An instrument for measuring the level of sound pressure in DECIBELs. Such measurements are called sound level readings. See: SOUND LEVEL.
Sound level meters usually contain different weighting networks (designated A, B, C, D) to discriminate against different frequencies. The very low frequencies are discriminated against quite severely by the A network, in a manner similar to the response of the ear (see EQUAL LOUDNESS CONTOURS), but only moderately by the B network and hardly at all by the C network. Therefore, if the measured sound level on the C network is much higher than that on the A network, much of the sound energy is concentrated in the low frequency region (see INFRASONIC).
Since the A weighting network corresponds to the tendency of the ear to discriminate against low frequency sounds, it is often employed in decibel measurements of community noise. Such measurements are designated dBA (see NOISE LEVEL). The sound level meter, however, does not take into account the predominant frequency region of the noise, or other factors which may cause annoyance.
Compare: COMMUNITY NOISE EQUIVALENT LEVEL, EQUIVALENT ENERGY LEVEL, NOISE POLLUTION, NOISE RATING, NOISE AND NUMBER INDEX, PERCEIVED NOISE LEVEL, SOUND INTRUSION, SOUND POLLUTION. Compare also: LEVEL RECORDER, SOUND ANALYSER, VU METER.
SPL readings (see SOUND PRESSURE LEVEL) refer to readings taken with a FLAT response network (i.e. unweighted level). In most cases, the C-scale readings will closely approximate the SPL value. Historically, the A, B, and C weighting networks were derived as the inverse of the 40, 70 and 100 dB EQUAL LOUDNESS CONTOURS, respectively, of Fletcher and Munson (1933). That is, the A network was to be used to measure low level sounds, the B for medium level and the C for high level. This distinction has been largely abandoned with a tendency to standardize to dBA measurements as a single number evaluator for noise.
When sounds have a similar frequency distribution of energy, dBA measurements may be used for ranking subjective responses to the sounds. When the sounds have very different energy distributions, or when there is a frequency dependence involved, such as with SOUND INSULATION or other noise control, then an analysis of the frequency distribution of the sound energy is required (see PERCEIVED NOISE LEVEL). In certain cases, average differences between PNL and dBA are commonly found, for instance, a 12 dB difference for jet aircraft, 13 dB for office noise.
A sound level meter will usually have fast and slow response modes indicating its sensitivity to rapid fluctuations and peak values of sound pressure. For a discussion of the measurement of peak and impulse levels, and their danger, see DAMAGE-RISK CRITERIA.
Sound level meter response characteristics for the A, B, and C weighting networks (from Handbook of Noise Measurement, General Radio, 1963, p. 9, used by permission).
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