GB1558641A

GB1558641A - Acoustic measuring system

Info

Publication number: GB1558641A
Application number: GB1146777A
Authority: GB
Original assignee: Telediffusion de France ets Public de Diffusion
Current assignee: Telediffusion de France ets Public de Diffusion
Priority date: 1976-03-31
Filing date: 1977-03-17
Publication date: 1980-01-09
Also published as: DE2711792A1; FR2346696A1; ES457318A1; DE2711792C3; IT1091944B; DE2711792B2; FR2346696B1

Description

(54) AN ACOUSTIC MEASURING SYSTEM (71) We, TELEDIFFUSION DE FRANCE, a Public Corporation, of 10, rue d'Oradour-sur-Glane, 75732, Paris, France, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- The invention relates to an acoustic measuring system for evaluating the acoustic power of a noise source and the absorption factor of a material in a reverberation chamber.

It is known that the acoustic pressure p of a reverberation field and the reverberation time T measured in a reverberation chamber are defined by the following relations: T=af(a) and p=bWg(a) where a and b are coefficients depending on the dimensions of the reverberation chamber; f (r) and , (a) are simple functions of the sound-absorption factor a of the reverberation chamber and W is the power of the noise source.

If, for example, a measurement is made of the reverberation time T and the acoustic pressure p from a microphone positioned in the reverberation chamber, the acoustic power W of the noise source can easily be deduced from the preceding relations. This example of measurement is recommended in French Standard NF S 31 024 entitled "A method of evaluation adapted to special reverberation chambers". In this method, a number of measurements, with different microphone positions, are required for accurately evaluating the average acoustic power of the noise source. In that case, very lengthy measurements are required for evaluating a reverberation time. As a result, the microphone positions in the direct field of the noise source are inaccurate and substantially different for similar measurements with (a) a standard noise source and (b) a noise source having a power to be determined.At present the measurements are made using a microphone moving in a circle in a substantially horizontal plane. In such systems, the direct sound fields coming from the noise sources and received by the microphone are not constant during all the measurements.

Similarly, if the reverberation time T is measured in a reverberation chamber of given dimensions, the absorption factor a of a material is easily deduced from the first aforementioned relation, if the absorption factor of the chamber without the material is known and if the reverberation time of the room is measured when the material is present. This method is at present used, in accordance with French Standard NF S 31 003. Like the other methods, it requires very lengthy measurements and numerous microphone positions, which are usually inaccurately known.

Other acoustic measurements are based on the level of acoustic pressure received by a microphone.

Measurements of this kind, in which the acoustic pressure of a noise source are evaluated, are described e.g. in the Article by C. E. Ebbing and G. C. Maling in "U.S.

Journal of the Acoustic Society of America", Volume 54, No. 4, October 1973, pages 935-949. This article relates to a qualification procedure for accurately determining the average acoustic pressure in a reverberation chamber. During a first or a "callibration" phase, the average acoustic pressure level is determined, using a single microphone placed in a number of positions in the reverberation field of the chamber, which contains a fixed reference noise source such as a loudspeaker. Next, during a second measuring phase proper, the average acoustic level is determined in the same manner as in the first phase, using a stationary noise source having an acoustic power to be measured, instead of the reference niose source.

A method of measuring the average acoustic absorption coefficient of a reverberation chamber before and after the insertion of a sound-absorbing material is described e.g. in U.S. Patent 2 356 478. In this method, a sound energy density measuring device is placed, during each measurement, near a source of pulsed sound waves and at given distances from the source. These distances are difficult to determine since the acoustic centre of the source is always inaccurately known. In addition the method does not allow for the directivity factor of the source in the chosen direction of measurement. It is also necessary to check that the noise source is omnidirectional at all frequencies.

As in the preceding Article, the average sound pressure level is measured by means of a single microphone in a number of positions using a stationary noise source.

In view of the very long measuring times required in prior art methods, the user often has to be content with a relatively small number of measurements. This results in (a) measuring errors due to inaccurately known microphone positions in the direct sound fields of the noise sources and (b) incorrect evaluation of the average values of the average acoustic power of a noise source and of the average absorption factor of a material.

An object of the invention is to provide an acoustic measurement system for making measurements to the aforementioned French Standard but with much shorter measuring times, while giving very accurate average values for the power and absorption factor.

The measurements made by the system according to the invention are based entirely on the second aforementioned relation and, like the prior art measurements, require a number of microphone positions in the reverberation field for various positions of the noise sources. More specifically, the system is adapted to measure differences in the acoustic pressure levels from a transmission microphone positioned near a noise source and from a receiving microphone placed successively at different positions in the reverberation field of a reverberation room, the positioning being accurate with respect to the direct acoustic field of the noise source.

In this method, use may be made of an apparatus for measuring sound absorption and insulation coefficients and described in British Patent Specification 1 423 756. This apparatus is used for comparing the sound pressure (or intensity) levels of the transmission microphone and the receiving microphone, by evaluating the differences between the levels. The average pressure of the reverberated acoustic field is then measured directly. If required, the pressure of the direct acoustic field of a noise source can be indicated by gain controls, calibrated in decibels and tens of decibels.

According to the present invention, there is provided an acoustic measuring system for use in a reverberation chamber to evaluate the acoustic power of a noise source or the acoustic absorption factor of a material, the system comprising an acoustic chamber noise source having a plurality of loudspeakers, a first microphone for positioning in the reverberation field of the reverberation chamber to detect the acoustic pressure level in the reverberation field, a second microphone secured to one of the loudspeakers of the acoustic chamber to detect the acoustic pressure level of direct sound emitted by the acoustic chamber noise source, a bar whose ends are secured to the acoustic chamber and to the first microphone respectively, mechanical means for moving the bar in any desired manner around a stationary point and simultaneously moving the first microphone and the acoustic chamber carrying the second microphone, during which movements the distance of the acoustic chamber from the first microphone and the orientation of the acoustic chamber with respect to the first microphone remain constant, and processing means for processing the output electrical signals of the first and second microphones to evaluate the acoustic power of a noise source under test or the acoustic absorption factor of material under test.

In order to measure the acoustic power of a noise source, the processing means may evaluate the acoustic pressure level from the first microphone positioned in the reverberation field of the reverberation chamber, first using a noise source having an acoustic power to be measured and positioned near the acoustic chamber and subsequently using a noise source of given power inserted in the acoustic chamber, and, when the acoustic pressure level from the first microphone with the noise source of given power reaches the level with the noise source to be measured, the calibration of the noise source of given power directly indicates the acoustic power of the noise source to be measured.

In a first manner of measuring the absorption factor of a material, the processing means may evaluate the differences between the acoustic pressure level from the second microphone, which is secured to the acoustic chamber surrounding a noise source of given power, and the first microphone, which is positioned in the reverberation field of a reverberation chamber, first in the absence and subsequently in the presence of a material having an absorption factor which is directly deduced from the differences in the acoustic pressure levels evaluated with and without the material in the reverberation chamber.

In a second manner of measuring the absorption factor of a material, the processing means may evaluate the phase shifts between the sound signals from the transmission microphone and the receiving microphone, the signals being generated by a noise source having a modulated sound power. The absorption factor a of the material is then determined, since it is proportional to the average value of the phase shifts.

Standard noise source having a given sound power and used according to the invention may inter alia be that described in British Patent Specification No. 1472713.

A calibration noise source of the aforementioned kind comprises inter alia a weighting network proving standardised weighting A in the reverberation region and in a place giving average reverberation.

The weighting A is obtained from the attenuation along an "international standardization" curve A, which is given in "Recommendations for Sound-Meters" publication No. 123 by the International Electronics Commission.

Thus, during all the measurements made by an acoustic measuring system embodying the invention, the variation in acoustic pressure indicated by the apparatus for measuring the sound absorption and insulation coefficients accurately represents the variation in the intensity of the reverberated sound, since the intensity of the direct sound remains constant.

Furthermore, the second microphone, which is secured to the acoustic chamber, is used to indicate the sound power level of a noise source.

In order that the invention may be readily understood, an embodiment thereof will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a diagram of an acoustic measuring system embodying the invention for simultaneously moving a receiving microphone and the reference noise source or transmission microphone; and Figure 2 is a simplified block diagram of processing apparatus for processing the output signals of the receiving and transmission microphones so as to measure the phase shifts between acoustic signals.

Figure 1 diagrammatically shows the components, inter alia the mechanical components, of a system embodying the invention. A telescopic foot I fixed on the floor of the reverberation chamber receives a sliding vertical rod 2, the free end of which is secured to a universal joint 5.

Joint 5 includes a U-shaped component 51, the ends of which rotate in cooperation with two opposite apices of a frame-like component 52 having substantially rectangular inner and outer contours. A cylindrical shaft 53 is secured between the other two opposite apices of member 52.

The free end of rod 2 is secured near the centre of the base of component 51.

A bar 3 is rotatably mounted on shaft 53, which extends with friction through a transverse aperture in bar 3. Thus, the universal joint 5 enables the central part of shaft 53 to move in any direction in the three-dimensional space in the reverberation chamber around a pivot point 54. Point 54 also divides bar 3 into two arms 20 and 30, one on each side of joint 5. Arm 20 has a constant length x2. Arm 30 has a length x3 which can be varied by means of a rod 4 slidably mounted in the body of bar 3.

As a rule, the lenght x3 of arm 30 is always greater than the length x2 of arm 20.

A standard noise source in an acoustic chamber 6, as described e.g. in British Patent No. 1472713, is secured to the end of arm 20. A transmission microphone 21 is centrally and rigidly secured, by means of a spider 8 secured to chamber 6, in front of one of the loudspeakers 7 of the chamber 6.

A receiving microphone 31 is secured to the free end of the rod 4 of arm 30. A weight 9 sliding on rod 4 of arm 30 is positioned so as horizontally to balance the gravitational forces applied on each side of the pivot point 54 of bar 3 in joint 5, for selected distances x2 and xs and in the general case where the microphone 31 is much lighter than the assembly comprising the acoustic chamber 6 and the transmission microphone 21.

An electric motor 10 drives a crank 11 in rotation around an imaginary axis extending through the pivot point 54 of bar 3. One end of crank 11 is secured to the driving shaft of motor 10. The other end of crank 11 rotates in cooperation with the bore of a small hollow cylinder 12 projecting from one surface of chamber 6. Motor 10 is mounted at the top end of a vertical rod 13. Rod 13 can slide in a telescopic foot 14 fixed on the floor of the reverberation chamber and similar to foot 1, so that the acoustic chamber 6 of the standard noise source can be positioned e.g. near a noise source having a power which is to be determined.

Wires 15 connected to the inputs of a general power supply unit 16 directly deliver the voltages required for operating the standard noise source. Two pairs of wires 22, 32 connect the transmission microphone 21 and the receiving microphone 31 respectively to the inputs of a power attenuator 23 and the receiving circuit of an acoustic comparator 17. The output of attenuator 23 is connected to the input of the transmission circuit of comparator 17.

As a result of the use of the universal joint 5, the rigid assembly comprising chamber 6 and microphone 31 does not rotate on itself when motor 10 rotates the crank 11.

Consequently, wires 15, 22 and 32 hang freely without the risk of winding around the components held by bar 3.

In a preferred embodiment of the invention, the processing apparatus or acoustic comparator 17 is similar to that described in the aforementioned British Patent Specification No. 1472713. The standard noise source, the variation in the power of which has been calibrated in an anechoic chamber, generates a pressure level at microphone 21 which is higher than the pressure level representing the powel level of the calibrated source. Consequently, attenuator 23 has to be suitably inserted in the transmission circuit of comparator 17, e.g. at its input as shown in Figure 1. Thus, the pressure level from microphone 21 is reduced to the pressure level representing the power level of the standard noise source.

The acoustic power of a given noise source is measured in two phases.

In the first phase, the standard noise source is disconnected from the supply unit 16. The acoustic chamber 6 is positioned in a reverberation chamber, so as to describe a circle in the immediate neighbourhood of the noise source under test, and so that the receiving microphone 31 describes another cycle, having the largest possible radius, in the reverberation field of the reverberation chamber. In the receiving circuit of comparator 17, which operates as a sonometer, microphone 31 generates a signal having an amplitude proportional to the acoustic pressure level in the reverberation field. Comparator 17 indicated the variation in the acoustic pressure level.

In the second phase, the noise source under test is disconnected and the standard noise source is energised by unit 16. The power of the standard source is increased to a value corresponding to an acoustic pressure variation identical with that indicated by the comparator during the first phase. A single measurement of the acoustic pressure level, picked up by microphone 21 in the transmission circuit of comparator 17, directly indicates the power of the source under test. This value is indicated e.g. by gain controls calibrated in decibels and inserted in the transmission circuit.

In a first embodiment of the invention the absorption factor of a material is measured in two phases. In both phases the standard noise source is connected to the supply unit 16. In the first phase, the average difference between the acoustic pressure levels from the two microphones 21 and 31 is used for calculating the average absorption factor of the reverberation room without the material. In the second phase, the material is inserted in the reverberation chamber and the average absorption factor of the chamber is evaluated when the material is present. The absorption factor a of the material under investigation is found from the average difference between the acoustic pressure levels indicated by the comparator during the two phases.

In a second embodiment of the invention, the absorption factor of a material is measured by means of a comparator and a standard noise source which are substantially different from those described previously.

Referring to Figure 2, a comparator for measuring the phase shift of two acoustic signals generated by microphones 21 and 31 respectively comprises, firstly the following known elements.

A preamplifier 24 and two gain controls 25, 26 calibrated in tens of decibels and in decibel units respectively in the transmission circuit II and behind the transmission microphone 21, which is connected to the input of circuit II by conducting wires 22; A preamplifier 34 in the receiving circuit III downstream of microphone 31, which is connected to the input of circuit III by conducting wires 32; and Respective automatic gain control amplifiers 27, 37 and band-pass filters 28, 38 in the two circuits, the filters being insertable into each circuit by switching as required.

In addition to the known elements, these circuits comprise demodulators 29, 39 downstream of filters 28, 38 respectively, the demodulator outputs being interconnected at the inputs of a phasemeter 40.

In the second embodiment, the acoustic power of the standard noise source is modulated e.g. at a frequency of 1 to 2 Hz with a modulation depth of 40 to 50 /n. There is consequent modulation of the acoustic signals which have amplitudes proportional to the acoustic pressures picked up by microphones 21 and 31. The modulation envelope of the acoustic signals generated by microphone 31 is phase-shifted with respect to the envelope of the acoustic signals generated by microphone 21. The signal demodulators 29, 39 are used in conjunction with phase-meter 40 to indicate the phase shift between the demodulated acoustic signals coming from circuits II and III respectively, the shift being proportional to the absorption factor of the reverberation chamber.

Two average values of the phase shifts of the demodulated signals are obtained in similar manner to the first embodiment, i.e.

first without and then with the material in the reverberation chamber, in order to calculate the absorption factor of the material under investigation.

By way of example, in the case of the various kinds of measurements described hereinbefore, the invention provides a bar 3 comprising an arm 20 having a constant length x2 of 0.6 m and a telescopic arm 30 having a length x3 varying between 1.2 m and 2.4 m. The length of the crank is adjustable between 0.3 and 0.6 m. Under these conditions, the acoustic chamber 6 and the receiving microphone 31 simultaneously describe circles having a radius between 0.3 and 0.6 m and between 0.6 and 2.4 m respectively.

The value of x3 is chosen in dependence on the dimensions of the reverberation chamber. Usually, microphone 31 is always positioned in front of the walls of the room at a distance at least equal to a quarter of the wavelength of the lowest frequency of the noise coming from the noise sources.

Preferably, the average values of the powers and factors under investigation are accurately obtained at each complete revolution of crank 11. In the case, for example, of a parallelepipedal reverberation chamber, the evaluations will be more accurate if the measurements are made when bar 3 is positioned so that one of the major diagonals of the parallelepipedal chamber coincides with the common axis of the opposite cones whose apices coincide with the pivot point 54, the cone envelopes being described by arms 20 and 30.

An advantage of the acoustic measuring system embodying the invention, as shown in Figure 1, is that it is transportable, can easily be dismantled, and is relatively cheap.

The system can be used inter alia for measuring the power of sound sources and absorption factors of materials used in dwellings.

WHAT WE CLAIM IS: 1. An acoustic measuring system for use in a reverberation chamber to evaluate the acoustic power of a noise source or the acoustic absorption factor of a material, the system comprising an acoustic chamber noise source having a plurality of loudspeakers, a first microphone for positioning in the reverberation field of the reverberation chamber to detect the acoustic pressure level in the reverberation field, a second microphone secured to one of the loudspeakers of the acoustic chamber to detect the acoustic pressure level of direct sound emitted by the acoustic chamber noise source, a bar whose ends are secured to the acoustic chamber and to the first microphone respectively, mechanical means for moving the bar, in any desired manner around a stationary point and simultaneously moving the first microphone and the acoustic chamber carrying the second microphone, during which movements the distance of the acoustic chamber from the first microphone and the orientation of the acoustic chamber with respect to the first microphone remain constant, and processing means for processing the output electrical signals of the first and second microphones to evaluate the acoustic power of a noise source under test or the acoustic absorption factor of material under test.

2. An acoustic measuring system according to claim 1, in which the processing means evaluates the acoustic pressure level from the first microphone positioned in the reverberation field of the reverberation chamber, first using a noise source having an acoustic power to be measured and positioned near the acoustic chamber and subsequently using a noise source of given power inserted in the acoustic chamber, and, when the acoustic pressure level from the first microphone with the noise source of given power reaches the level with the noise source to be measured, the calibration of the noise source of given power directly indicates the acoustic power of the noise source to be measured.

3. An acoustic measuring system according to claim 1, in which the processing means evaluates the differences between the acoustic pressure level from the second microphone, which is secured to the acoustic chamber surrounding a noise source of given power, and the first microphone which is positioned in the reverberation field of the reverberation chamber, first in the absence and subsequently in the presence of a material having an absorption factor which is directly deduced from the differences in the acoustic pressure levels evaluated with and without the material in the reverberation chamber.

4. An acoustic measuring system according to claim I, in which the processing means evaluates the phase shifts between the acoustic signals from the second microphone, which is secured to the acoustic chamber surrounding a noise source having modulated power, and the first microphone, which is positioned in the reverberation field of a reverberation chamber, first in the absence and subsequently in the presence of a material having an absorption factor which is directly deduced from the phase shifts between the acoustic signals, evaluated with and without the material in the reverberation room.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. the demodulated signals are obtained in similar manner to the first embodiment, i.e. first without and then with the material in the reverberation chamber, in order to calculate the absorption factor of the material under investigation. By way of example, in the case of the various kinds of measurements described hereinbefore, the invention provides a bar 3 comprising an arm 20 having a constant length x2 of 0.6 m and a telescopic arm 30 having a length x3 varying between 1.2 m and 2.4 m. The length of the crank is adjustable between 0.3 and 0.6 m. Under these conditions, the acoustic chamber 6 and the receiving microphone 31 simultaneously describe circles having a radius between 0.3 and 0.6 m and between 0.6 and 2.4 m respectively. The value of x3 is chosen in dependence on the dimensions of the reverberation chamber. Usually, microphone 31 is always positioned in front of the walls of the room at a distance at least equal to a quarter of the wavelength of the lowest frequency of the noise coming from the noise sources. Preferably, the average values of the powers and factors under investigation are accurately obtained at each complete revolution of crank 11. In the case, for example, of a parallelepipedal reverberation chamber, the evaluations will be more accurate if the measurements are made when bar 3 is positioned so that one of the major diagonals of the parallelepipedal chamber coincides with the common axis of the opposite cones whose apices coincide with the pivot point 54, the cone envelopes being described by arms 20 and 30. An advantage of the acoustic measuring system embodying the invention, as shown in Figure 1, is that it is transportable, can easily be dismantled, and is relatively cheap. The system can be used inter alia for measuring the power of sound sources and absorption factors of materials used in dwellings. WHAT WE CLAIM IS:

1. An acoustic measuring system for use in a reverberation chamber to evaluate the acoustic power of a noise source or the acoustic absorption factor of a material, the system comprising an acoustic chamber noise source having a plurality of loudspeakers, a first microphone for positioning in the reverberation field of the reverberation chamber to detect the acoustic pressure level in the reverberation field, a second microphone secured to one of the loudspeakers of the acoustic chamber to detect the acoustic pressure level of direct sound emitted by the acoustic chamber noise source, a bar whose ends are secured to the acoustic chamber and to the first microphone respectively, mechanical means for moving the bar, in any desired manner around a stationary point and simultaneously moving the first microphone and the acoustic chamber carrying the second microphone, during which movements the distance of the acoustic chamber from the first microphone and the orientation of the acoustic chamber with respect to the first microphone remain constant, and processing means for processing the output electrical signals of the first and second microphones to evaluate the acoustic power of a noise source under test or the acoustic absorption factor of material under test.

5. An acoustic measuring system

according to claim 4, in which the processing means comprises two demodulators of the acoustic signals, connected to the output of the transmission channel and the receiving channel respectively, and a phasemeter connected between the demodulators to indicate the phase shifts between the acoustic signals from the first and second microphones.

6. An acoustic measuring system according to any of claims 1 to 5, in which the mechanical means comprises a rod sliding in a telescopic foot and secured to a universal joint having a pivot point identical with the stationary point of the bar.

7. An acoustic measuring system according to claim 1 or claim 6, in which the bar comprises a telescopic rod, one end of which is secured to the first microphone, so that the distance between the fixed point and the first microphone is adjustable and can be considerably greater than the distance between the fixed point and the acoustic chamber.

8. An acoustic measuring system according to any one of claims I to 7 comprising a motor driving a shaft which cooperates in rotation with one end of the bar, via a crank, and a rod sliding in a telescopic foot and bearing the motor at its end, so that the rotating shaft extends towards the fixed point of the bar, and the motion of the bar describes two opposite cones, the apices of which are at the fixed point, without the bar rotating on itself.

9. An acoustic measuring system substantially as hereinbefore described with reference to the accompanying drawings.