Some devices, particularly digital tape recorders, apply A-weighting to all their data 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 unweight the data or remove one weighting factor and apply another.
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 weighting already applied will inform the WEIGHT module to treat it accordingly.
Figure 1 above shows four DATS signals. Each one is the frequency spectrum of a broadband random input. The first (dark blue) is unweighted, and the 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 the 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.
Dr Colin Mercer
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