JPH03276031A - Method and device for measuring sound absorption rate - Google Patents

Abstract

PURPOSE:To increase the measurement accuracy of the sound absorption rate by finding the spatial gathering mean of reverberation when a sound source is interrupted, finding the extent of curvature of the reverberation waveform and the reverberation time of the linear attenuation in a complete diffused sound field to the attenuation rate, and then finding the sound absorption rate of a material. CONSTITUTION:When the impulse response by a single-sound generating device 1 is generated from a speaker 3, a microphone 4-1 absorbs the sum of an impulse and a noise. This signal is converted by an A/D converting circuit 7 and squared by a squaring circuit 8, a subtracting circuit 23 subtracts the effective value of the noise, and an accumulator 11 perform sequential accumulation. Sounds are collected by microphones 4-2 - 4-n in order and accumulated values are added in a RAM 14. The arithmetic result indicates the spatial gathering mean value, which is compressed logarithmically in a ROM 17 to find a reverberation curve. The measurement is performed when a reverberation room 2 is empty and when chairs (b) are put in, and the reverberation time and the sound absorption rate of the sound absorber are found.

Description

DETAILED DESCRIPTION OF THE INVENTION "Field of Industrial Application" The present invention relates to a sound absorption coefficient measuring method and apparatus that can accurately determine the sound absorption coefficient of flat sound absorbing materials, theater chairs, and the like.

``Prior art'' Measurement of the sound absorption coefficient α of a flat member such as a sound-absorbing material, or the sound absorption coefficient a of a chair, etc. (the sound absorption coefficient of a non-flat sample such as a chair is sometimes expressed as absorption force) is carried out in theaters, etc. This is extremely important when designing.

Sound absorption coefficient (or sound absorption power) measurement using the conventional reverberation coefficient method is carried out using samples such as flat sound absorbing material a, chair, b... This has been done by placing a sound source on the floor of the reverberation chamber 2, generating a To7 burst or the like from a sound source in the reverberation chamber, and measuring the reverberation.

"Problem to be Solved by the Invention" However, the conventional measurement method has the following causes of error.

■Sound diffusion conditions deteriorate due to the placement of the sample.

■Since the entire side surface of a chair etc. has the same area as the top surface, the incidence of sound from the side surface of the sample (engine/nozzle incident) is large.

■ Area effect cannot be removed.

Due to the above-mentioned causes of error, a large difference has arisen between the effective value and the measured value in actual construction. Also,
There were problems such as variations in measured values depending on the measuring institution.

This invention was made in view of the above-mentioned problems.
It is an object of the present invention to provide a sound absorption coefficient measuring method and apparatus that eliminate the above-mentioned error causes and enable highly accurate sound absorption coefficient measurement.

"Means for Solving the Problem" In order to solve the above problem, in the invention according to claim 1, the reverberation chamber is empty, the sample whose sound absorption coefficient is to be measured, and the lateral surroundings of the sample. The curvature of the reverberation waveform can be determined by calculating the spatial collective average of the reverberation when the sound source is cut off and comparing this reverberation waveform with the theoretical value in the case where a reflective enclosure surrounding the sound source is placed in the reverberation room. The present invention is characterized in that the reverberation time T O+ T with respect to the attenuation rate of linear attenuation in a completely diffuse sound field is determined from this degree, and the sound absorption coefficient of the sample is determined based on these T O+ T .

Further, in the invention according to claim 2, there is provided a reverberation chamber, a reflective enclosure means for surrounding the sides of a sample whose sound absorption coefficient is to be measured, and a spatial collection of reverberations when a sound source is cut off in the reverberation chamber. a spatial collective average measuring means for calculating an average; a curvature determining means for determining the degree of curvature of the reverberant waveform by comparing the reverberant waveform measured by the spatial collective average measuring means with a logical value; reverberation time calculation means for calculating the reverberation time with respect to the attenuation rate of linear attenuation in a completely diffuse sound field from the degree of attenuation; and Calculation result T of the reverberation time measuring means. The present invention is characterized by comprising a sample sound absorption coefficient calculation means for calculating the sound absorption coefficient of the sample from +TI.

"Operation" By providing a reflective enclosure, sound incidence from the sides is blocked and the area effect is eliminated. Also,
By determining the spatial collective average of the reverberant waveform and comparing this reverberant waveform with the theoretical value, the degree of curvature of the reverberant waveform is determined, and from this degree, the reverberation time relative to the attenuation rate of linear attenuation in a completely diffuse sound field is calculated. By determining T, , T, and determining the sound absorption coefficient of the sample based on these T, , T1, it is possible to compensate for insufficient diffusion and determine the same sound absorption power as in the case of complete diffusion.

"Embodiments" Hereinafter, embodiments of the present invention will be described with reference to the drawings.

A: Principle of invention First, the measurement principle of this invention will be explained.

Here, α (in the case of a flat sound absorbing material) and a (in the case of a sample such as a chair) are defined as sound absorption coefficients that do not include errors.

First, the following relationship exists regarding α.

α, = α, 10βE ... (1) (Here, β is a coefficient determined by the sound source wavelength and the sample,
E is the circumference a1 per sample area S, or s/Q) βE in equation (1) is a term due to the aforementioned error factors ■ and ■, and α is a value that does not include error, that is, infinite This value is obtained by assuming diffuse incidence of sound on a sample with a large area. The above equation (1) also holds true for the sound absorption coefficient a of a sample such as a chair. Also, when α1,α is multiplied by the sample area S, the sound absorption force of the whole sample becomes A,, A-, and when al,a is multiplied by the number of samples n, the sound absorption force of the whole sample becomes A1, A, becomes.

Next, the question is how to find α, (or a,).

As shown in FIG. 2 (a), if the sample is placed on the entire floor of the reverberation chamber 5 and the reverberation characteristics are measured, α.

a is calculated as −6. However, in reality, the diffusion of the sound field in the reverberation room decreases due to the concentration of the sample on the floor.
Additionally, accurate measurements cannot be made due to the large sample area (it is difficult to place sound absorbers such as theater chairs on the entire floor surface).

Therefore, in this invention, first, as shown in FIG. 2(b), a chair as a sample is placed in a reverberation chamber, and its surroundings are covered with a side wall (deep-well) c.

The height H of this side wall C is set higher than the chair.

This eliminates sound incidence from the side and also eliminates area effects. It should be noted that if the height of the side wall C is less than the height of the chair, the area effect cannot be sufficiently removed due to the sound coming around from above as shown in FIG. Further, the side wall C is made of a reflective material, and if the sample is a flat sound absorbing material, it is arranged as shown in FIG. 2 (2).

By the above procedure, error factors other than insufficient diffusion can be removed. Next, a method for removing errors due to insufficient diffusion will be explained.

First, we will discuss the spatial collective average of reverberant waveforms. A spatial collective average is a spatial average added to the collective average. The method to measure the collective average of reverberant waveforms is as follows:
The so-called impulse response square integration method of Mr. M. R. Sc'hroeder is known. The principle is
The transient characteristics of the sound receiving point when the sound source band noise is cut off from the steady state, that is, the woody transient response curve of the sound receiving point corresponding to the average of ω times of the reverberation (attenuation) drum shape <s ” (t)>
is the impulse response r (x) between the sound source and the sound receiving point.
According to it, a certain point t in the transition period
The sound pressure level 5(t) at is expressed as a set average as follows.

<S"(t)>=N f tr"(x)dx--(
2) Where, N: sound source band noise power, r(x),
Impulse response In this way, by squaring and integrating the impulse response r (x) in the integral interval [1, +ω, the set average of infinite times of the square of the sound pressure level S (t) at the time point is first obtained. To take this as a spatial average (S'(t)l), see, for example, Japanese Patent Application Laid-Open No. 55-54414.

In addition, considering that the actual measurement environment is under the influence of dark noise, the true reverberant waveform cannot be observed unless noise correction is performed. Therefore, it is necessary to perform noise correction on equation (2). In other words, R(X)=r(X)+n(x) is defined as the sum of impulse responses r(x) and noise (background noise, etc.) n(x),
Also, the square value of the effective value of the noise n (x) is 011. shall be. Then, instead of integrating the above equation (2), f CR"(x)-n",tr"J d x・--
= (3) was calculated and noise correction was performed (<s'(t)
>l I'm looking for a NO. Specifically, please refer to, for example, Japanese Patent Application Laid-open No. 154423/1983.

Then, as described above, the spatial ensemble average f<5t(t)>) is obtained by removing irregular fluctuations without performing noise correction.

The curvature of the curve reflects the lack of diffusion in the decay process. Therefore, focusing on a parameter indicating the degree of curvature, the value of this parameter is determined as follows.

First, the spatial collective average (〈S'(
t)>l No. to reverberation thin line 10・log+of<S
Find '(t)>lso. Then, a theoretical value is approximated to this reverberation curve. For example, the curve 110-1o+of<S"(
t)>two is the power given by 5chroeder
It is well approximated by the logical formula L(t) of r-1aw decay. This theoretical value is expressed, for example, by the following formula.

L, 1(t) knee 4.34(N-1)In(1+a
t/N)・=−(4)(Note that Ln(t)・f(N
-'1)/? ll L(t)) where N is the number of independent points taken on the sample. This N is an index related to the degree of curvature, and the larger N is, the closer to a straight line it is. Also,
100 is an index related to the attenuation rate of the initial part of the reverberation curve, and t is time.

By the way, fs''(t)) No. in the process of spatial averaging processing
loses all other information and the resulting reverberation curve reflects only the initial decay rate and curvature. Therefore, if the values of 100 and N are known, it is possible to evaluate the diffusion of the attenuation process and to determine the sound absorption coefficient observed under diffusion (therefore, the sound absorption power of 5abine).

Next, to select the theoretical value, convert equation (4) into the function LN(1) of t.
This is done by selecting the curve closest to the reverberation curve from the group of curves obtained when i and N are used as parameters (see Figure 3), and by this selection the values of N and 100 are determined. be done.

Further, by differentiating equation (4), the attenuation rate L'(t) can be obtained as follows.

L'(t)=-4,34((N-1)/Nla/(
1+100t/N)... (5) Therefore, the initial damping rate L' (0) is L'(0)・-4,34t(N
-1)/N) 100 ......(6). By the way, the reverberation time is T. is generally 60dB from the initial volume.
It refers to the time it takes to decay. In addition, the reverberation curve is curved due to various error factors, and the slope becomes smaller as t increases, so ideally linear attenuation (the slope is L'
(0)) t, then, as is clear from the graphical relationship shown in FIG. 3, the reverberation time T, 4° is expressed by the following equation.

TNo=-60/Lm'(0)=13.8fN/(N-
1) l/a ・----(7) Then, in order to make the condition for a completely diffuse sound field, N in equation (7) should be set to a silver-free value, and thereby, a completely diffuse sound field is obtained. The reverberation time T0° corresponding to the attenuation rate of linear attenuation in is determined as follows.

T,,=13.8/N −−−−・−(8) Here,
Since 100 is determined when selecting the logical value Lr+(t), by substituting it into equation (8), the reverberation time T. .

is required.

This reverberation time is T. 0 is determined respectively when the reverberation chamber is empty and when the sample is placed in the reverberation chamber, and each is T. ,
Let it be TI. Then, α and a can be found using the following formula introduced mathematically.

α, -55・3V (yo-yo) ・・・・・・(9) C3
T, T.

a-one special”ヱ(11) ・・・・・・(1o)C;n
・T1 T.

Here, S is the sample area, n is the number of samples, ■ is the volume of the reverberation chamber, and C is the speed of sound.

In addition, in actual measurements, the reverberation curve (〈52(t)
>l Time t for NO to attenuate by a predetermined amount ((dB),
It is relatively easy to realize the method of determining the parameters 100 and N described above from , and the degree of curvature Q of the curve.

That is, tl and Q are defined as follows.

t x・(N(exp(f2/4.34(N-1)))
-1)/a +++++・(11)Ql・L, /(N-1
)...(12) Therefore, T 00 can be expressed as in the following equation using these tt and Qx without directly finding a and N.

T , o□(1,38Q x・t z)/(((exp
(QQ/4.34)-1))...(13) Then, Top is calculated from TNo as follows.

Too-(N-1)・Tso/N--(1
'4) Here, when f2dB is set to 30 dB, this corresponds to a region where curvature becomes noticeable, so the value of Q30 becomes a value that reflects the curvature well. Here, in FIG. 4, Q20 and t, assuming Q=30. A group of reverberation curves is shown when is set as a parameter. In reality, a large number of curve groups as shown in FIG. 4 are stored in a table, and the one closest to the reverberation curve obtained by measurement is selected to determine the values of Q3G and t30.

B. Configuration and operation of the embodiment FIG. 1(A) is a block diagram showing the configuration of an embodiment of the present invention.

In FIG. 11(A), 1 is a single tone generator that generates an impulse (or a tone burst), and 2 is a reverberation chamber. Inside this reverberation chamber 2, as shown in FIG. There's also a chair.

When b... is placed, a reflective side wall C made of plywood or the like is placed around it, as shown in the same figure (c).
will be arranged. The height of the side wall C is higher than the height of the chair. For example, when the height of the chair is 900 mm, the height of the side wall C is set to 1800 mm. Further, the thickness of the side wall C is about 20 mm.

Next, in the circuit shown in FIG. 1(a), before the impulse is generated, the noise n(x) in the reverberation chamber 2 is picked up by one of the microphones 4-1 to 4-n,
After being converted into a digital signal by the A/D conversion circuit 7, it is squared by the squaring circuit 8. The output n'(x) of this square circuit 8 is sequentially accumulated by an accumulator 9 to obtain fn''(x)dx. This multiplication result is divided by the accumulation time τ by a division circuit 25 to eliminate noise n(x)
The average of the squares of , that is, the square n'art of the effective value of the noise n (x) is determined.

Next, before the first impulse response is generated from the speaker 3, the common terminal 5-C0M of the changeover switch 5 and the contact 5-1 are connected, and the accumulator 9 and the RAM 1
4 and register 12 are cleared. When the impulse response r, (x) by the single sound generator l is generated from the speaker 3, the sum of the impulse r, (x) and the noise n (x) from the microphone 4-1 is R(x) - r
, (x)+n(x). This R(x) is A/
It is converted into a digital signal by the D conversion circuit 7, and squared by the squaring circuit 8 to become R''(x).Then, the square n2 of the effective value of the noise n(x) mentioned above is converted to R'(x) by the subtraction circuit 23. ), R2(x) n
'off'. This subtraction result, accumulator 1
1 is sequentially accumulated, and f o CR'(x)-n"-r+] dx is calculated. This accumulation result is manually input to the register 10 sequentially at a predetermined timing, and the contents of this register 10 are added. circuit 1
1 is added to the contents of register 12. The result of this addition is stored in a predetermined location of RA and M14, and also inputted into the register 13 manually.

In this case, the contents of the accumulator 9 are manually entered into the register 10 every time a predetermined period of time elapses from the start of measurement. At the first timing, the contents (S, . . . ) of the accumulator 9 are input to the register 10. At the same time, the contents of address 1 of RAM 1.4 are manually input to register 12. At this time, RAM 14 has been cleared, so the value entered manually in register 12 is also "
o''. Then, the contents of the register 12 and the contents 51-1 of the register 10 are added by the adder circuit 11 and [
S, , +O] is rewritten to address 1 of RAMI 4 and input to register 13. Then, at the next timing, the contents of the accumulator 9 (S
, -2) is input to the register 10, RAM 1
The contents of address 2 of 4 (-“'O”) are input to the register 12, and the addition circuit 11 adds [51-2+O:], and this addition result (S, -2) is stored in the RAM 14 again.
is written to the second address and register 13. below,
Similarly, data is written to the RAM 14 and the register 13 at the end of the accumulation. That is, at the time when the writing of the accumulation end point is completed, each address of the RAM 14 is filled with the following address: 51-1.2 address -+S, -2, 3 address -+S,
-, . . . address m→S, -ka are written, and S and - are written in the register 13.

In this way, when the data collection power of the impulse response r, (x) in the microphone 4-1 is completed, the common terminal 5-COM of the changeover switch 5 and the contact 5-2 are connected, and the accumulator 9 is also cleared. be done.

Then, the second impulse response r from the speaker 3
, (X) are sequentially processed in the same manner as above.

Now, when the first accumulated value (S2.□I) of the impulse response r2(X) is input to the register 10, the RAM
The contents of address 1 of No. 14 (51-1 is stored in the above process) are input to the register 12, and the adder circuit 11
CS2-1+S+-+'J is calculated by CS2-1+S+-+'J, and the result of this calculation is stored at address 1 of RAM 14 and /
It is input again to the meta 13. Thereafter, the accumulated value of impulse response r2(X) is sequentially added to the contents of RAM14 in the same manner, and at the end of the accumulated value, the accumulated value of impulse response r2(X) is added to the contents of RAM14.
For each address of 4, address 1 → [S I-+ + 32-I
] 2nd address → [S, -t + S t-2] 3rd address → [
The value of address S+-s+St-Jm→[S+-wa+S2-a+] is stored.

Next, microphone 4-3.4-4...4
−n gives impulse response r*(X), r.

(x)...r, (x) are sequentially collected, and the accumulated value is added to the RAM 14. That is, n impulse responses r, (x), r, (x)...
When the processing of r, , (x) is completed, R
The contents of AM14 are address 1 → [s, -1+s! -1+ ・++ ＋・+
s n-1= T S l) 1st address → (S l-t+
S t-t+...S , -, = T S
, ) Address 1 - [S I-, + S *-, 10-・-・-
8,,-,=TS,:], and TS is written in the register 13.

The contents TS- of the register 13 are then stored in the latch circuit 16.
The signal is input to the subtracted input terminal of the subtraction circuit 15 through the subtraction circuit 15.

On the other hand, the contents of the RAM 14 are sequentially read out from address 1 and supplied to the subtraction input terminal of the subtraction circuit 15. Therefore 1,
In the subtraction circuit 15, the following operations are performed in sequence.

[:TS, -TS,=Zl), [TS, -TS2-
Z, E... [TS=-TS. =2. ] This operation result Z, ~Z. is the spatial ensemble mean (<s'(t)>
),. This shows that. Then, the calculation results Z1 to Z,
, is logarithmically compressed (1012og) by the ROM 17, and a reverberation curve is obtained. This reverberation curve is displayed/recorded over time by a display/recording device 19 via an interface 18 .

Next, 30 shown in FIG. 1(a) is, for example, the above-mentioned Q3
This is a chart table ROM that stores a group of curves (see FIG. 4) having 0Sj3° as a parameter. Also, 3
Q1 compares the reverberation curve output from the ROM 17 with the curve in the chart table ROM 30, selects the closest curve, and indicates the characteristics of the curve. , t3
This is a comparison calculation circuit that determines the value of 0. This comparison calculation circuit 31 compares the determined values of Q go, tso with the above-mentioned (1
From equations 3) and 14, To. seek.

The above measurements are performed when the reverberation room 2 is empty and when a chair is inserted, and the reverberation time a is determined by using equation (7). In addition, if the sample is a sound absorbing material, (6)
Find α using the formula.

Here, an example of actual measurement results is shown in FIG. Same figure (a)
Figure (B) shows the case where there is no side wall and no correction is made for perfect diffusion, Figure (C) shows the case where there is a side wall but no correction is made for perfect diffusion. ,
) shows the case (in the case of the embodiment) in which side walls are provided and complete diffusion is corrected. It can be seen that the measurements according to the example match the actual measurements in the hall, but there is a large difference in other measurements.

Note that although multiple speakers 3 are shown in FIG. Select speaker 3.

"Effects of the Invention" As explained above, according to the present invention, when the reverberation chamber is empty, a sample whose sound absorption coefficient is to be measured and a reflective enclosure surrounding the sides of the sample are placed in the reverberation chamber. By calculating the spatial collective average of the reverberation when the sound source is cut off, and comparing this reverberant waveform with the theoretical value, the degree of curvature of the reverberant waveform is calculated. Find the reverberation times T o, T 1 for the attenuation rate of linear attenuation in a diffuse sound field, and calculate these T. , T
Since the sound absorption coefficient of the sample is determined based on 1, the error factors such as insufficient diffusion, sound incidence from the side of the sample, and area effect are eliminated, and the advantage is that extremely accurate sound absorption coefficient can be measured. is obtained.

[Brief explanation of drawings]

Figure 1 (a) is a block diagram showing the configuration of an embodiment of the present invention, Figure 1 (b) is a schematic diagram showing the inside of the reverberation chamber 2, and Figure 1 (c) is the arrangement of chairs in the reverberation room. A perspective view showing
FIG. 2 is a schematic diagram for explaining the measurement principle of this invention,
Fig. 3 is a characteristic diagram for explaining the measurement principle, Fig. 4 is a diagram showing an example of a group of curves used as a reference, Fig. 5 is a graph showing an example of measurement results in the example, and Fig. 6 is a graph showing the conventional FIG. 2 is a schematic diagram for explaining a measurement method. 2... Reverberation room, 7... A/D conversion circuit, 8...
・Square circuit, 9...Accumulator, 10・
...Register, 11...Addition circuit, 12...
...Register, 13...Register, 14.
...RAM, 15... Subtraction circuit, 16.
...Latch circuit, 17...ROM (hereinafter "near" to "17" are spatial collective average measurement means), 30...
Chart table ROM (curvature determination means) 31...
Comparison calculation circuit (curvature determination means, reverberation time calculation means, sample sound absorption coefficient calculation means) C...Side wall (reflective enclosure means).

Claims (2)

[Claims] (1) Reverberation when the sound source is cut off when the reverberation chamber is empty and when a reflective enclosure surrounding the sample whose sound absorption coefficient is to be measured and the side of the sample is placed inside the reverberation chamber. The degree of curvature of the reverberant waveform is determined by calculating the spatial collective average of , and comparing this reverberant waveform with the theoretical value.Furthermore, from this degree, the reverberation time T_0 for the attenuation rate of linear attenuation in a completely diffuse sound field is calculated. A sound absorption coefficient measuring method characterized in that T_1 is determined, and the sound absorption coefficient of the sample is determined based on these T_0 and T_1. (2) a reverberation chamber; reflective enclosure means for surrounding the sides of a sample whose sound absorption coefficient is to be measured; and a spatial ensemble mean measuring means for determining a spatial ensemble mean of reverberation when the sound source is cut off in the reverberation chamber; , a curvature determining means for determining the degree of curvature of the reverberant waveform by comparing the reverberant waveform measured by the spatial collective average measuring means with a logical value; and a straight line in a completely diffuse sound field based on the degree determined by the curvature determining means. a reverberation time calculation means for calculating a reverberation time with respect to an attenuation rate; a calculation result T_0 of the reverberation time measurement means when the sample and reflective enclosure means are arranged in the reverberation room and when nothing is arranged; A sound absorption coefficient measuring device comprising: sample sound absorption coefficient calculation means for calculating the sound absorption coefficient of the sample from T_1.