Acoustic Glossary


 

Frequency Weightings : Definitions, Terms, Units, Measurements ..

Frequency Weightings also know as Frequency Networks and Frequency Filters correlate measured sound pressure levels with the subjective human response The human ear is frequency selective, between 500 Hz and 6 kHz our ears are very sensitive compared with lower and higher frequencies. 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 subjective human response


A-weighting : the A-weighting filter covers the full audio range - 20 Hz to 20 kHz and the shape is similar to the response of the human ear at the lower 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 for most noise applications, supported by regulations, does cause concern in some quarters.

Also 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 between the A and C-weighting networks. It was also 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 general decline in popularity.


C-weighting : a standard frequency weighting for sound level meters, commonly used for higher 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 : 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 : 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 Weighting : similar to the Flat Weighting above, superseded by the Z-weighting.


Z-weighting : Z for 'Zero' frequency weighting, which implies no frequency weighting. 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

page up