The ear is relatively insensitive to very low and very high frequencies and sound level meters have, by international agreement, weighted frequency responses. Hence different frequency weightings will produce, for the same sound source, different Lp readings.
A series of different frequency responses are designed for different uses in sound level measurement. In accordance with IEC 61672:2003 sound level meters will now have the following frequency weightings:
– This curve is designed to best express the response by the human ear and is subsequently the most commonly used frequency response. The use of the A weighted frequency curve is mandatory for measurement of environmental and industrial noise as well as assessing potential hearing damage and health effects of noise.
– This curve is used to ascertain the acoustic emissions of machines and has a broader spectrum than that of the A weighting curve. Although the C weighting frequency response is less commonly used all Class 1 sound level meters are required to have the necessary filters.
– Also known as Linear or Flat weighting. This curve gives a flattened response to different frequencies with cut-offs at the very high or very low frequencies. These cut-offs will vary in different manufactures instruments. Often used in conjunction with octave band filters, a typical linear response would be flat from 20Hz to 16KHz.
There are also two older frequency response curves, B and D. These two frequency curves are rarely used. D weighting frequency responses were primarily used in aircraft noise measurements, however A weighting curves are now mandated for use in all civilian aircraft measurements.