Frequency-weightings correlate sound level meter (objective) measurements with the subjective human response.
Our ears are frequency selective, being most sensitive between 500 Hz and 6,000 Hz, compared with the full range from 20 Hz up to 20,000 Hz.
Our subjective response is also level (loudness) dependent, so early sound level meters included A, B, C and Linear or Flat frequency-weightings, to take both factors into account, across the frequency range. At one stage D-weighting filters were also popular for rating aircraft noise levels.
A-weighting is the standard frequency-weighting for sound level meters, covering the full audio range, 20 Hz to 20 kHz. 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 measurements correlate well with the perceived loudness in many applications, as originally intended. However the current use of the A-weighting for most environmental noise applications, supported by regulations, does cause concern in some quarters, as the 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.
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L, Lin and Linear-weightings, are similar to the Flat Weighting above, also superseded by the Z-weighting.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