JPS63153433A - Acoustic power level measuring method - Google Patents

Abstract

PURPOSE:To measure the titled level while reducing a measurement error by placing a pair of microphones in two symmetrical parts to a vertical axis of a hemispherical space for surrounding a sound source to be measured, and deriving an acoustic power level. CONSTITUTION:The surface of a hemispherical space 3a for surrounding a sound source to be measured 1 is axially symmetrical around a vertical axis 3b. At two positions on the surface of the hemispherical space that are axially symmetric with respect to the vertical axis, a pair of microphones 2a, 2b are placed, and an acoustic intensity at the respective positions is determined simultaneously. A simultaneous measurement at such two symmetrical position is executed with regard to all split points which are set to the hemispherical space surface. Measured values of sound pressure levels which are measured, respectively are inputted to frequency analyzers 5a, 5b, respectively, in which a sound pressure level is derived at every frequency contained in the sound pressure level, a sound pressure level is derived at every frequency contained in this sound pressure level, and this sound pressure level is inputted to a computing element 6 and synthesized at every same frequency, and the acoustic intensity is calculated at every frequency.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention is a method for measuring sound power level using sound intensity, in which a microphone pair in which two microphones are coupled with a small interval between them is used. The two microphones are installed vertically on the surface of a closed space surrounding the sound source to detect the sound pressure, and then the sound pressure of the sound source to be measured is calculated using the detected sound pressure and air density. [Prior art] Sound power level is a sound pressure level that is greatly affected by the environment in which the sound source to be measured is installed. Unlike , it represents the total energy of sound radiated from a sound source, and is likely to become widely used as an absolute evaluation measure of equipment noise.

The measurement of sound power level has conventionally been carried out by bringing a sound source to be measured into an anechoic chamber or a reverberation chamber and measuring the sound pressure level. However, it was not possible to accurately measure large machines or products installed on-site because they were being measured in an environment where measurement errors due to background noise and sound reflections were unavoidable.

However, as is well known, recently it has become possible to easily determine the sound intensity, and this has been used to determine the sound power level with relatively high accuracy even at sites with high background noise levels. This method will be explained below.

As shown in FIG. 3, the acoustic power W radiated from the sound source 1 to be measured is transmitted to the surface of a closed space surrounding the sound source 1 by using a microphone pair 2 in which two microphones are coupled with a small distance between them. Detect the sound pressure level at 3, and integrate the acoustic intensity ■ calculated from the relationship between the sound pressure level and the air density on the surface of the closed space, that is, W = ff5I - dA = ff =, In - d
A ・・・・・・・・・・・・・・・・・・(1) Actually, the area element dA of the surface of the closed space divided into many parts and the center position of this area cumulative are found. . The acoustic power can be found by summing the product with the acoustic intensity In perpendicular to the spatial surface in a form such as (Equation 21).

W=, riI i @ dAi ・・・・・・・・・・・・
・・・・・・・・・・・・・・・・・・・・・・・・
・・・・・・・・・(2)l暉1 Here, n is the number of divisions of the closed space surface. Also, if the sound power level is measured by dividing the surface of the closed space into N equal parts, then 10LOglo N ・・・・・・・・・・・・・・・
・・・・・・・・・・・・・・・・・・・・・・・・
...It is shown in (3). However, ■#: Fixed point acoustic intensity at each measurement point θν rent) IO = Basic sound intensity (1 pW/go) S: Surface area of the hemisphere surrounding the sound source (2πr2.r is the radius of the hemisphere) So: 1 Go.

Therefore, when measuring the acoustic power level under background noise generated from the background noise source 4 as shown in FIG. However, measurement errors due to background noise appear (2).

(As shown in Equation 31, this error is canceled out in order to sum up the sound intensity at each measurement point.

For this reason, sound power level measurement based on sound intensity has the advantage that measurement errors due to the influence of background noise can be reduced.

[Problem that the invention seeks to solve]

However, the conventional measuring method has several problems. That is, in the conventional measurement method, 10 to 40 points are selected as the number of measurement points, and the measurement time per measurement point is several tens of seconds. Therefore, it takes a considerable amount of time to complete measurements at all measurement points. However, the microphone pair 2, 2 used for measurement
a # 2b (Fig. 3+4), frequency analyzer 5, and computing unit 6 are expensive, so if you try to complete measurements in a short time by equipping a large number of these devices, the measurement equipment will become extremely expensive and you will suffer a lot of damage. Conventionally, measurements were taken one measurement point at a time. This has caused the following problems.

FIG. 5 shows a typical pattern of the relationship between the background noise level and the measurement elapsed time from the first measurement point to the Nth measurement point on the N-divided spatial surface.

In the figure, when there is no change in the background noise level during the measurement time, as in ial, measurement results with small errors can be easily obtained using the conventional measurement method, and as in ibl, when there is no change in the background noise level. If it only occurs for a short time, the measurement error can be reduced by increasing the number of measurement points. However, in the case of [cl], which most commonly appears as a time change in the background noise level, the background noise level differs from measurement point to measurement point, so the measurement error is calculated using the above (al,
(The problem arises that the measurement error increases because it cannot be canceled out like bl.) One way to solve this problem is to end the measurement while the fluctuation in the background noise level is small. However, the time change of this variation cannot be known in advance, and in cases where the sound source to be measured is a large machine or in the case of on-site measurement where workability is poor, microphone pair 2 (Fig. 3) should be used. Because it is not easy to move, it is difficult to universally solve the problem using this method.

An object of the present invention is to provide a method for measuring a sound power level from the sound intensity obtained using a pair of microphones and the area of the surface of a closed space where the microphones are installed. Regardless of the size, and even if the temporal change in background noise level is unknown in advance,
It is an object of the present invention to provide a measurement method that reduces measurement errors.

[Means for solving problems]

In order to achieve the above object, according to the present invention, a pair of microphones in which two microphones are coupled with a small interval is installed perpendicularly on the surface of a closed space surrounding a sound source to be measured, Each sound pressure is detected, and then the sound intensity of the sound source to be measured is calculated using each of the detected sound pressures and air density, and the sound power level is measured from this sound intensity and the surface area of the closed space. In the method, the closed space surrounding the sound source to be measured is a hemispherical space, and the microphone pair is installed at two appropriate locations on the surface of the hemispherical space that is symmetrical with respect to the vertical axis of the hemispherical space, and the microphone pair is set at two appropriate locations at these two locations simultaneously or It is assumed that the sound pressure is detected at each location in time series without substantially any time interval.

[Effect]

In this way, if a pair of microphones is installed at two appropriate locations on the surface of the hemispherical space that is symmetrical with respect to the vertical axis of the hemispherical space surrounding the sound source to be measured, and the sound intensity is determined simultaneously at these two locations, the sound intensity will be Since the background noise levels involved are substantially equal in magnitude and opposite in polarity, simultaneous measurements using such a symmetrical arrangement of microphone pairs are performed on the entire surface of the hemispheric space, and the above-mentioned (2) and (3) are carried out. When the sound power or sound power level was calculated according to the formula, the background noise component was canceled out at every two symmetrical locations, and the background noise component was almost completely eliminated as a whole. The sound power level of only the sound source to be measured can be obtained. Furthermore, since the background noise level usually does not change very quickly over time, instead of measuring at two symmetrical locations at the same time, it may be measured one location at a time and chronologically without any substantial time interval.

Measurement errors due to background noise are so small that they can be ignored in practice.

〔Example〕

FIG. 1 shows an embodiment of the sound power level measuring method according to the present invention. In the figure, 3a indicates a hemispherical space surrounding the sound source 1 to be measured, 3b is a vertical axis perpendicular to the bottom surface of this hemispherical space, and the hemispherical space surface is axially symmetrical around this vertical axis. A pair of microphones 2a and 2b is placed at two positions on the surface of the hemispherical space that are axially symmetrical with respect to this vertical axis, and the sound intensity at each position is determined simultaneously.

Such simultaneous measurements at two symmetrical locations are performed at all dividing points set on the hemispherical space surface.

The sound pressure levels measured at two symmetrical locations in this way are each measured by a frequency analyzer 5a.

5b, where the sound pressure level is determined for each frequency included in the sound pressure level, and this sound pressure level is manually input to the calculator 6, where the same frequency is synthesized and the sound intensity is calculated for each frequency. Ru. Therefore, when calculating the sound power level, the error due to the background noise has already been canceled out and reduced.

FIG. 2 shows a comparison of measurement accuracy between the conventional measurement method and the measurement method according to the present invention. Figure 5 shows the sound intensity found at the dividing points when the arc of the circumference of one section perpendicular to the vertical axis of the hemispherical space surrounding the sound source to be measured is divided into 10 parts, and the sound intensity near the sound source to be measured. The relationship with measured background noise is shown.

In the figure, (al indicates the magnitude of the background noise level at the time of measurement at each division point, and b indicates the magnitude of the background noise level at the time of measurement at each division point. This shows that the noise level is different. (bl shows the sound intensity determined by the conventional measurement method under such background noise. Curve C shows the sound intensity when the background noise does not change over time like straight line A.) In this case, the measurement error based on background noise is extremely small.On the other hand, curve 2 is for the case where the background noise changes like a curved line, and the background noise level changes at each measurement point. Therefore, when the sound pressure levels at two symmetrical points are combined, the background noise level cannot be canceled out and an error remains. tel indicates the sound intensity when measurements at two symmetrical points are performed simultaneously based on the present invention. In this case, as shown in curve E, the background noise level is almost completely canceled out, and there is no substantial difference from curve II.

In addition, in FIG. 1, if a simple switch is used in front of the frequency analyzers 5a and 5b, it becomes possible to measure two symmetrical locations in a short time with one frequency analyzer.

Similarly, measurement errors are reduced.

〔Effect of the invention〕

As described above, according to the present invention, in order to obtain the acoustic power level of a sound source to be measured, the microphone pair is arranged symmetrically with respect to the vertical axis of the hemisphere in order to obtain the sound intensity on the surface of the hemispherical space surrounding the sound source to be measured. (1) Large equipment that cannot be brought into an anechoic chamber. The sound power level can be easily and reliably measured at the site where this equipment is installed.

(Since high measurement accuracy can be obtained without simultaneously installing microphone pairs at all measurement positions on the spatial surface of the 21 hemispheres, the measurement equipment becomes inexpensive.

(3) In addition to temporal changes in the background noise level, - In an environment where the direction of the flow of background noise is changing, that is, when the position of the background noise source is moving or the number of background noise sources is changing. Accurate sound power level measurement is possible even when

There are many effects such as.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram of a sound power level measuring method according to an embodiment of the present invention, FIG. 2 is a diagram showing the effect of reducing measurement errors by the method of the present invention in comparison with a conventional method, and FIG. FIG. 2 is an explanatory diagram showing a sound power level measuring method using sound intensity, which is the object of the present invention. FIG. 4 is an explanatory diagram showing that the method of FIG. 3 can reduce measurement errors due to background noise, and FIG. 5 is a diagram showing an example of a temporal change in the background noise level. Wavenumber analyzer 56...Sound intensity calculator. Fig. 1 1≧ '-τR'12i! ” Figure 3 Figure 4 (G) (b) (C) Figure 5

Claims (1)

[Claims] 1) A microphone pair consisting of two microphones coupled with a small distance between them is installed vertically on the surface of a closed space surrounding the sound source to be measured, and the sound pressure is detected by each of the two microphones, and then this detected sound pressure is In this method, the sound intensity of the sound source to be measured is calculated using each sound pressure and air density, and the sound power level is measured from this sound intensity and the surface area of the closed space. The microphone pair is installed at two appropriate locations on the surface of a hemispherical space that is a space and is symmetrical with respect to the vertical axis of the hemispherical space, and the microphone pairs are placed at two locations simultaneously or at one location at a time substantially apart from each other in time. A sound power level measuring method characterized by detecting sound pressure in time series without any noise.