JP2000065635A - Apparatus for measuring acoustic characteristic - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain an acoustic characteristic in a state in which an acoustic material changes by calculations by setting a sound sensor for measuring a sound pressure of a predetermined position at the side of a front face of the acoustic material filled in a pipe and another sound sensor for measuring a sound pressure of a predetermined position in a space at the side of a rear face. SOLUTION: An acoustic material 20 to be measured is filled in an acoustic pipe 11 at a part of X=0 to X=d. A speaker 12 radiates sound waves towards a front face 21 of the acoustic material 20 in the acoustic pipe 11. A piston 13 forms a space at the side of a rear face 22 of the acoustic material 20. A length of the space is changed by moving the piston 13 in an X direction. A microphone 14 is a sound sensor for measuring a sound pressure of a position of the front face 21 of the piston 13. A microphone 15 is a sound sensor for measuring a sound pressure of a position of the piston 13 at the side of the rear face 22 of the acoustic material 20. An arithmetic operating part 16 obtains an acoustic characteristic of the acoustic material 20 on the basis of sound pressures obtained by microphones 14, 15 under a plurality of different conditions in which the length of the space at the side of the rear face 22 of the acoustic material 20 is different.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an acoustic characteristic measuring device for measuring acoustic characteristics of an acoustic material.

[0002]

2. Description of the Related Art Sound propagation characteristics in a porous acoustic material such as a sound absorbing material that absorbs sound are represented by four materials including an equivalent density ρ, an equivalent bulk modulus K, a resistance coefficient γ, and a leakage coefficient g. Determined by material properties. In general, it is difficult to separate and measure these material properties, so that a sound absorption coefficient and a propagation constant determined by a combination of a plurality of material constants among those material constants are measured. For example, as an example of a method of measuring the sound absorption coefficient, an acoustic tube is applied to the surface of an acoustic material, a sound wave is emitted into the tube to form a standing wave in the tube, and the sound pressure at each point along the longitudinal direction in the tube. A method is known in which the minimum sound pressure and the maximum sound pressure are measured to measure the sound absorption coefficient from the minimum sound pressure, the highest sound pressure, and the position where the lowest sound pressure is obtained.

[0003]

However, even if the material of the acoustic material is the same, the values of the sound absorption coefficient and the propagation constant are different if the thickness of the material is different or the structure of the back side of the acoustic material is different. Moreover, even with the same acoustic material, the sound absorption coefficient and propagation constant of an acoustic material of another thickness cannot be known from the sound absorption coefficient and propagation constant measured for an acoustic material of a certain thickness. The same is true when the structure on the back side of the material is different, and there is a problem that the measurement must be performed again for each state where the thickness of the acoustic material and the structure on the back side are different.

According to the present invention, when the state changes, the acoustic characteristics in the changed state, such as the sound absorption coefficient and the propagation constant for sound waves incident not only vertically but also obliquely, can be obtained by calculation. It is an object of the present invention to provide an acoustic characteristic measuring device for measuring the acoustic characteristics.

[0005]

An acoustic characteristic measuring apparatus according to the present invention for achieving the above object comprises a tube in which a portion of a longitudinal direction is filled with an acoustic material to be measured, and a surface of the acoustic material filled in the tube. A sound source that radiates sound waves into the tube toward the tube, a space forming member that forms a space on the back side of the acoustic material filled in the tube so that the length of the space can be changed, and a tube inside the tube, A first sound sensor for measuring a sound pressure at a predetermined position on the front side of the acoustic material, and a sound pressure at a predetermined position in the space formed by the space forming member on the back side of the acoustic material filled in the tube. And a second sound sensor for measuring.

[0006] An acoustic characteristic measuring apparatus of the present invention has the above-described configuration.
By obtaining sound pressures with the first sound sensor and the second sound sensor under a plurality of conditions having different lengths of the space, acoustic characteristics based on those sound pressures, that is, the above-described four The material constants can be determined, and calculations based on them allow for acoustic properties such as the sound absorption of the acoustic material in that state, even in different states, for example, when the thickness of the acoustic material is different from the thickness of the acoustic material at the time of measurement. Characteristics can be obtained. In addition, according to the sound pressure measurement using the acoustic characteristic measuring device of the present invention, it is possible to obtain all the four material constants described above. However, it is not always necessary to obtain all the four material constants. For example, if the acoustic characteristic that you want to finally find is sound absorption,
It goes without saying that only the material properties related to the sound absorption coefficient need to be obtained.

Here, in the acoustic characteristic measuring apparatus according to the present invention, the sound pressure obtained by the first sound sensor and the second sound sensor under a plurality of conditions where the lengths of the spaces are different from each other. It is preferable to include a calculation unit for determining the acoustic characteristics of the acoustic material filled in the pipe based on the calculation.

When such an arithmetic unit is provided, the acoustic characteristics of the acoustic material are automatically obtained from the sound pressure.

Here, in the acoustic characteristic measuring apparatus of the present invention, the space forming member can freely change the space including the length of zero on the back side of the acoustic material filled in the tube. The first sensor may measure a sound pressure at a surface position of the acoustic material filled in the tube, and the second sensor may further include:
The sound pressure at a position farthest from the back surface of the acoustic material filled in the tube in the space may be measured.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Before describing embodiments of the present invention, the theory of a measurement method will be described based on an equation representing a one-dimensional wave phenomenon in an acoustic tube filled with a lossy medium. Induce the underlying expression. (One-dimensional Wave Theory) FIG. 1 is a schematic diagram showing a part of a sound tube having a loss accompanying sound propagation, and FIG. 2 is an equivalent electric circuit diagram of the sound tube shown in FIG. is there.

As shown in FIG. 1, consider the motion of a medium in an acoustic tube having an infinitely hard wall and having a sectional area S when an acoustic wave propagates in an axial direction. Taking the spatial coordinate x in the axial direction, if the dimension in the cross-sectional direction of the tube is sufficiently smaller than the wavelength, the wavefront of the sound wave in the tube is flat and propagates only in the direction of x. Here, equations (1) and (2), which are equations representing waves, when the acoustic tube is filled with a lossy acoustic material, are set as starting points.

[0012]

(Equation 1) Here, p (x, t) is a sound pressure and a function of distance x from the origin x and time t v (x, t) is a particle velocity and a function of the distance x and time t from the origin r is a resistance coefficient (to speed) Ρ is the density of the medium (corresponding to the inductance of the electric circuit)

[0013]

(Equation 2) Here, g is a leakage coefficient (a proportional coefficient of loss proportional to sound pressure and corresponds to a leakage conductance of an electric circuit) K is a bulk modulus of a medium (corresponding to a reciprocal of the capacitance of an electric circuit) Equation (1) And (2) generally hold for lossy transmission lines represented by cascaded electrical circuits as in FIG.

(1) and (2) Fourier transform of both equations and replace them with the description in the frequency domain. The differentiation by time in the time domain is multiplied by jω (j = √ (−1) is an imaginary unit) in the frequency domain, and therefore, the equations (1) and (2) are replaced as follows.

[0015]

(Equation 3) Here, P (x) is a Fourier transform of p (x, t) V (x) is a Fourier transform of v (x, t) Z = Z (jω) = r + jωρ, G = G (jω) = g + j
Rewrite the above equation as ω / K.

[0016]

(Equation 4) Both equations can be rewritten as:

[0017]

(Equation 5) here,

[0018]

(Equation 6) Γ = γ (jω) = α + jβ (10), where α is an attenuation constant and β is a wavelength constant.

When there is no loss, that is, in air,

[0020]

(Equation 7) Here, c = √ (K / ρ) is a sound velocity (phase velocity) (12) k = ω / c is a wave number (wave number) Equations (5) and (6) or Equations (7) and (8) The solution of) is as follows.

P = Cexp (−γx) + Dexp (γx) (13) V = (1 / W) {Cexp (−γx) −Dexp (γx)} (14) where C and D are Integration constant

[0022]

(Equation 8) If r = g = 0 in the air,

[0023]

(Equation 9) (Boundary condition) FIG. 3 shows an acoustic tube filled with an acoustic material having a propagation constant γ ₁ from x = 0 to x = d, an air layer from x = d to x = L, and a closed acoustic tube at x = L. FIG.

As shown in FIG. 3, an acoustic material having a propagation constant γ ₁ is filled from x = 0 to x = d and an air layer is formed from x = d to x = L. Think. x = 0
To x = d is referred to as a first section, and the air layer from x = d to x = L is referred to as a second section. Then, the solutions of the wave equation in the first section and the second section are as follows.

From the first section x = 0 to x = d, if a constant 1 is added to the constant, P = C ₁ exp (−γ ₁ x) + D ₁ exp (γ ₁ x) (17) V = (1 / W ₁ ) {C ₁ exp (−γ ₁ x) −D ₁ exp (γ ₁ x)} (18) For the second section, a subscript 2 is added to the constant, and the second section x = D to x = L is an air layer, so γ ₂ = jω
/ C = jk, and P = C ₂ exp (−jkx) + D ₂ exp (jkx) (19) V = (1 / ρc) {C ₂ exp (−jkx) −D ₂ exp (jkx)} (20) The boundary condition is that the sound pressure at x = 0 is P ₀ , a sound tube closed at x = d is connected, and x = d
L means that the particle velocity is 0. Further, a condition that both the sound pressure and the particle velocity are continuous is required on the connection surface between the first section and the second section.

(Input impedance of the second section) First,
Input impedance of the second section that is a terminal impedance of the first section, i.e., to calculate the impedance Z _T viewed right from the left end of the acoustic tube length L-d. This is the input impedance of a closed tube of length Ld. Here, only the main points are shown because the method of obtaining the coordinates is different.

The sound pressure and the particle velocity distribution in the pipe when the length L-d at x = L left x = d the sound pressure P _d of the acoustic tube end is closed of by the following formula Can be

[0028]

(Equation 10) Therefore, the impedance of this acoustic tube viewed from the end of x = d is

[0029]

[Equation 11] Becomes

This is the terminal impedance of the first section.

(Sound Pressure Distribution in First Section) Next, the sound pressure in the first section when the first section is driven by the sound pressure P ₀ from the left end, and the impedance Z _T is connected to the right end thereof. The expression of the velocity distribution is shown.

[0032]

(Equation 12) (Derivation of material constant calculation formula by P ₀ and P _d ) If the sound pressure distribution in the acoustic tube described above is used, x = 0 to x
If an unknown material is placed between = d and an acoustic tube with a closed end at the length Ld is connected to the back, an unknown material constant should be obtained from the measurement of the sound pressure at two points. . Here, the measured quantity used therefor, and the sound pressure P _d of the point x = sound pressure point -a P _a and x = d. It is assumed that the length L-d of the sound tube in the second section can be changed.

The sound pressure in the first section (0 ≦ x ≦ d) is P
Equation (33), (3) where ₁ (x) and the particle velocity are V ₁ (x).
Rewriting 4) is as follows.

[0034]

(Equation 13) The sound pressure P ₂ (x) and the particle velocity V ₂ (x) in the second section (d ≦ x ≦ L) are given by equations (25) and (26). I will repeat it next.

[0035]

[Equation 14] Although the relationship of the sound pressure P _d of the point of the sound pressure P ₀ and x = d the point x = 0 is required, in order that the terminating impedance of the first section, i.e. it is necessary to input impedance of the second section . It has already been given as equation (27). Z _T = Z (d) = − jpc · cotk (L−d) (27) When this is substituted into equations (33a) and (34a), the sound pressure and speed in the first section (0 ≦ x ≦ d) It becomes a distribution formula.

[0036]

(Equation 15) If x = d in equation (35), the relationship between P ₀ and P _d at this time is obtained.

[0037]

(Equation 16)Measured P₀And P_dTo γ₁And W₁To find
P on both sides of (37) _d ^*Multiply the average
You. Then

[0038]

[Equation 17] become that way. Here, the cross spectrum of the P _d and P ₀ P
_{Dividing by} the power spectrum of d produces T _d-0 which is formally a transfer function of sound pressure from x = d to x = 0.

[0039]

(Equation 18) If L = d in this equation, the following is obtained.

T _d−0 = cosh (γ ₁ d) (40)

[0041]

[Equation 19] Since the denominator in the inverse hyperbolic function of this equation is a real number, if the real part of T _d-0 is written as R and the imaginary part as I,

[0042]

(Equation 20) And the real part α ₁ and the imaginary part β _{1 of} γ ₁ are calculated as follows (see Iwanami Zensho “Mathematical Formula II”)

[0043]

(Equation 21) Since γ ₁ is obtained by the above procedure, the characteristic impedance W ₁ of the material in the first section is obtained from P ₀ and P _d when L is set to an appropriate value such that L> d. The transfer function of the sound pressure from x = d to x = 0 when the second section x = d to x = L is an air layer is also calculated from Expression (39) as follows.

[0044]

(Equation 22) Substituting γ ₁ = α ₁ + jβ ₁ in equation (10) gives

[0045]

(Equation 23) Decomposing the complex cosh function and sinh function gives

[0046]

(Equation 24) Accordingly, when L = d, x = 0, ie, the sound pressure P ₀ on the material surface, and x = d, ie, the sound pressure P _d on the back surface of the material, are measured.
Next, if the sound pressure P ₀ at x = ₀ and the sound pressure P _d at the point x = d when L> d are measured, the propagation constant γ ₁ and the characteristic impedance W _It can be seen that ₁ can be calculated.

(Calculation of Material Constants Using P ₀ and P _L ) Up to now, the propagation using the measured values of x = 0, ie, the sound pressure P ₀ on the material surface and x = d, ie, the sound pressure P _d on the back surface of the material, It is shown that the constant and characteristic impedance can be calculated. However, assuming that the thickness of the sound-absorbing material to be measured is not always constant, it is not practical to make holes in the acoustic tube wall at x = 0 and x = d, and it is not practical. It is considered that the sound pressure P ₀ is often difficult to measure. The length L of the acoustic tube
There is also a need for a way to change

Therefore, an embodiment of the present invention will be described below, and a continuation of the above description will be described with reference to the embodiment.

FIG. 4 is a diagram showing a state in which no space exists on the back surface of the acoustic material (a space having a length Ld = 0) in one embodiment of the acoustic characteristic measuring apparatus of the present invention. FIG. 5 is a view showing a state in which an air layer having a predetermined length Ld is present on the back surface of the acoustic material in the embodiment shown in FIG.

The acoustic characteristic measuring device 10 shown in FIGS.
Is a sound tube 11, a speaker 12, a piston 13,
It is composed of two microphones 14 and 15 and an operation unit 16.

A portion of the acoustic tube 11 between x = 0 to d is filled with an acoustic material 20 to be measured.

The speaker 12 is a sound source that emits a sound wave into the acoustic tube 11 toward the surface 21 of the acoustic material 20 filled in the acoustic tube 11.

The piston 13 is a means for forming a space on the back surface 22 side of the acoustic material 20 filled in the acoustic tube 11. By moving the piston 13 in the x direction, the length of the space on the back surface 22 side of the acoustic material 20 can be changed, including the length = 0.

One microphone 14 of the two microphones 14 and 15 is located at a predetermined position inside the acoustic tube 11 on the surface 21 side of the acoustic material 20 filled in the acoustic tube 11 (in the present embodiment, the acoustic material 20). This is a sound sensor for measuring the sound pressure of the surface 21 (position of the surface 21).

The other microphone 15 is located at a predetermined position in the space formed by the piston 13 on the side of the back surface 22 of the acoustic material 20 filled in the acoustic tube 11 (in this embodiment, the back surface of the acoustic material 20). This is a sound sensor that measures the sound pressure at the position farthest from the position 22 (ie, the surface position of the piston 13).

Further, the arithmetic section 16 operates the piston 13 to move the piston 13 so that the space on the back surface 22 side of the acoustic material 20 has different lengths (here, the condition of the space length = 0 shown in FIG. 4). Of the acoustic material 20 filled in the acoustic tube 11 based on the sound pressures obtained by the two microphones 14 and 15 under the condition of the length of the space = Ld shown in FIG. It is a means to obtain a characteristic.

Based on the above description of the acoustic characteristic measuring device 10 as an embodiment of the present invention shown in FIGS.

Here, as shown in FIGS. 4 and 5,
On the opposite side of the speaker 12 using the acoustic tube 11, ie, x
The piston 13 having the microphone 15 attached to the surface is inserted from the end on the = L side so that the value of L can be adjusted.

[0059] Thus, (length = 0 of the space) of the piston 13 most if pushed to the back L = d, and the measured by the microphone 15 fitted with a sound pressure P _d to the piston 13 at this time, when the Equation (43) and Equation (4) together with P _{0 of}
By substituting in 4), the propagation constant γ ₁ can be obtained.

[0060] then measured P ₀ when the value of the appropriate L, is measured by the microphone 15 fitted with a sound pressure P _L in x = L to the piston, it is possible to obtain the characteristic impedance W ₁ It is considered something.

A method of obtaining the characteristic impedance W ₁ from the sound pressure P ₀ at x = ₀ and the sound pressure P _{L at} x = L will be considered.

The sound pressure P _{L at} x = L is as follows from equation (25).

[0063]

(Equation 25) This P _d is given a relationship with P ₀ by equation (37).

[0064]

(Equation 26) The average number of times over the complex conjugate P _L ^* of P _L to both sides of the equation (48).

[0065]

[Equation 27] Sound pressure transmission function T _L-0 from the P _L to P ₀ is,

[0066]

[Equation 28] From this equation, W ₁ is calculated as follows.

[0067]

(Equation 29) Thus, x = 0 That is characteristic of the sound absorbing material of the first section by measurement of the sound pressure P _L of the surface of the sound pressure P ₀ and x = L In other words the piston 13 is inserted in the acoustic pipe 11 of the surface 21 of the acoustic material 20 it can be seen that the impedance W ₁ is required.

(Calculation of Other Material Constants) It has been described above that the propagation constant γ ₁ and the characteristic impedance W ₁ of the sound absorbing material in the first section can be obtained. Next, it will be shown that more basic material constants such as the resistance coefficient r ₁ , density ρ ₁ , leakage coefficient g ₁ , bulk modulus K ₁ or sound velocity c _{1 of} the medium can be obtained therefrom.

The product of the propagation constant γ _{1 in} equation (9) and the characteristic impedance W _{1 in} equation (15) is as follows.

[0070]

[Equation 30] That is, the real part is the resistance coefficient r ₁ and the imaginary part is the product of the angular frequency ω and the density ρ ₁ .

Also,

[0072]

(Equation 31) That is, when the propagation constant γ ₁ is divided by the characteristic impedance W ₁ , the real part is the leakage coefficient g ₁ and the imaginary part is the product of the angular frequency ω and the reciprocal of the bulk modulus K ₁ .

Therefore, all the basic material constants can be obtained.

The sound speed c is calculated in a lossless medium by the above formula.

C = √ (K / ρ) is the speed of sound... (12) If this form is applied to a lossy medium as it is,

[0076]

(Equation 32) Becomes a complex number.

That is, if there is no loss, the propagation constant is a pure imaginary number, and both the wave number k and the sound speed c are real numbers. However, in a lossy medium, the propagation constant is a complex number. If this is expressed as jω / c in the same way as when there is no loss, the sound velocity is expressed by a complex number as in Expression (53). This is the concept of the speed of complex sound. (Calculation of sound absorption coefficient) The normal incidence sound absorption coefficient of this acoustic material can be calculated from the measurement results obtained above and the material constants and impedances calculated thereby.

In order to calculate the sound absorption coefficient, the impedance looking right from the plane of x = 0 is necessary. The impedance viewed from an arbitrary coordinate value x in the section 1 is calculated as a ratio of the equations (35) and (36) as follows.

[0079]

[Equation 33] L = d, that is, when there is no space behind the acoustic material,
It looks like this:

[0080]

(Equation 34) The reflectance R of the sound pressure with respect to the incident wave from the medium having the characteristic impedance ρc to this medium is given as follows.

[0081]

(Equation 35) The sound absorption coefficient α is as follows.

Α = 1−R ² (57) As described above, according to the present embodiment, any acoustic characteristics of the acoustic material to be measured can be obtained. It is not always necessary to obtain it, but only the desired one may be obtained.

In the embodiment shown in FIG. 4 and FIG.
In step 6, based on the sound pressure obtained by the two microphones 14 and 15, an operation based on the above-described theory is performed, and a desired acoustic characteristic is obtained.

[0084]

As described above, according to the present invention,
Acoustic characteristics that allow calculation of acoustic characteristics in different states, even if the state of the acoustic properties is different from that at the time of measurement, such as when the thickness of the acoustic material is different or when sound waves are incident from an oblique direction. Can be measured.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing a part of an acoustic tube having a loss accompanying acoustic propagation.

FIG. 2 is an equivalent electric circuit diagram of the acoustic tube shown in FIG. 1 when loss is considered.

FIG. 3 shows that an acoustic material having a propagation constant γ ₁ is filled from x = 0 to x = d, an air layer is formed from x = d to x = L, and x = d
It is a schematic diagram which shows the acoustic tube closed by L.

FIG. 4 is a diagram illustrating an acoustic characteristic measuring apparatus according to an embodiment of the present invention, in which there is no space (length L−d =
FIG. 11 is a diagram illustrating a state where a space of 0 exists).

5 is a diagram showing a state in which an air layer having a predetermined length Ld exists on the back surface of the acoustic material in the embodiment shown in FIG. 4;

[Explanation of symbols]

 Reference Signs List 10 acoustic characteristic measuring device 11 acoustic tube 12 speaker 13 piston 14, 15 microphone 16 arithmetic unit 20 acoustic material 21 front surface 22 back surface

 ──────────────────────────────────────────────────の Continuing on the front page (72) Inventor Masaru Ogoshi 1-16-1 Hakusan, Midori-ku, Yokohama-shi, Kanagawa Prefecture Ono Sokki Co., Ltd. F-term (reference) 2G064 AA11 AB01 AB02 AB12 AB16 BD05 BD20 CC13 CC29 CC41

Claims (5)

[Claims] 1. A tube in which a portion of a longitudinal direction is filled with an acoustic material to be measured, a sound source that emits a sound wave into the tube toward a surface of the acoustic material filled in the tube, and a tube that fills the tube. A space forming member that forms a space so that the length of the space can be changed on the back side of the acoustic material that has been formed, and a sound pressure at a predetermined position on the surface side of the acoustic material filled in the tube in the tube. A first sound sensor for measuring, and a second sound sensor for measuring a sound pressure at a predetermined position in the space formed by the space forming member on the back side of the acoustic material filled in the tube. An acoustic characteristic measuring device, characterized in that: 2. An acoustic material filled in the pipe based on sound pressures obtained by the first sound sensor and the second sound sensor under a plurality of conditions having different lengths of the space. 2. The acoustic characteristic measuring device according to claim 1, further comprising a calculation unit for calculating acoustic characteristics of the acoustic characteristic. 3. The method according to claim 1, wherein the space forming member is formed on the back side of the acoustic material filled in the tube so as to freely change the length of the space including the length of zero. The acoustic characteristic measuring device according to claim 1, wherein: 4. The acoustic characteristic measuring apparatus according to claim 1, wherein said first sensor measures a sound pressure at a surface position of the acoustic material filled in said tube. 5. The sound sensor according to claim 1, wherein the second sensor measures a sound pressure at a position in the space farthest from a back surface of the acoustic material filled in the tube.
An acoustic characteristic measuring apparatus as described in the above.