Sound waves in air can be described in terms of the cyclic variation in pressure, in particle velocity, or in particle displacement; for a complete description, frequency and wave-form data are also required.
Sound pressure is the cyclic variation superimposed upon the steady or atmospheric pressure; usually it is the RMS value. An RMS value is determined by taking the square root of the arithmetic mean of the instantaneous values over one complete cycle for a sine wave, or for as many cycles of a nonsinusoidal wave form as are necessary for a reliable sample. The units of sound pressure are force per unit area—dynes per square centimeter or newtons per square meter. Particle displacement is in cen timeters. Most sound pressures are given in RMS values, and most sound level meters display RMS values.
To describe the range of sound pressures in a logarithmic scale is convenient, the unit of SPL is the dB, described by
where P is measured sound pressure, and Pref is the reference pressure ordinarily used. The customary reference pressure is 0.0002 dynes/cm2, or 0.0002 / bars. (One standard atmosphere is equal to 1,013,250 microbars, so 1 microbar is nearly 1 dyne/cm2. The reference level should always be stated when sound pressure levels are given, as dB re 0.0002 dynes/cm2.)
Sound power—the acoustic power produced by a source—is described in watts. Again a logarithmic scale is used to accomodate the wide range involved, without inconveniently clumsy figures. The unit again is the dB. The PWL is expressed by
PWL = 10 log10 (W/Wref), or 10 log10 (W/10~12) dB 6.4(2)
where W is the acoustic power in watts, and Wref is the reference level which should always be stated; the reference level ordinarily used is 10~12 watt. Since the power ratio 10~12 can also be written as 120 dB, Equation 6.4(3) is convenient to write as:
Sound pressures and sound power values are physical magnitudes, expressed in physical terms. SPLs and PWLs are ratios (the ratio of a measured value to a reference value) expressed in logarithmic terms called dBs.
Other terms and quantities used in noise control work will be defined as they are used. Sound pressures and sound powers are basic. The ear responds to sound pressure waves, and nearly all sound magnitude measurements are in terms of sound pressure. Sound power determines the total noise produced by a machine and, thus, is important in machine design.
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