvantage that the number of microphones and measurement channels is high, but 4 microphones. Although the frequency range of the type vertical incident sound absorption coefficient measuring device is narrower than that of the 8-microphone type vertical incident sound absorption coefficient measuring device, there is an advantage that the cost of the measuring device can be suppressed.

[3] Scattering component monitoring method Next, a scattering component monitoring method (a method for calculating the reflectance of a sound wave reflected in the (1,0) order acoustic mode with respect to a vertically incident sound wave) will be described. As already described, it is possible to extract the vertical traveling component in each cross section by adding the signals of two or four microphones installed in each cross section. By obtaining the vertical traveling component for the two cross sections, the vertically incident sound absorption coefficient can be measured up to the band in which the (1,0) order acoustic mode and the (2,0) order acoustic mode propagate.

However, this is based on the premise that a vertically incident plane wave is reflected vertically, and if a vertically incident sound wave is reflected (scattered) in an oblique direction, an accurate evaluation should be performed. I can't. When there is scattering, the component reflected diagonally is not measured by the above-mentioned 8-microphone type or 4-microphone type vertical incident sound absorption coefficient measuring device, so that the sound absorption coefficient is apparently measured to be large. However, in the case of a sample that has scattering in the first place, it is considered that it is impossible to obtain the vertical incident sound absorption coefficient because scattering occurs even in the actual use situation. From this, we investigated a method for evaluating whether or not scattering occurs on the sample surface during measurement. If the influence of scattering can be monitored, the reliability of the measured vertical incident sound absorption coefficient can be evaluated, and the measurement can be performed while ensuring the reliability of the measurement result. It is also considered to be an index as to whether or not the sample itself is suitable for measuring the vertical incident sound absorption coefficient.

[3.1] Separation method of (1,0) order acoustic mode Here, the (0,0) order acoustic mode, the (1,0) order acoustic mode and the (2,0) order acoustic mode are the same as before. Consider the frequency domain in which the three acoustic modes propagate, and consider a method of extracting only the (1,0) order acoustic mode component. When the difference between the signals of the two microphones facing each other is obtained for each of the cross section A and the cross section B, the following formulas (29) to (20) are obtained from the above formulas (12) to (15) and the above formulas (17) to (20). 32) is obtained.

From the above equations (29) to (32), by taking the signal difference between two microphones facing each other, the (0,0) order acoustic mode and the (2,0) order acoustic mode can be determined from the symmetry of each acoustic mode. It can be seen that the component of (1,0) is canceled and only the (1,0) order acoustic mode component can be separated. In the above "[1] Measurement principle of 8 microphone type vertical incident sound absorption coefficient measuring device" and "[2] Measurement principle of 4 microphone type vertical incident sound absorption coefficient measuring device", (0) is obtained by summing the signals. , 0) The method of extracting only the order-order acoustic mode component was shown, but here, it was clarified that only the (1,0) order-order acoustic mode component can be extracted by a simple process of taking a signal difference.

[3.2] Vertical incident (1,0) order acoustic mode reflectivity calculation method (simplified method)
Next, a method of obtaining the reflectance of a vertically incident sound wave reflected in the (1,0) order acoustic mode by using the signal of the (1,0) order acoustic mode extracted in the previous section will be described.

First, it is assumed that the sound wave (speaker) is placed in the center of the acoustic tube and only the vertically incident plane sound wave is generated, and further, it is considered that the sound wave side is sufficiently absorbed, and the sound wave incident on the sample is (0,0). It is assumed that there is only the next acoustic mode (B ^σ ₁₀ = 0, B ^τ ₁₀ = 0). Consider the frequency transfer function of the sound pressure of the signal p _s and two opposite points of the sum of the four points of the sound pressure at this time. _First, (but may be either section A and section B, and target a section A here) transfer function _{H h {1}} of Δp _{h {1}} for _{p s {1}} is the following formula (33) Will be shown.

However, in the above equation (33), r _{0 → 0} is the ratio A ₀₀ / B _{of the reflected wave amplitude A 00 in} the (0, 0) order acoustic mode to the incident wave amplitude B _{00 in} the (0, 0) order acoustic mode. _{Represents 00} (r _{0 → 0} = A ₀₀ / B ₀₀ ), where r ^σ _{0 → 1} is the (1, 0) order sound in the + direction of θ with respect to _{the incident wave amplitude B 00 in} the (0, 0) order sound mode. _Represents the ratio A ^σ ₀₁ / B ₀₀ of the mode amplitude A ^{σ 01 (r σ} _{0 → 1} = A ^σ ₀₁ / B ₀₀ ), where r ^τ _{0 → 1} is the incident wave amplitude B of the (0,0) order acoustic mode. ₀₀ for the theta - represents the direction (1,0) the ratio a ^tau ₀₁ / _{B 00} of the next acoustic modes amplitudes ^{_{a τ 01 (r τ 0 →}} 1 = a τ 01 / B 00).

From the above formula (33), the following formula (34) can be obtained.

Next, considering the transfer function H _{v {1}} of Δp _{v {1} with respect to p s} , the following equation (35) can be obtained in the same manner.

From the above formula (35), the following formula (36) can be obtained.

Therefore, the following formula (37) and the following formula (38) can be obtained from the above formula (34) and the above formula (36).

That is, according to the above equations (37) and (38), the amplitude ratio of the sound wave reflected as the (1,0) order acoustic mode to the sound wave vertically incident (incidented in the (0,0) order acoustic mode). Can be asked. _{Since r 0 → 0} in the above equation (37) and the above equation (38) is separately obtained from the sum of the microphone signals, the sum of the microphone signals in only one cross section is obtained in order to obtain _{r 0 → 1.} The ratio of the difference between the signals of the opposing microphones and the signal of the opposite microphone may be measured.

Next, consider the incident sound power and the reflected sound power. Now, since it is considered that the incident sound has only a vertically incident component, the sound pressure p _i and the particle velocity v _i of the incident sound can be expressed by the following equations (39) and the following equations (40), respectively.

Therefore, the incident sound power _Wi is represented by the following equation (41).

(0,0) when only the following acoustic modes and (1,0) following acoustic mode is considered a frequency domain propagation, the sound pressure p _r and z direction of the particle velocity v _r of the reflected sound, the following formulas (42) And can be expressed by the following equation (43).

Therefore, the reflected sound power _Wr is expressed by the following equation (44).

When the total reflectance (here, referred to as "intra-tube incident power reflectance") R is obtained from the above equation (41) and the above equation (44), it becomes the following equation (45).

The first term on the right side in the above equation (45) represents the reflectance of the vertical reflection component with respect to the vertically incident component, and the second term on the right side and the third term on the right side represent the (1,0) order acoustic mode with respect to the vertically incident component. Represents reflectance. Therefore, if r ^σ _{0 → 1} and r ^τ _{0 → 1} are obtained from the measurement, it is possible to evaluate the effect of the sound wave scattered by the sample and reflected in the (1,0) order acoustic mode with respect to the vertical incident. It is possible.

[3.3] Vertical incident (1,0) order acoustic mode reflectivity calculation method (detailed method)
In the "simple method" in the previous section, it is assumed that the incident sound wave has no diagonally traveling component. _{In this case, r 0 → 1} could be obtained by measuring the ratio of the difference between the signals of the four microphones to the sum of the signals of the four microphones for only one cross section. However, if there is scattering on the surface of the sample, the sound waves reflected diagonally are repeatedly reflected and stopped at the sound reflected at the tube end on the sound source (speaker) side or in the cross-sectional direction of the acoustic tube, and then enter the sample again. It is thought that there are ingredients that do. From this, it is considered that the incident sound wave also has an oblique traveling component, and a method (detailed method) for measuring _{r 0 → 1 is considered.} In the detailed method, a method of finding the ratio of the difference between the signals of the microphones to the sum of the signals of the microphones in the two cross sections of the cross section A and the cross section B was examined.

for p _s, the equation (29), (30), (31), taking each of the ratio (32), the following equation (46), (47), (48) is obtained (49).

^{However, β σ} _{0 → 1} and β ^τ _{0 → 1} in the above equations (46) to (49) represent the ratio of the amplitude of the incident (1,0) order acoustic mode to the incident (0,0) order acoustic mode. , Are given by the following equation (50) and the following equation (51), respectively.

From the above equations (46) to (49), r ^σ _{0 → 1} , r ^τ _{0 → 1} , β ^σ _{0 → 1} and β ^τ _{0 → 1} are obtained, and the following equations (52), (53), respectively. (54) and (55).

Therefore, _{by measuring H h} ^{1} , H _v ^{1} , H _h ^{2} and H _v ^{2} , the reflection (1,0) acoustic mode with respect to the incident (0,0) order acoustic mode The reflectance can be obtained. It can be seen that it is possible to obtain _{r0 → 1} even when an oblique component is present in the incident sound by obtaining the ratio of the difference between the signals of the microphones and the opposite signals to the sum of the signals of the microphones for the two cross sections. It can also be seen that the reflectance can be calculated even when the microphone positions are _{displaced by an angle θ m in the two cross sections.}

The power reflectance can be obtained as follows, as in the simple method described in the previous section. That, considering the incident sound (1,0) and contains up to components of the next acoustic modes, the incident acoustic power W _i can be obtained as the following equation (56).

Further, the reflected sound power _Wr can be obtained by the following equation (57).

Therefore, the incident power reflectance in the tube is obtained by the following equation (58).

However, in the above equation (58), _{R0 → 0} is the power reflectance of the (0,0) order acoustic mode with respect to the acoustic power of the incident (0,0) order acoustic mode, and is given by the following equation (59). R _{0 → 1} is the power reflectance of the (1,0) order acoustic mode with respect to the acoustic power of the incident (0,0) order acoustic mode, and is given by the following equation (60), and _{Br0 → 1} is , The ratio of the incident acoustic power by the (1,0) order acoustic mode to the incident acoustic power by the (0,0) order acoustic mode, which is given by the following equation (61).

The above equation (58) is an in-tube incident power reflectance (overall power reflectance) including an oblique incident component and an oblique reflection component according to the (1,0) order acoustic mode. When the vertical incident component and the vertical reflection component are large and the influence of the (1,0) order acoustic mode is small, the vertical incident power reflectance r _{0 → 0} squared (=) obtained by the above equation (24). | R _{0 → 0} | ² ) and the in-tube incident power reflectance R of the above equation (58) match. Therefore, monitoring the incident power reflectance R in the tube is considered to be an index for determining whether or not there is an influence of scattering. Of course, r _{0 → 1} and β _{0 → 1} may be monitored, but it is considered that monitoring the in-tube incident power reflectance R can more intuitively evaluate the effect of scattering on the vertical incident sound absorption coefficient.

[4] Experiment An experiment was conducted to confirm whether the vertical incident sound absorption coefficient measuring device of the present invention can measure up to a high frequency according to the above theory.

[4.1] Experimental equipment The outline of the experiment is shown in FIG. 4, and the state of the experiment is shown in FIG. In the experiment, an acrylic pipe having an inner diameter of 100 mm was used as the acoustic tube, and a sound absorbing material was inserted on the sound source (speaker) side of the acoustic tube. Table 2 below shows the cut-on frequency of each acoustic mode determined by the diameter of the acoustic tube and the upper limit frequency determined by the distance s (= 25 mm) between microphones.

Eight microphones with a diameter of 1/4 inch (B & K4985) were used. White noise was used as the sound source (speaker). In order to sum the signals of the microphones in the same cross section, in order to correct the characteristic mismatch between the microphones, the processing in the frequency domain by FFT was performed in this experiment.

[4.2] Measurement result of vertical incident sound absorption coefficient when scattering can be ignored (porous material)
First, FIGS. 6 to 9 show the measurement results of the vertical incident sound absorption coefficient when the experiment was performed using a porous material as a sample. FIG. 6 shows the measurement result when antihair felt (20 mm thick) was used as the sample, FIG. 7 shows the measurement result when melamine foam (25 mm thick) was used as the sample, and FIG. 8 shows the glass wool as the sample. FIG. 9 shows the measurement results when (50 mm thick) was used, and FIG. 9 shows the measurement results when PET felt (12 mm thick) was used as the sample. These measurement results are obtained from Mic. 1 and Mic. When using only the two microphones of 5, and when using Mic. 1. Mic. 3. Mic. 5 and Mic. When using the four microphones of No. 7 and Mic. 1 to Mic. Each of the cases where all eight microphones up to 8 are used is shown. In addition, the measurement result (tube diameter 29 mm) when an acoustic tube (B & K4206) manufactured by a measuring instrument manufacturer is used is also shown.

Looking at the measurement results of FIGS. 6 to 9, when any of anti-hair felt (FIG. 6), melamine foam (FIG. 7), glass wool (FIG. 8) or PET felt (FIG. 9) was used as the sample. However, in the measurement result (2-mic) when only two microphones are used, the curve of the sound absorption coefficient is disturbed from around 2 kHz, which is near the cut-on frequency of the (1,0) order acoustic mode. On the other hand, in the measurement result (4-mic) when four microphones are used, the curve of the sound absorption coefficient changes smoothly up to around 3.3 kHz near the cut-on frequency of the (2,0) order acoustic mode. There is. Further, in the measurement result (8-mic) when eight microphones are used, a smooth curve is obtained up to around 4 kHz. In the region where the sound absorption coefficient appears as a smooth curve, it is in good agreement with the measurement result of the acoustic tube (tube diameter 29 mm) manufactured by a manufacturer with a small tube diameter. From these results, when measuring the vertical incident sound absorption coefficient of a general porous sound absorbing material, by using the vertical incident sound absorption coefficient measuring device of the present invention, the frequency can be measured up to about twice the frequency of the conventional measuring device. It turned out to be possible.

[4.3] Measurement result of vertical incident sound absorption coefficient when there is scattering (complete reflection condition)
Subsequently, it was confirmed by an experiment what kind of effect would appear on the measured vertical incident sound absorption coefficient when there was scattering on the sample surface. In this experiment, it is assumed that the wall surface that completely reflects the sound wave is installed diagonally inside the acoustic tube, and as shown in FIG. 10, oil clay is placed inside the acoustic tube where the sample is installed. After packing, the wall surface (back wall surface) of the oil clay was tilted 30 degrees with respect to the cross section of the acoustic tube. For comparison, an experiment was also conducted when the wall surface (back wall surface) of the oil clay was made parallel (0 degrees) to the cross section of the acoustic tube. FIG. 11 shows the measurement results of the vertical incident sound absorption coefficient when the back wall surface is tilted by 30 degrees and when the back wall surface is set to 0 degrees.

From FIG. 11A showing the measurement result when the back wall surface is tilted by 30 degrees with respect to the cross section of the acoustic tube, it can be read that the vertical incident sound absorption coefficient shows a high value in the region of about 2 kHz or more. However, as can be seen from FIG. 11 (b) showing the measurement results when the back wall surface is parallel to the cross section of the acoustic tube (0 degree), the oil clay reflects sound waves almost completely, so that the back wall surface is substantially completely reflected. If is tilted, everything is reflected in the same way, and the vertical incident sound absorption coefficient should be close to zero. Nevertheless, the reason why the vertically incident sound absorption coefficient in FIG. 11A shows a high value is that the back wall surface is tilted, so that almost all sound waves are reflected diagonally by the back wall surface and are reflected in the vertical direction. It is thought that this is because it is hardly reflected. That is, since the sound wave reflected diagonally is not detected in this experiment, it is considered that the vertical incident sound absorption coefficient was apparently measured to be high.

The results of calculating the in-tube incident power reflectance by obtaining the vertical incident (1,0) order acoustic mode reflectance R _{0 → 1 are shown in FIGS. 12 and 13.} FIG. 12 shows the result by the above-mentioned simplified method, and FIG. 13 shows the result by the above-mentioned detailed method. In any of FIGS. 12 and 13, whether at the time the rear wall is 30 degrees, the influence of scattering and appear in the reflectance R _{0 → 1,} scatter if you look at the values of reflectance R _{0 → 1} occurs It is possible to judge. On the other hand, when the back wall is 0 degrees, _{the value of the reflectance R 0 → 1} is approximately 0 up to around 4 kHz, and it can be confirmed that scattering does not occur.

When the back wall is 30 degrees, the in-tube incident power reflectance calculated by the detailed method (FIG. 13) is approximately 1 at about 3.3 kHz or less, and the original reflectance of the wall surface is high. It is probable that the measured and reasonable results were obtained. The difference between the in-tube incident power reflectance obtained by the detailed method and the measured vertical incident reflectance represents the effect of scattering on the measured vertical incident sound absorption coefficient, and by looking at this difference, the effect of scattering is quantitative. It is thought that it will be possible to grasp. On the other hand, in the simple method, the incident power reflectance in the tube is often a value larger than 1, and although it can be determined that scattering has occurred, it is somewhat difficult to quantitatively grasp the effect of scattering. It is thought that there is.

[4.4] Measurement result of vertical incident sound absorption coefficient when there is scattering (sound absorbing material)
Next, in order to verify whether or not the influence of scattering can be evaluated when the sample is a sound absorbing material, when there are variations in the density distribution of the sound absorbing material (sample) or when the reflecting material is a sound absorbing material (sample). ) Was attached to a part of the surface of the surface. FIG. 14 shows a sound absorbing material (sample) to be verified in this experiment.

FIG. 14A shows a sample made of a melamine resin foam having a thickness of 25 mm (left), a sample having a hole having a diameter of 29 mm in the melamine resin foam having a thickness of 25 mm (center), and a melamine resin foam having a thickness of 25 mm. This is a photograph of a sample (right) with a hole of 63.5 mm. The holes were arranged offset from the center of the sample so as to be close to the inner wall surface of the acoustic tube. FIG. 14B is a photograph of a sample in which a disk (diameter 40 mm) made of polypropylene (PP) having a thickness of 0.4 mm is attached to the surface of a melamine resin foam having a thickness of 25 mm, and the sample (left). ) Is when the same disk is attached to the center of the sample, the sample (center) is the same disk when the same disk is attached at a position 15 mm off the center of the sample, and the sample (right) is the same disk. This is the case when the sample is attached at a position deviated by 30 mm from the center of the sample. FIG. 14 (c) is a photograph of a sample in which a disk (diameter 50 mm) made of polypropylene (PP) having a thickness of 0.4 mm is attached to the surface of a melamine resin foam having a thickness of 25 mm, and the sample (left). ) Is the case where the disk is attached to the center of the sample, and the sample (right) is the case where the disk is attached to a position 25 mm off the center of the sample.

FIG. 15 shows the results of measuring the vertical incident sound absorption coefficient and the like using the sample of FIG. 14 (a). Further, FIG. 16 shows the results of measuring the vertical incident sound absorption coefficient and the like using the sample of FIG. 14 (b). Further, FIG. 17 shows the results of measuring the vertical incident sound absorption coefficient and the like using the sample of FIG. 14 (c).

Looking at FIG. 15A, it can be seen that in the case of “no hole”, _{the component of the reflectance R 0 → 1} is hardly measured and scattering does not occur. However, as shown in FIG. 15B, when the power reflectance (intra-tube incident power reflectance) is calculated by a simple method, a reflectance peak of about 0.05 appears at around 2.2 kHz. .. In the case of the simple method, it is considered that this is an error caused by calculating the _{reflectance R 0 → 1} under the assumption that the incident sound does not contain the high-order acoustic mode component. Further, even in the case of "hole 29 mm", _{the value of the reflectance R 0 → 1} is small, and it is considered that a large influence due to scattering does not appear with such a variation in the material. On the other hand, in the case of "hole 63.5 mm", the reflectance R _{0 → 1 of} about 0.03 is measured in a wide band around 2 to 4 kHz, and although it is not large, it is affected by this degree of scattering. Is considered to have been affected by the vertical incident sound absorption coefficient.

Looking back at the measurement results of the vertical incident sound absorption coefficient in FIG. 15 (a), the vertical incident sound absorption coefficient (2-mic.) Measured by the two microphones is the cut-on frequency of the (1,0) order acoustic mode. It can be seen that it is greatly disturbed in the range exceeding. This tendency becomes more pronounced as the holes become larger. It is considered that this is because the vertical traveling component could not be measured accurately due to the influence of the sound wave traveling diagonally inside the acoustic tube. From this, when the number of microphones is two, the measurement result of the vertical incident sound absorption coefficient is large in the range exceeding the cut-on frequency of the (1,0) order acoustic mode even if the scattering is slight. It turns out that it will be affected. On the other hand, when the vertical incident sound absorption coefficient is measured with eight microphones, the curve of the sound absorption coefficient is relatively smooth even in the case of "hole 63.5 mm". From this, it was reconfirmed that the measurement using eight microphones is effective.

Further, when a polypropylene (PP) disk having a diameter of 40 mm is attached to the surface of the sample, as shown in FIG. 16, the reflection calculated as the attachment position of the disk is shifted from the center. It can be seen that the value of the rate R _{0 → 1 increases.} When a disk is attached to the center of the sample, even if scattering occurs due to the disk, the (1,0) mode and (2,0) mode in which the node enters the center of the disk are not excited, so it is affected. Has not appeared. On the other hand, when the sticking position of the disk is shifted from the center, the (1,0) mode and the (2,0) mode are excited, and the influence of scattering appears.

Further, when a disk made of polypropylene (PP) having a diameter of 50 mm is attached to the surface of the sample, as shown in FIG. 17, when the disk deviates from the center of the sample, it is about 0.05 at 2 kHz or higher. Scattering occurs, and larger scattering occurs than when a disk made of polypropylene (PP) having a diameter of 40 mm is attached to the surface of the sample (FIG. 16). Therefore, there is a difference between the incident power reflectance in the tube and the vertical incident reflectance. This difference indicates the effect of scattering, and it is considered that the reliability of the measurement result of the vertical incident sound absorption coefficient can be evaluated by monitoring this difference. It was also found that the reflectance measurement results tended to be larger in the simple method than in the detailed method. This is because the influence of the higher-order mode in the incident sound is ignored. In the simple method, the incident sound is calculated to be smaller by the amount of the obliquely incident component, so that the reflectance is considered to be large.

Claims (7)

An acoustic tube in which the shape of the cross section perpendicular to the axial direction forms a tubular shape with line symmetry and point symmetry, and the sample is placed inside the tip side in the axial direction.
A speaker mounted on the axial base end side of the acoustic tube,
Multiple microphones installed on the peripheral wall of the acoustic tube,
It is a vertical incident sound absorption coefficient measuring device equipped with
Multiple microphones
At least one set of microphones MA _{. 1} , MA _. The first microphone group consisting of _{2 and}
At least one set of microphones MB arranged point-symmetrically on another cross section B separated by a predetermined distance in the axial direction from the cross section A of the acoustic tube _{. 1} , MB _. A second microphone group consisting of _2,
Consists of
Microphones that make up the first microphone group MA _{. 1} , MA _{. 2} , ... output signals SA _{. 1} , SA _{. 2} , ... sum SA _{. 1} + SA _{. 2} + ... (hereinafter, referred to as "[sigma] s _A".) And a microphone _{M B.} constituting the second Micron Group ₁ , MB _{. 2} , ... output signals SB _{. 1} , SB _{. 2} , ... sum SB _{. 1} + SB _{. From 2} + ... (hereinafter referred to as "ΣS _B "), the transfer function between the first microphone group and the second microphone group is obtained, and the transfer function method is applied to this transfer function to sample. The vertical incident sound absorption coefficient calculation means for calculating the vertical incident sound absorption coefficient of
At least, the microphones MA _{. 1} , MA _. Output signal of _{2 SA. 1} , SA _. Difference of _{2 SA. 1-} S _A. Reliability evaluation parameter calculation means for calculating _{2 and}
A vertical incident sound absorption coefficient measuring device characterized by being further equipped with.
The first microphone group is at least
A set of microphones MA _{. 1} , MA _{. 2} and
Microphone MA _{. 1} , MA _. Are line symmetry arranged on section A for _two pair of microphones M _{A. 3} , MA _{. 4} and
Consists of
The second microphone group is at least
A set of microphones arranged point-symmetrically on the cross section _{B. 1} , MB _{. 2} and
Microphone MB _{. 1} , MB _{. A} set of microphones MB arranged line-symmetrically on the cross section B with respect to _{2. 3} , MB _{. 4} and
Consists of
The means for calculating the vertical incident sound absorption coefficient is
Microphones that make up the first microphone group MA _{. 1} , MA _{. 2} , MA _{. 3} , MA _{. 4} , ... Output signals SA _{. 1} , SA _{. 2} , SA _{. 3} , SA _{. 4,} the sum [sigma] s _A of ..., microphones _{M B.} constituting the second microphone group ₁ , MB _{. 2} , MB _{. 3} , MB _{. 4} , ... Output signals SB _{. 1} , SB _{. 2} , SB _{. 3} , SB _{. From} the sum ΣS _B of 4, ..., the transfer function between the first microphone group and the second microphone group is obtained, and the transfer function method is applied to this transfer function to calculate the vertical incident sound absorption coefficient of the sample. The vertically incident sound absorption coefficient measuring device according to claim 1.
The reliability evaluation parameter calculation method is
To the sum ΣS _A, microphone _{M A. 1} , MA _. Output signal of _{2 SA. 1} , SA _. Difference of _{2 SA. 1-} S _{A. 2} ratio _(S A.1 _{-S A.2)} / and [sigma] s _A,
To the sum ΣS _A, microphone _{M A. 3} , MA _. Output signal of _{4 SA. 3} , SA _. Difference of _{4 SA. 3-} S _{A. 4} ratio and _{(S A.3 -S A.4) / ΣS} A,
The vertical incident sound absorption coefficient measuring device according to claim 2, wherein the above-mentioned device is to be calculated.
The reliability evaluation parameter calculation method is
To the sum ΣS _A, microphone _{M A. 1} , MA _. Output signal of _{2 SA. 1} , SA _. Difference of _{2 SA. 1-} S _{A. 2} ratio _(S A.1 _{-S A.2)} / and [sigma] s _A,
To the sum ΣS _A, microphone _{M A. 3} , MA _. Output signal of _{4 SA. 3} , SA _. Difference of _{4 SA. 3-} S _{A. 4} ratio and _{(S A.3 -S A.4) / ΣS} A,
To the sum ΣS _B, microphone _{M B. 1} , MB _. Output signal of _{2 SB. 1} , SB _. Difference between _{2 SB. 1-} S _{B. 2} ratio _(S B.1 _{-S B.2)} / and [sigma] s _B,
To the sum ΣS _B, microphone _{M B. 3} , MB _. Output signal _{SB of 4. 3} , SB _. Difference of _{4 SB. 3-} S _{B. 4} ratio _(S B.3 _{-S B.4)} / and [sigma] s _B,
The vertical incident sound absorption coefficient measuring device according to claim 2, wherein the above-mentioned device is to be calculated.
The vertical incident sound absorption coefficient measuring device according to any one of claims 1 to 4, wherein the reliability evaluation parameter calculating means also calculates the incident power reflectance in the acoustic tube.
Microphones that make up the first microphone group MA _{. 1} , MA _{. 2} , ..., and the microphones MB that make up the second micron group _{. 1} , MB _{. 2.} The vertically incident sound absorption coefficient measuring device according to any one of claims 1 to 5, wherein the two, ... Are arranged so as not to overlap each other when viewed from the axial direction of the acoustic tube.
A vertical incident sound absorption coefficient measuring method for measuring a vertical incident sound absorption coefficient of a sample using the vertical incident sound absorption coefficient measuring device according to any one of claims 1 to 6.

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