The human ear is frequency selective. Between 500 Hz and 6,000 Hz our ears are very sensitive compared with the lower and higher frequencies. The full range for young people is 20 to 20,000 Hz.
This subjective response is also loudness dependent and as a result early sound level meters included A, B, C and Linear or Flat settings to 'correlate' the objective meter readings with the very wide subjective human response
A-weighting filters cover the full audio range, 20 Hz to 20 kHz, but the filter 'shape' is similar to the response of the human ear at the lower sound levels - see the equal loudness contours entry.
A-weighted noise measurements are widely used and the meter class confirms the accuracy of the meter including the filters.The preferred convention is to write LA = x dB, however dBA and dB(A) are often used, as are LAeq, LAmax etc., for more information on the dB (decibel) click here
A weighted measurements correlate well with the perceived loudness at low sound levels, as originally intended. However the current use of the A-weighting for most noise applications, supported by regulations, does cause concern in some quarters. For example low frequency noise, which disturbs many people, is suppressed by A-weighted meters. Currently there are thousands of wind turbines, built in quiet rural areas, which are highlighting this low frequency anomaly.
B-weighting, no longer in common use, was initially developed to cover the mid-range loudness scale. It was more 'critical' of lower frequencies than the A-weighting network which probably accounts for it's use by the motor industry for many years after it's decline in general popularity.
C-weighting is a standard frequency weighting for sound level meters, commonly used for high level measurements and peak sound pressure levels. Approximately follows the 100 phon curve - also written as dB(C) or dBC.
The A-weighting curve is used extensively for general purpose noise measurements but the C-weighting correlates better with the human response to high noise levels.
D-weighting, a sound level meter frequency weighting developed for measuring aircraft noise especially non-bypass military engines.
No longer in common use since IEC 61672 2003. More recent ISO standards recommend A-weighting for commercial aircraft noise.
Flat weighting, a sound level meter frequency weighting, flat over a frequency range that must be stated. No longer appears in the Standards, which prefer the newer Z-weighting.
L, Lin and Linear Weightings, are similar to the Flat Weighting above, also superseded by the Z-weighting.
Z-weighting, the Z is for 'zero' frequency weighting, implying no weighting across the audio spectrum. In reality the range is 10 Hz to 20 kHz ±1.5 dB.
Introduced in 2003 (IEC 61672) to replace the Flat or Linear Filters.
Written as dB(Z) or dBZ, see other examples of Z-weighted information LZeq, LZF, LZS, etc.
See also vibration weighting networks