When sounds have a similar frequency distribution of energy, dBA measurements may be used for ranking subjective responses to the sounds. This distinction has been largely abandoned with a tendency to standardize to dBA measurements as a single number evaluator for noise. 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. 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). In most cases, the C-scale readings will closely approximate the SPL value. SPL readings (see SOUND PRESSURE LEVEL ) refer to readings taken with a FLAT response network (i.e. Compare also: LEVEL RECORDER, SOUND ANALYSER, VU METER. The sound level meter, however, does not take into account the predominant frequency region of the noise, or other factors which may cause annoyance.Ĭompare: COMMUNITY NOISE EQUIVALENT LEVEL, EQUIVALENT ENERGY LEVEL, NOISE POLLUTION, NOISE RATING, NOISE AND NUMBER INDEX, PERCEIVED NOISE LEVEL, SOUND INTRUSION, SOUND POLLUTION. Such measurements are designated dBA (see NOISE LEVEL ). 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. 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 ). 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. Sound level meters usually contain different weighting networks (designated A, B, C, D) to discriminate against different frequencies. Such measurements are called sound level readings. An instrument for measuring the level of sound pressure in DECIBEL s.

A weighted decibel scale