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Simple sound power measurements
SIMPLE SOUND POWER MEASUREMENTS
JAMES Morn and W. R. STEVENS James Mob- & Associates, ChipperfieM, Herts, WD4 9JJ (Great BritahO
(Received: 14 June, 1973)
SUMMA R Y
A simple method of measuring soundpower is described. Noise is added from a second calibrated source until the room sound pressure lee'el is increased by 3 d B. The noise output of the device is then equal to the added power. A simple loudspeaker, modified to reduce the effect of the environment on the sound power output, is used as the calibrated source of sound power. Checks show that the sound power readings are substantially independent of the room in which they are made.
INTRODUCTION
When assessing the ~noisiness' of an appliance it has been customary to quote the sound pressure level, measured using the 'A' weighted scale of a soufid level meter. This implies that the sound pressure level is a unique aspect of the machine's operation and independent of the environment, although this is obviously not true. The sound pressure produced in any enclosure by a given amount of sound power is a function of the point in the room at which the sound pressure is measured, and the size and acoustic treatment of the room. Thus sound pressure level data should really be accompanied by acoustic data on the room in which they are measured. This, however, is very rarely done. A sound pressure level quotation without data on the acoustic environment is substantially meaningless; indeed it lends itself to manipulation by the unscrupulous advertiser, sound pressure levels being measured under the most advantageous environmental conditions and then being quoted as applying in every other situation. 15 Applied Acoustics (7) (1974)--© Applied Science Publishers Ltd, England, 1974 Printed in Great Britain
16
JAMES /VlOIR, W. R. STEVENS
The dependence on the environment has been obvious from basic theory for many years and it is slowly becoming appreciated by industry that it is the sound power output of the appliance that is the fundamental indication of the noise output. The sound power output of a machine is substantially independent of the environment in which it is placed, although the location of the machine in a corner may increase the loudness of the noise. The advantages of specifying sound power, rather than sound pressure, have been appreciated in some industries for many years. Thus the heating and ventilation engineer almost always expresses the sound output of a fan or air handling unit in terms of sound power, generally in dB with respect to 10- 12 watt. In the absence of any simple method of sound power measurement, the obvious alternative approach of measuring the sound pressure level in a standardised environment has been used, but at present there is standardisation of a typical domestic environment only within the gas industry. Achieving a standard environment representative of a typical domestic living-room or kitchen has been found to require the expenditure of something in the region of £2,000 (1972 costs) and every worker in the field must reproduce the acoustic aspects of this environment if his data are to be compared with those obtained by any other worker. In spite of the basic advantages of the sound power emission concept, any protagonist of sound power specification faces two difficulties:
O) The hearing system is sound
pressure sensitive, so at the end of any chain of manipulation of sound power data, the sound power remaining must be translated into sound pressure data in order to determine the human reaction to the available sound power. (2) There is an almost complete absence of instrumentation for the direct measurement of sound power. Currently available methods of measuring the sound power output of any device are indirect techniques, based on measurements of the sound pressure level, the geometry of the situation and the acoustic properties of the environment in which the device is measured.
The protagonists of sound pressure measurement have very similar problems, for the measured data on sound pressure level apply only to the particular room in which the measurement has been made. When it is necessary to deduce the sound pressure level the machine would produce in any other room, an arithmetical manipulation is required that is no less tedious than is necessary when sound power level is measured. This problem is usually avoided by ignoring its presence. It would seem that the general preference for sound pressure level measurements is really based on the relative simplicity of the available sound pressure level measurement techniques as compared with those currently available for measuring sound power level.
SIMPLE SOUND POWER MEASUREMENTS
• 17
SOUND POWER MEASUREMENTS
The current methods of measuring sound power have been in use for many years and are well understood, so there is no point in a detailed discussion of them here. In outline they consist of making between ten and twenty measurements of the sound pressure level at a fixed distance from the source, either in the open air or in an environment of known acoustic characteristics. The sound pressure level readings are averaged to obtain a value of sound pressure that is assumed to exist over a sphere or hemisphere having a radius equal to the source/microphone spacing. Interested readers can consult BS4196 or, for example, Section 17, Handbook of Noise Control, Harris, McGraw-Hill. The purpose of the present contribution is to describe a method of measuring sound power that is surprisingly uncomplicated and accurate and uses only the simple sound level meter as the indicating device. In summary, the procedure consists of adding to the sound power output of the device to be measured, an equal amount of sound power from a calibrated source, the state of power equality being shown by using a standard sound level meter to indicate when the sound pressure level has been increased by 3 dB. The added sound power then equals the machine sound power and the value is read, either from a calibration chart, or directly from a meter reading the electrical input of the calibrated noise source. Exactly the same procedure can be used for measuring the octave band sound power levels of any device. The octave band spectrum and level of the noise source being known, the octave band spectrum of the unknown noise can be determined by using an octave band analyser in the circuit of the sound level meter and noting the noise source output that produces a 3 dB increase in the octave band sound pressure level of the machine. If the calibration noise source and the machine being measured are close together, the standing wave system characteristic of the enclosure is similarly excited by both sources of noise and, as will be shown, the enclosure has almost no effect upon the measured value of sound power. There appears to be little need to make several determinations of power output using different microphone positions, although this might be advisable if the machine spectrum included a particularly prominent pure tone or if the source were highly directive. The additional sound power is provided by a loudspeaker placed adjacent to the appliance under test and supplied by a shaped noise signal from a separate amplifier. In so far as sound powers are being summed, the frequency spectrum of the added power is not of great significance, but there are some minor advantages in having the spectrum of the added noise power generally similar to that of the devices to be measured. The immediate application was the measurement' of the noise emission of typical domestic appliances and, as a guide to shaping the spectrum of the loudspeaker source, the spectrum of a number of typical domestic appliances was checked.
JAMES MOIR, W. R. STEVENS
18
These data were supported by examining published data on the spectra of other domestic appliances. It is found that typical domestic appliances have normalised sound pressure spectra falling within the broad limits shown in Fig. l, so the spectrum of the noise power generator was shaped to the median of the group shown. In fact, the frequency
~3dB Extremes found in : :'" f r~:tue~..y spectrum Ideol noise
o,~o~,,a.ces\ box~..."
IOHz
,/
/
."
-.
,/"
.."
1
".
1
iO0 Hz
°~
"'X. ~
IkHz
"-~
"~
"'"°
I0 kHz
Frequency (Hz)
Fig. 1. Graph showing the spectrum variations of a cross-section of domestic appliances.
response characteristics of the loudspeaker used were fortuitously such that the desired spectral response was obtained from a driving source having a white noise pressure characteristic with only a small high frequency cut off. It is obviously necessary to know the sound power radiated by the 'standard source' and to know that the source is stable and, indeed, this places one limit on the accuracy of the method. The better examples of the loudspeaker used by hi-fi enthusiasts have been developed to have a high standard of performance in respect of frequency response, amplitude linearity and long-term stability. The requirements for this purpose far exceed those for a speaker employed as a relatively low power source over the limited frequency band of interest in noise measurement. Several of the better examples of l0 in and 12 in diameter speakers were tested for response and linearity and all were found to be more than adequate. Modified* * Provisionally patented.
SIMPLE SOUND POWER MEASUREMENTS
19
to make the sound output less dependent on their position in the room, the source speakers were then calibrated in terms of sound power output as a function of drive power, by the standard technique of measuring the sound pressure levels over a surface of known radius, the calibration being carried out in a large open field. As a first check, the sound pressures were measured at two different radii and the power of the two radii compared. As a second check the sound power output was compared with that of a standard I L G source. In the experimental model, a calibration chart was prepared, relating sound power and drive current, but there is no reason why the drive power meter should not be directly calibrated in terms of radiated sound power.
MEASURING TECHNIQUE In use for determining the sound power output of an appliance, the loudspeaker noise source is set up near the appliance and the sound pressure level read off a standard sound level meter, the instrument being placed at some convenient p o i n t - usually in the room's reverberant field---equidistant from the machine and the sound power sources. The distance between the appliance and the sound level meter is not of any great significance providing the enclosure is moderately reverberant, but it should be not less than about 2 m. The acoustic conditions prevailing in the average laboratory, lacking carpet and curtains, appear perfectly acceptable. The sound p r e s s u r e level produced by the appliance at the chosen point is noted and the loudspeaker drive power contrbi advanced until the sound pressure level is increased by 3 dB. The sound power output of the appliance is then read directly off the calibration chart. In practice, the only difficulty that has arisen is due to the unsteadiness of the noise emission of the average appliance.
PROVING PROCEDURE As a check of the effect of the room characteristics on the measured sot~nd power level, a typical domestic vacuum cleaner (without its nozzle and brush) was measured under five different conditions. Outline data on the room and furnishings are given in Table 1. TABLE I Sound power (d B)
Room
1. 2. 3. 4. 5.
Open air--two acre field. Well furnished lounge 20 ft x 14 ft x 8 ft high. Typical laboratory 20 ft x 12 ft. No carpet. Laboratory/office 14 ft × 12 ft. Part carpeted. Small office 13 ft x 9 ft. Carpet and curtains.
84 83.9 84.4 84.1 83.6
20
JAMES MOIR, W. R. STEVENS
It will be seen that the variations in measured sound power are not significant even though the acoustic characteristics of the environment varied over a much wider range than would be normally encountered. The first unit was intended for use in a large laboratory where it i.s impossible to have the appliance to be measured in one standard position, or to have it well away from similar appliances when measuring the sound power output. The earlier comparison had been carried out in a series of small rooms having dimensions of the order of 6 m x 4 m, but to obtain confirmation under the conditions in which the sound power meter would be used, a second series of tests was carried out in a much larger laboratory. For this purpose the sound power output from a second domestic vacuum cleaner was measured under conditions that differed as widely as possible: (a) When it was standing on top of a washing machine. (b) On the floor between two similar washing machines 1.5 m apart. (c) At a point about 3 m away from a line of washing machines. (d) In a small, almost cubical, photometry room that included no sound absorbents except that due to some small timber benches. The sound power levels obtained were all within a band I dB wide. The only limitation that has been observed is the need to avoid having the appliance to be measured within about 3 ft of any wall.
SOUND POWER/SOUND PRESSURE TRANSLATION
It would seem appropriate to outline the procedure to be followed in translating the sound power emitted by an appliance into sound pressure at any point in the room. The sound pressure level in an enclosure is related to the sound power output of an omnidirectional source by the following relation: SPL=PWL+
101ogto(4__~r2 + 4 )
(1)
where PWL is the sound power of source, re: 10-12 watt and r = the radius at which the SPL is measured. R = Sct/(l - ct), a parameter describing the acoustic characteristics of the environment, S being the total surface area and ct the average absorption coefficient of that area. The first term inside the brackets l/4nr 2 is the direct component of the sound field, obeying the inverse square law, and only requires a tape measure for its evaluation. Evaluation of the second term 4/R necessitates a knowledge of the total sound absorption in the room and the total area of the room boundaries. Obtaining the room boundary area is simple but approximating the total absorption present is much more troublesome. It may be outlined in any of the following ways.
SIMPLE SOUND POWER MEASUREMENTS
21
(a) By calculation. The area of each boundary surface is determined and the absorption of that surface obtained by multiplying the area by the absorption coefficient appropriate to the material of the surface. These coefficients are tabulated in many textbooks. (For example see: Noise and Vibration Handbook, Harris, McGraw-Hill.) The absorptions of all the surfaces are summed and to this is added the absorption values appropriate to the room furnishings, chairs, desks, etc. (b) From measurements of the reverberation time. Where the necessary equipment is available, this is a more accurate method because it includes any absorption introduced by structural vibration. At the lower end of the frequency spectrum, the absorption as a result ofvibration of the basic building structure tends to be large, and even a major part of the total-absorption. (c) From measurements of sound pressure level developed by a source of known acoustic power. Equation (1) is used in reverse, all the terms on the right hand side being known actual measurements. When a source of known acoustic power and spectrum is available this is the obvious procedure to adopt. When a series of machines a're to be measured in one room, calculation or measurement of the value of R need only be carried out once. When this has been done either the sound power output of the machines or the sound pressure level plus the value of the parameter R, can be quoted. Except in those instances where the machine noise is largely radiated in one direction, the addition of the 'R' value of the measuring room to the sound pressure level quotation is an adequate specification of the noise output, in that it allows the sound pressure level that would be produced at any distance and in any other room to be calculated. When the sound power measuring technique described in this paper is used the sound pressure level is automatically obtained during the sound power measuring process. The value obtained is that at the preferred location, 2 m from the appliance being tested, but the sound pressure level so obtained is subject to the same limitations as any other sound pressure measurement. The standard sound level meter provides a choice of 'A', 'B' or 'C' weighted network, the purpose of these being well understood. The total sound power is more accurately measured by using the sound level meter on its 'C' weighting. However, it is well established that use of the 'A' weighted sound pressure values gives readings that are in better agreement with subjective estimates of the noisiness of machines. In measuring the noise power Output there is no very obvious reason why the sound level meter should not be used on its 'A' scale when establishing the equality of machine noise and added sound power. There is then the usual necessity of specifying that the power level being quoted is the 'A' weighted power to distinguish that value from the generally higher value obtained when the 'C' weighting network is used. This is an instance where there is some advantage in using a calibrated noise source having a frequency spectrum generally similar to that of the machine. It should be obvious that the sound power meter requires a separate
22
JAMES MOIR, W. R. STEVENS
calibration chart when used to measure ' A ' and ' C ' weighted powers. The sound power meter has proved its value in such diverse applications as the measurement of the sound power emitted by a lecturer, the on-site checking of the noise output of the ventilating fans in a lecture theatre, and the measurement of reverberation time of a room.
JAMES Morn and W. R. STEVENS James Mob- & Associates, ChipperfieM, Herts, WD4 9JJ (Great BritahO
(Received: 14 June, 1973)
SUMMA R Y
A simple method of measuring soundpower is described. Noise is added from a second calibrated source until the room sound pressure lee'el is increased by 3 d B. The noise output of the device is then equal to the added power. A simple loudspeaker, modified to reduce the effect of the environment on the sound power output, is used as the calibrated source of sound power. Checks show that the sound power readings are substantially independent of the room in which they are made.
INTRODUCTION
When assessing the ~noisiness' of an appliance it has been customary to quote the sound pressure level, measured using the 'A' weighted scale of a soufid level meter. This implies that the sound pressure level is a unique aspect of the machine's operation and independent of the environment, although this is obviously not true. The sound pressure produced in any enclosure by a given amount of sound power is a function of the point in the room at which the sound pressure is measured, and the size and acoustic treatment of the room. Thus sound pressure level data should really be accompanied by acoustic data on the room in which they are measured. This, however, is very rarely done. A sound pressure level quotation without data on the acoustic environment is substantially meaningless; indeed it lends itself to manipulation by the unscrupulous advertiser, sound pressure levels being measured under the most advantageous environmental conditions and then being quoted as applying in every other situation. 15 Applied Acoustics (7) (1974)--© Applied Science Publishers Ltd, England, 1974 Printed in Great Britain
16
JAMES /VlOIR, W. R. STEVENS
The dependence on the environment has been obvious from basic theory for many years and it is slowly becoming appreciated by industry that it is the sound power output of the appliance that is the fundamental indication of the noise output. The sound power output of a machine is substantially independent of the environment in which it is placed, although the location of the machine in a corner may increase the loudness of the noise. The advantages of specifying sound power, rather than sound pressure, have been appreciated in some industries for many years. Thus the heating and ventilation engineer almost always expresses the sound output of a fan or air handling unit in terms of sound power, generally in dB with respect to 10- 12 watt. In the absence of any simple method of sound power measurement, the obvious alternative approach of measuring the sound pressure level in a standardised environment has been used, but at present there is standardisation of a typical domestic environment only within the gas industry. Achieving a standard environment representative of a typical domestic living-room or kitchen has been found to require the expenditure of something in the region of £2,000 (1972 costs) and every worker in the field must reproduce the acoustic aspects of this environment if his data are to be compared with those obtained by any other worker. In spite of the basic advantages of the sound power emission concept, any protagonist of sound power specification faces two difficulties:
O) The hearing system is sound
pressure sensitive, so at the end of any chain of manipulation of sound power data, the sound power remaining must be translated into sound pressure data in order to determine the human reaction to the available sound power. (2) There is an almost complete absence of instrumentation for the direct measurement of sound power. Currently available methods of measuring the sound power output of any device are indirect techniques, based on measurements of the sound pressure level, the geometry of the situation and the acoustic properties of the environment in which the device is measured.
The protagonists of sound pressure measurement have very similar problems, for the measured data on sound pressure level apply only to the particular room in which the measurement has been made. When it is necessary to deduce the sound pressure level the machine would produce in any other room, an arithmetical manipulation is required that is no less tedious than is necessary when sound power level is measured. This problem is usually avoided by ignoring its presence. It would seem that the general preference for sound pressure level measurements is really based on the relative simplicity of the available sound pressure level measurement techniques as compared with those currently available for measuring sound power level.
SIMPLE SOUND POWER MEASUREMENTS
• 17
SOUND POWER MEASUREMENTS
The current methods of measuring sound power have been in use for many years and are well understood, so there is no point in a detailed discussion of them here. In outline they consist of making between ten and twenty measurements of the sound pressure level at a fixed distance from the source, either in the open air or in an environment of known acoustic characteristics. The sound pressure level readings are averaged to obtain a value of sound pressure that is assumed to exist over a sphere or hemisphere having a radius equal to the source/microphone spacing. Interested readers can consult BS4196 or, for example, Section 17, Handbook of Noise Control, Harris, McGraw-Hill. The purpose of the present contribution is to describe a method of measuring sound power that is surprisingly uncomplicated and accurate and uses only the simple sound level meter as the indicating device. In summary, the procedure consists of adding to the sound power output of the device to be measured, an equal amount of sound power from a calibrated source, the state of power equality being shown by using a standard sound level meter to indicate when the sound pressure level has been increased by 3 dB. The added sound power then equals the machine sound power and the value is read, either from a calibration chart, or directly from a meter reading the electrical input of the calibrated noise source. Exactly the same procedure can be used for measuring the octave band sound power levels of any device. The octave band spectrum and level of the noise source being known, the octave band spectrum of the unknown noise can be determined by using an octave band analyser in the circuit of the sound level meter and noting the noise source output that produces a 3 dB increase in the octave band sound pressure level of the machine. If the calibration noise source and the machine being measured are close together, the standing wave system characteristic of the enclosure is similarly excited by both sources of noise and, as will be shown, the enclosure has almost no effect upon the measured value of sound power. There appears to be little need to make several determinations of power output using different microphone positions, although this might be advisable if the machine spectrum included a particularly prominent pure tone or if the source were highly directive. The additional sound power is provided by a loudspeaker placed adjacent to the appliance under test and supplied by a shaped noise signal from a separate amplifier. In so far as sound powers are being summed, the frequency spectrum of the added power is not of great significance, but there are some minor advantages in having the spectrum of the added noise power generally similar to that of the devices to be measured. The immediate application was the measurement' of the noise emission of typical domestic appliances and, as a guide to shaping the spectrum of the loudspeaker source, the spectrum of a number of typical domestic appliances was checked.
JAMES MOIR, W. R. STEVENS
18
These data were supported by examining published data on the spectra of other domestic appliances. It is found that typical domestic appliances have normalised sound pressure spectra falling within the broad limits shown in Fig. l, so the spectrum of the noise power generator was shaped to the median of the group shown. In fact, the frequency
~3dB Extremes found in : :'" f r~:tue~..y spectrum Ideol noise
o,~o~,,a.ces\ box~..."
IOHz
,/
/
."
-.
,/"
.."
1
".
1
iO0 Hz
°~
"'X. ~
IkHz
"-~
"~
"'"°
I0 kHz
Frequency (Hz)
Fig. 1. Graph showing the spectrum variations of a cross-section of domestic appliances.
response characteristics of the loudspeaker used were fortuitously such that the desired spectral response was obtained from a driving source having a white noise pressure characteristic with only a small high frequency cut off. It is obviously necessary to know the sound power radiated by the 'standard source' and to know that the source is stable and, indeed, this places one limit on the accuracy of the method. The better examples of the loudspeaker used by hi-fi enthusiasts have been developed to have a high standard of performance in respect of frequency response, amplitude linearity and long-term stability. The requirements for this purpose far exceed those for a speaker employed as a relatively low power source over the limited frequency band of interest in noise measurement. Several of the better examples of l0 in and 12 in diameter speakers were tested for response and linearity and all were found to be more than adequate. Modified* * Provisionally patented.
SIMPLE SOUND POWER MEASUREMENTS
19
to make the sound output less dependent on their position in the room, the source speakers were then calibrated in terms of sound power output as a function of drive power, by the standard technique of measuring the sound pressure levels over a surface of known radius, the calibration being carried out in a large open field. As a first check, the sound pressures were measured at two different radii and the power of the two radii compared. As a second check the sound power output was compared with that of a standard I L G source. In the experimental model, a calibration chart was prepared, relating sound power and drive current, but there is no reason why the drive power meter should not be directly calibrated in terms of radiated sound power.
MEASURING TECHNIQUE In use for determining the sound power output of an appliance, the loudspeaker noise source is set up near the appliance and the sound pressure level read off a standard sound level meter, the instrument being placed at some convenient p o i n t - usually in the room's reverberant field---equidistant from the machine and the sound power sources. The distance between the appliance and the sound level meter is not of any great significance providing the enclosure is moderately reverberant, but it should be not less than about 2 m. The acoustic conditions prevailing in the average laboratory, lacking carpet and curtains, appear perfectly acceptable. The sound p r e s s u r e level produced by the appliance at the chosen point is noted and the loudspeaker drive power contrbi advanced until the sound pressure level is increased by 3 dB. The sound power output of the appliance is then read directly off the calibration chart. In practice, the only difficulty that has arisen is due to the unsteadiness of the noise emission of the average appliance.
PROVING PROCEDURE As a check of the effect of the room characteristics on the measured sot~nd power level, a typical domestic vacuum cleaner (without its nozzle and brush) was measured under five different conditions. Outline data on the room and furnishings are given in Table 1. TABLE I Sound power (d B)
Room
1. 2. 3. 4. 5.
Open air--two acre field. Well furnished lounge 20 ft x 14 ft x 8 ft high. Typical laboratory 20 ft x 12 ft. No carpet. Laboratory/office 14 ft × 12 ft. Part carpeted. Small office 13 ft x 9 ft. Carpet and curtains.
84 83.9 84.4 84.1 83.6
20
JAMES MOIR, W. R. STEVENS
It will be seen that the variations in measured sound power are not significant even though the acoustic characteristics of the environment varied over a much wider range than would be normally encountered. The first unit was intended for use in a large laboratory where it i.s impossible to have the appliance to be measured in one standard position, or to have it well away from similar appliances when measuring the sound power output. The earlier comparison had been carried out in a series of small rooms having dimensions of the order of 6 m x 4 m, but to obtain confirmation under the conditions in which the sound power meter would be used, a second series of tests was carried out in a much larger laboratory. For this purpose the sound power output from a second domestic vacuum cleaner was measured under conditions that differed as widely as possible: (a) When it was standing on top of a washing machine. (b) On the floor between two similar washing machines 1.5 m apart. (c) At a point about 3 m away from a line of washing machines. (d) In a small, almost cubical, photometry room that included no sound absorbents except that due to some small timber benches. The sound power levels obtained were all within a band I dB wide. The only limitation that has been observed is the need to avoid having the appliance to be measured within about 3 ft of any wall.
SOUND POWER/SOUND PRESSURE TRANSLATION
It would seem appropriate to outline the procedure to be followed in translating the sound power emitted by an appliance into sound pressure at any point in the room. The sound pressure level in an enclosure is related to the sound power output of an omnidirectional source by the following relation: SPL=PWL+
101ogto(4__~r2 + 4 )
(1)
where PWL is the sound power of source, re: 10-12 watt and r = the radius at which the SPL is measured. R = Sct/(l - ct), a parameter describing the acoustic characteristics of the environment, S being the total surface area and ct the average absorption coefficient of that area. The first term inside the brackets l/4nr 2 is the direct component of the sound field, obeying the inverse square law, and only requires a tape measure for its evaluation. Evaluation of the second term 4/R necessitates a knowledge of the total sound absorption in the room and the total area of the room boundaries. Obtaining the room boundary area is simple but approximating the total absorption present is much more troublesome. It may be outlined in any of the following ways.
SIMPLE SOUND POWER MEASUREMENTS
21
(a) By calculation. The area of each boundary surface is determined and the absorption of that surface obtained by multiplying the area by the absorption coefficient appropriate to the material of the surface. These coefficients are tabulated in many textbooks. (For example see: Noise and Vibration Handbook, Harris, McGraw-Hill.) The absorptions of all the surfaces are summed and to this is added the absorption values appropriate to the room furnishings, chairs, desks, etc. (b) From measurements of the reverberation time. Where the necessary equipment is available, this is a more accurate method because it includes any absorption introduced by structural vibration. At the lower end of the frequency spectrum, the absorption as a result ofvibration of the basic building structure tends to be large, and even a major part of the total-absorption. (c) From measurements of sound pressure level developed by a source of known acoustic power. Equation (1) is used in reverse, all the terms on the right hand side being known actual measurements. When a source of known acoustic power and spectrum is available this is the obvious procedure to adopt. When a series of machines a're to be measured in one room, calculation or measurement of the value of R need only be carried out once. When this has been done either the sound power output of the machines or the sound pressure level plus the value of the parameter R, can be quoted. Except in those instances where the machine noise is largely radiated in one direction, the addition of the 'R' value of the measuring room to the sound pressure level quotation is an adequate specification of the noise output, in that it allows the sound pressure level that would be produced at any distance and in any other room to be calculated. When the sound power measuring technique described in this paper is used the sound pressure level is automatically obtained during the sound power measuring process. The value obtained is that at the preferred location, 2 m from the appliance being tested, but the sound pressure level so obtained is subject to the same limitations as any other sound pressure measurement. The standard sound level meter provides a choice of 'A', 'B' or 'C' weighted network, the purpose of these being well understood. The total sound power is more accurately measured by using the sound level meter on its 'C' weighting. However, it is well established that use of the 'A' weighted sound pressure values gives readings that are in better agreement with subjective estimates of the noisiness of machines. In measuring the noise power Output there is no very obvious reason why the sound level meter should not be used on its 'A' scale when establishing the equality of machine noise and added sound power. There is then the usual necessity of specifying that the power level being quoted is the 'A' weighted power to distinguish that value from the generally higher value obtained when the 'C' weighting network is used. This is an instance where there is some advantage in using a calibrated noise source having a frequency spectrum generally similar to that of the machine. It should be obvious that the sound power meter requires a separate
22
JAMES MOIR, W. R. STEVENS
calibration chart when used to measure ' A ' and ' C ' weighted powers. The sound power meter has proved its value in such diverse applications as the measurement of the sound power emitted by a lecturer, the on-site checking of the noise output of the ventilating fans in a lecture theatre, and the measurement of reverberation time of a room.