JPH0515972B2 - - Google Patents

Description

[Detailed Description of the Invention] Industrial Application Field This invention relates to a calibrator for correcting the characteristics of a microphone used in a sound intensity measurement device used for noise control, architectural acoustics, measurement of audio equipment, etc. .

Conventional technology The time-averaged sound intensity I is, for example, as disclosed in "Identification of Noise Sources by Sound Intensity Method" (Acoustical Society of Japan Vol. 39, No. 10, 1983), I = (t) (t) (1 ). Here, p(t) is the sound pressure, and u(t) is the particle velocity in the direction of interest. At present, there is no transducer that can directly measure particle velocity with high precision, so two microphones 6a and 6b are lined up in the direction of measurement as shown in Fig. 3, and their sound pressure signals p ₁ (t), From p ₂ (t), U(t)=-1/ρ∫ ^t ∽p ₂ (t') - p ₁ (t')/△rdt'
(2) uses the value that can be found recently. Here ρ is the density of air. The sound pressure is the average value of the outputs of the two microphones, and is approximated by P(t)=p ₁ (t)+p ₂ (t)/2 (3). By substituting equations (2) and (3) into equation (1), the time-averaged sound intensity can be determined.

Here, as shown in equations (2) and (3), if the characteristics between the two channels, including the characteristics of the two microphones, are the same, and the absolute sensitivities are not equal, there will be an error in equations (2) and (3). is included. For example, when a plane wave arrives in the axial direction of the microphone in Figure 3, the measurement error when the phase difference of the two-channel measurement system including the microphone is △θ and the amplitude sensitivity is the same is Ie/I = sin(k・△r−△θ)/
It is given by k・△r, k=2πf/c(4). Here, Ie is the acoustic intensity including error, I is the true acoustic intensity, f is the plane wave frequency, and c
is the speed of sound. To give an example, △r=0.05m,
When △θ=1°, f=50Hz, Ie/I0.63, 4dB
There will be a measurement error. Correcting the characteristic difference between the two channels, especially the phase difference, in this way is very important in improving the accuracy of sound intensity measurement.

Conventional solutions to this problem have typically involved the use of two microphones with matching characteristics. It is relatively easy to manage the characteristics of an electric circuit, but it is difficult to manage the characteristics of a microphone that includes a mechanical structure, and the difference in characteristics between channels is almost determined by the difference in characteristics of the microphones. However, selecting two microphones with similar characteristics not only impairs production efficiency, but it is also highly conceivable that the characteristics of the microphones may change during long-term use.

As a method to solve these problems, a calibrator for measuring the difference in characteristics between microphones as shown in FIGS. 4 and 5 is used.

FIG. 4 shows a method of directly measuring the difference in characteristics between two microphones to be calibrated. 1 is a calibrator, 2 is a small space inside the calibrator, 3 is a speaker for generating a calibration sound pressure in the small space, 4 is a signal generator that supplies a calibration signal to the speaker, and 5 is its amplifier. . 6a and 6b are two microphones to be calibrated, and 7 is a device for measuring the difference in their characteristics. Since the two microphones 6a and 6b are positioned symmetrically with respect to the speaker in the small space, they receive the same sound pressure. If two signals from the microphone are applied to the characteristic difference measuring device 7, 2
It is possible to measure the difference in characteristics between two microphones. By supplying it to the sound intensity measuring device, the accuracy of sound intensity measurement can be improved.

Figure 5 shows the microphone to be calibrated.
This method indirectly measures the characteristic difference between microphones by inserting them into a calibrator one by one.
In the figure, the same parts as in FIG. 4 are given the same numbers. In the figure, first, the microphone 6a is inserted into a small space, and the amplitude ratio with the input signal to the speaker is
Measure A ₁ and phase difference Δθ ₁ . Next, the microphone 6b is inserted into the small space at the same position as the microphone 6a, and the amplitude ratio A ₁ and phase difference Δθ ₂ with respect to the input signal of the speaker are measured.

If these are obtained, the microphones 6a and 6b
The amplitude ratio and phase difference are given as A ₂ /A ₁ and (θ ₂ −θ+), respectively. By supplying these characteristic differences to the sound intensity measuring device, measurement accuracy can be improved.

Problems to be Solved by the Invention However, the problem with the calibrators shown in FIGS. 3 and 4 is that the sound pressure at the sound receiving surface of the microphone changes greatly depending on the frequency inside the small space.

FIG. 6 shows an example. The horizontal axis is frequency, and the vertical axis is relative sound pressure (dB).

There is a peak around 5kHz and a deep valley at 9kHz. At the frequency of the sound pressure trough, sufficient sound pressure is not supplied to the microphone, making it impossible to measure the characteristic difference with sufficient accuracy.

Means to Solve the Problem By the way, the reason why deep valleys are seen in Figure 6 is that acoustic resonance occurs in the high frequency range in a small space, and the sound at the sound receiving part of the microphone at a specific frequency is As a result of various studies, it was found that this was due to a decrease in pressure.

The present invention aims to solve this problem and provide a calibrator in which changes in sound pressure with respect to frequency in a small space inside the calibrator are reduced. That is, the first aspect of the present invention is a microphone characteristic difference having a small space into which each microphone is inserted or two microphones are inserted at the same time, and a speaker which generates a signal sound field for calibration in the small space. In the correction calibrator, a secondary small space for high frequency compensation is formed in communication with the small space and has a resonant frequency higher than that of the small space. Further, the second invention further improves the effect of the first invention by providing an acoustic damping material in the acoustic path connecting the two small spaces of the first invention.

Function The acoustic system of the present invention will be equivalently represented by an electric circuit, and its function will be explained.

First, FIG. 7 shows an equivalent electric circuit of the calibrator shown in FIG. 4 or 5 in relation to the prior art. The resonance in the high frequency range without the correction circuit of the calibrator is
As shown in FIG. 7, it is determined by L ₀ , C ₀ and L ₁ , C ₁ . where the inductance L ₀ corresponds to the vibrating mass of the speaker and the capacitance C ₀
corresponds to the spring constant of the speaker's vibration system. Also, the inductance L ₁ and capacitance C ₁
corresponds to the mass and spring constant of the small space. E ₀ is an electromotive force corresponding to the driving force of the speaker. At this time, the voltage E ₁ generated across the capacitance C ₁
corresponds to the sound pressure in a small space. Voltage
E ₁ rapidly decreases above the resonance frequency f ₁ = 1/2π√( ₀ + ₁ ) ( ₀ + ₁ ) _{1 0} ≒ 1/2π√1 ( ₀ + ₁ ) ₁ (C ₀ ≫C ₁ ). In other words, the impedance due to capacitance C ₁ is inductance (L ₀ + L ₁ )
This is because the impedance is smaller than the impedance due to Therefore, as shown in FIG. 8, a new resonance system L ₂ and C ₂ is provided in parallel with C ₁ in order to prevent the impedance from decreasing. The calibrator represented by the equivalent circuit in FIG. 8 is the first invention, and the inductance L ₂ and capacitance C ₂ in the equivalent circuit correspond to the equivalent mass and equivalent spring constant of the secondary small space of the calibrator. do. Here, C ₁
The resonant frequency f ₂ = 1/2π√ _{2 2} determined by and L ₂ is
By setting f to be larger than f ₁ , the impedance at both ends of C ₁ increases at frequencies above f ₁ , and as a result, E ₁ ' becomes larger than E ₁ in the same frequency band.
That is, by forming the secondary small space, the sound pressure in the small space was increased.

However, in the equivalent circuit shown in FIG. 8 above, since the resonant circuit is composed only of inductance and capacitance, large resonance peaks and valleys occur. To prevent this, a resistor R ₂ is added in FIG. 9 to that in FIG. 8.
This damps resonance and achieves the effects of the invention over a wide band.

That is, this damping resistance corresponds to acoustic damping material added to the acoustic path connecting the small space and the secondary small space. Note that the acoustic damping material includes glass wool, nonwoven fabric, felt, etc. suitable for this purpose.

Embodiment FIG. 1 shows an embodiment of the second invention in which an acoustic damping material is interposed in the acoustic circuit of the first invention, and elements given the same numbers as those in FIG. It is similar to that of FIG. 4 and operates similarly. In FIG. 1, a secondary small space 8 for high-frequency compensation is provided on the wall facing the speaker 3 of the small space 2, and its size is about 1/10, and a sound absorbing space 8 is provided in the middle. A flexible material 9 is arranged.
This secondary small space 8 prevents resonance of the small space 2 as explained based on FIG. 8 above.

Figure 2 shows the sound pressure characteristics inside the calibrator, which has a secondary small space 8 in which a sound-absorbing material is provided in the acoustic path 9, and there is a sharp valley as seen in Figure 6. do not have. By using a calibrator in which the frequency change of sound pressure in a small space is small in this way, it becomes possible to correct characteristic differences with high accuracy, and the accuracy of sound intensity measurement itself can be improved.

Although the above embodiment is related to a calibrator that directly calibrates two microphones, the same applies to a case where the difference in characteristics of the microphones is indirectly measured as shown in FIG. Furthermore, although the secondary small space 8 may be placed anywhere in the small space, it is most effective to place it on the wall facing the speaker.
Further, in FIG. 1, the signal generator and amplifier are placed outside the calibrator, but similar results can be obtained even if they are placed inside the calibrator. Moreover, even if a characteristic difference measuring device is not specially prepared, if the function is included in the sound intensity measuring device itself, the calibrator of the present invention can be effectively used.

Effects of the Invention As described above, the present invention improves the frequency characteristics in the high frequency range, which cannot be achieved with the small space alone, by using the secondary small space for high frequency compensation installed in the small space. This allows for accurate calibration over a wide frequency range.

[Brief explanation of the drawing]

FIG. 1 is a sectional view showing an embodiment of the second invention of the present invention, FIG. 2 is a diagram showing an example of the frequency characteristics of sound pressure inside the calibrator shown in FIG. 1, and FIG.
A front view showing a sound intensity measurement probe using two microphones, FIGS. 4 and 5 are cross-sectional views showing a conventional calibrator for correcting microphone characteristic differences, and FIG. 6 is shown in FIGS. FIG. 3 is a diagram showing an example of frequency characteristics of sound pressure inside the calibrator. FIG. 7 is an equivalent electric circuit corresponding to FIG. 4 or FIG. 5, and FIGS. 8 and 9 are equivalent electric circuits corresponding to FIG. 1. 1: Calibrator, 2: Small space inside the calibrator, 3: Speaker, 4: Calibration signal generator, 5: Amplifier, 6a,
6b: Microphone, 7: Characteristic difference measuring device, 8:
Secondary small space, 9: Acoustic damping material.

Claims (1)

[Claims] 1. A microphone characteristic difference correction device having a small space into which each microphone or two microphones are inserted at the same time, and a speaker that generates a signal sound field for calibration in the small space. A calibrator for correcting characteristic differences, characterized in that a secondary small space is formed in communication with the small space and has a resonant frequency higher than the resonant frequency of the small space. 2. In a calibrator for microphone characteristic difference correction, which has a small space into which each microphone is inserted or two microphones are inserted at the same time, and a speaker that generates a signal sound field for calibration in the small space, the small A secondary small space is formed in communication with the space and has a resonance frequency higher than that of the small space, and an acoustic damping material is interposed in the acoustic path connecting the two small spaces. Calibrator for characteristic difference correction.