A, B & C WEIGHTING
By Dr Colin Mercer, Technical Director, Prosig
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 DATS it is a simple task to instruct the WEIGHT module to either simply unweight the data or remove one weighting factor and apply another.
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| Fig. 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.
The screenshot 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.
