A Weighting. And B. And C.

Some devices, particularly digital tape recorders, apply A-weighting to all their data in order to achieve acceptable data compression. This is fine unless you want to analyse the unweighted data or apply a different weighting factor. Using Prosig’s DATS software it is a simple task to instruct the WEIGHT module to either simply unweight the data or remove one weighting factor and apply another.

Example of A, B and C Weighting
Figure 1: Example of A, B & C Weighting

The presence of the Named Element $WEIGHT in a signal is used to tell DATS what weighting has been applied to a signal. Correctly setting this for data gathered with A-weighting will inform the WEIGHT module to treat it accordingly.

Figure 1 above shows four DATS signals. Each one is the frequency spectrum of a broad band random input. The first (dark blue) is unweighted and red trace shows the same data A-weighted. It can be easily seen how A weighting depresses frequencies below 500Hz whilst increasing slightly those above 1250Hz. For completeness the B-weighted signal is shown along with the C-weighted one. These weightings suppress frequencies below about 250Hz and 20Hz respectively.

D weighting, which for clarity is not shown, is similar to B weighting except that it significantly boosts frequencies in the 1250Hz to 10kHz region. It was designed specifically for assessment of aircraft noise.

Generally speaking the overall level found from A weighted spectrum correlates well with subjective assessment of loudness. The C weighting curve gives equal emphasis over the normal hearing range from 31.5Hz to 8kHz.

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Dr Colin Mercer

Chief Signal Processing Analyst (Retired) at Prosig
Dr Colin Mercer was formerly at the Institute of Sound and Vibration Research (ISVR), University of Southampton where he founded the Data Analysis Centre. He then went on to found Prosig in 1977. Colin retired as Chief Signal Processing Analyst at Prosig in December 2016. He is a Chartered Engineer and a Fellow of the British Computer Society.

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