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Sound Power Measurements on Large Compressors Installed Indoors - Two Surface Method G Purdue University Purdue e-Pubs International Compressor Engineering Conference School of Mechanical Engineering 1974 Sound Power Measurements on Large Compressors Installed Indoors - Two Surface Method G. M. Diehl Ingersoll-Rand Company Follow this and additional works at: https://docs.lib.purdue.edu/icec Diehl, G. M., "Sound Power Measurements on Large Compressors Installed Indoors - Two Surface Method" (1974). International Compressor Engineering Conference. Paper 126. https://docs.lib.purdue.edu/icec/126 This document has been made available through Purdue e-Pubs, a service of the Purdue University Libraries. Please contact [email protected] for additional information. Complete proceedings may be acquired in print and on CD-ROM directly from the Ray W. Herrick Laboratories at https://engineering.purdue.edu/ Herrick/Events/orderlit.html SOUND POWER MEASUREMENTS ON LARGE COMPRESSORS INSTALLED INDOORS - TWO-SURFACE METHOD - George M. Diehl Manager, Sound and Vibration Section Ingersoll-Rand Research, Inc. Phillipsburg, New Jersey Sound power ratings of machinery are be­ 3. Sound power levels of large machines coming important for a number of reasons. are difficult to obtain in most industrial Industrial installations must comply with locations. recently enacted state and local noise con­ trol codes, and sound power ratings are This situation is changing, and sound needed to predict compliance. In addi­ power ratings of machinery are becoming tion to this, the Federal Noise Control more important. Recognizing this, the Act of 1972 requires labeling of machinery International Organization for Stand­ to show maximum noise emission, and this, ardization has decided that all machinery logically, may be in terms of sound power. noise measurement standards must include a procedure for determination of sound The sound produced by a compressor cannot power level. be stated in terms of A-weighted sound level alone. Since sound level decreases The objective, then, is to develop a with distance from the source, this dis­ procedure for determining the sound power tance must be given, along with the height level of a machine when it is operating above the floor where sound measurements in an industrial environment. This can were taken. Furthermore, measured noise be done by the two-surface method. depends upon the environment where the compressor is installed. A product noise For accurate results, sound power level emission label in terms of A-weighted should be measured in either a free field, sound level would have to show all this an anechoic chamber, or a reverberant additional information. field. Most large compressors, and simi­ lar industrial machinery cannot be sound­ In contrast, the sound power level of a tested in any of these three environments. ~chine is independent of measurement They must be measured where they are distance from the source, and although installed, on a customer's property, or radiated sound power depends on the in some cases, on the machinery manu­ acoustic impedance into which it is ra­ facturer's test stand, prior to shipment. diated, it is practically independent of environment for most machinery installa­ Unfortunately, neither the customer's tions. These properties are the ones that plant nor the manufacturer's test make sound power so desirable in expressing facility is ideal for sound measurements. the noise rating of a machine. In fact, some machines are installed in locations where it is simply impossible In spite of this, sound power level is to conduct a sound test that has any not often stated, nor even measured for meaning. Some machines are nearly as large machinery. The air conditioning large as the room in which they are lo­ industry and certain gas turbine manu­ cated. Often other nearby equipment, facturers provide sound power information, which cannot be shut down makes more noise but almost all other large machinery is than the machine being tested. In other rated in terms of near-field sound pressure instances, it is too dangerous to try to levels. There are several reasons for take sound measurements over the top of this: large rotating machinery, as required in sound power tests. 1. The-Occupational Safety and Health Act states hearing damage criteria in Not all cases are imPossible. There are terms of A-weighted sound level. Most many machines that can be sound-tested on machinery sound specifications require the job and both octave-band sound this information. pressure levels and octave-band sound power levels can be obtained with reasonable 2. Until now industrial plant operators accuracy. Acousticians and noise control have had little or no need for sound engineers cannot back away from these power ratings. projects because they cannot be done with 230 laboratory accuracy. Something must be room done, and they must do it with the best accuracy possible under actual operating SPL - PWL 10 log~r2 + .!}+ 0.1 conditions. Measurements in these semi­ reverberant environments are of great where, practical importance to industry. SPL is sound pressure level, in decibels, 2 Sound power levels must be calculated re 2.0 X 10-SN/m : from measured sound pressure levels. Since sound pressure levels are influenced PWL is sound power level, in decibels, by the environment in which the measure­ ments are made, it becomes necessary to re lo-12w; know the effective room constant of the environment. There are several ways to r is the distance from the source, in determine this: meters; and, R is the room constant, in square meters. First, it may be calculated by estimating the average absorption coefficient of This equation can be plotted in a series the floor, ceiling, side walls, and of curves for various combinations of other items in the room, and using the r and R. appropriate equation for room constant. Although this technique can yield reason­ able accuracy for some relatively simple areas, it turns out in the case of most industrial plants that it is no better than simply looking at the location and est­ imating the room constant directly. In other words, it is not very accurate. Second, it may be determined by measuring the reverberation time with a microphone, sound level meter, and graphic level re- · corder. This method is usually not feasible in industrial area. It involves the use of a noise source that is stopped quickly; the time for the level to de­ crease by 60 dB is measured. High ><•DISTANCE FROM ACOUSTIC CENTER machinery noise precludes the use of most sound sources; attempts to get around it, such as by firing a gun as the noise It can be seen from these curves that in source, are usually not acceptable to the case of ·a small source, the decrease plant operators. in sound pressure level with distance provides a means for determining the Third, the effect of the environment may effective room constant of the area. be determined by either a calibrated sound source and an absolute comparison With this reasoning, a set of correction test, or by an auxiliary sound source in factor curves can be obtained by plotting SPL - SPL on the vertical axis and r, a relative comparison test. In these 1 r methods, the machine under test should be the distance from the source, on the moved out while measurements are being horizontal axis. SPL is the octave­ made on the reference source. This is band sound pressure level1 at a distance obviously impossible, and the technique of 1 m from the nearest major surface of loses its value when it is found that the machine, and SP.Lris the octave-band many different answers are obtained by sound pressure level at a distance of placing the reference sources at various r (in meters). locations with respect to the machine under test. Furthermore, the sound pro­ The curves, then, can be used to obtain duced by availabe reference sources is a correction factor to be subtracted much too low to make them usable in most from the octave-band sound pressure industrial areas. levels measured in the semi-reverberant room. From this, the approximate level There is one technique that is feasible that would be measured in a free field and that is to use the machine itself as can be obtained. the sound source. To check the correction factor's validity, First consider the equation relating an ILG sound source was tested in a free sound pressure level, sound power level, field. Next, it was tested indoors in and room constant in a semi-reverberant 231 10 -~ !--""' . - -·---.... / <6/ / ,,o V/ ~ c- //v .. I-"" L/'v - ? ..0 .1 a.. v-- ~ "'I V/ .1 a.. ~ "' 25 / l-- -~ -- f-'""" ~ ,_v- 4.0 ~-5 V::= ~ 5.0 ...... 5.5 1(WJ 1000 1.. 0. I--t- FRE:QUE::NCY IN CVCL5$ PER SECONO v::: 1--~0 QCT,i,VE ,AM MND~ IN CYCLES ron !!~ND 1 v::= 110 - ~JJ - 11" - !<II - ~100 - l601 - \IJOO 2 J 4 s 6 8 9 7 10 DISTANCE r, METERS various locations in a semi-reverberant .field. At each microphone location, mea­ surements were made at a distance of 1 m, and then the microphone was moved away from the sound source in a direction perpendicular to the axis of the machine. Then the maximum decibel drop-off was measured in each octave band of interest. At each microphone location, the distance from the 'source was noted where the max­ imum decibel drop-off occurred. The inter­ section of the maximum decibel drop-off and the distance from the source where it occurred determine the correction factor for that microphone location. When the corresponding correction factor was sub­ 100 1000 10000 tracted from the indoor measurement, the FREQUENCY IN CYCLES PER SECOND result came surprisingly close to the free-field measurement.
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