THE SCIENCE AND APPLICATIONS OF ACOUSTICS - H. H. Arnold ...

358 14. Machinery Noise ControlIn setting up noise control measures, the first step is to identify the noise sourcesand to measure the sound power output. Ideally, it would be desirable to takea source into a well-defined environment such as a reverberation chamber andmeasure its sound power output, but many stationary sources cannot be moved.But the sound power output for immovable sources can still be estimated fromsound pressure measurements made on a hemispherical or rectangular envelopingsurface (cf. Chapter 9). The sound pressure level will be increased by the presenceof background noise and room reverberation, so a correction factor for either/bothbackground noise and reverberation must be applied in such cases. For reasonswhich we shall see in the sections following, a spectrum analysis of noise fromspecific machines will often prove useful in tracing malfunctioning machineryparts so that they can be realigned or replaced.Many noise problems in the workplace can be avoided by heeding the old adagethat an ounce of prevention is worth a pound of cure. Prior to any purchase ofmachinery, sound power output of each unit should be obtained beforehand, eitherdirectly from the vendor or by conducting actual measurements on an existinginstallation. Wise planning of the plant layout includes not only promoting productionefficacy and personnel safety; it should always involve prediction of noiseoutput of all equipment in normal operation. Adjustments can then be made atthis stage in the choice of quieter equipment and/or incorporating noise-reducingdevices so that the planned facility will operate within the sound exposure limitsmandated for workers.14.3 Estimation of Noise Source Sound PowerA nondirectional point source in a free field will radiate sound uniformly andradially in all directions. Such a source L W represents the true octave-band soundpower level, with units of decibels based on the reference power 1 pW (10 –12 W).Most pieces of machinery, however, are not point sources, nor do they radiate soundpower uniformly. In planning of facilities, it is generally necessary to estimate theexpected sound power for individual machines that will affect the environment.For certain machines, a sound power conversion factor F n can be used to determinethe output on the basis of the total power rating of the machine,whereP = F n × P m (14.1)P = sound power of the machine, WP m = machine rated power, WThe relationship of Equation (14.1) applies to both mechanical and electrical machinery.Estimated conversion factors for a number of common machinery arelisted in Table 14.1. It should be noted that the ranges are quite large for each typeof machine.