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From Earl Geddes distortion papers

"3. Distortion by-products of any kind are likely
to be more perceptible at lower signal levels
than at higher signal levels."

I.e. masking is less broad at lower levels (see the graphs in the link)
http://www.gedlee.com/downloads/Distortion_AES_I.pdf
http://www.gedlee.com/downloads/Distortion_AES_II.pdf

Also of interest WRT to distortion audibility:

"An nth order nonlinearity generates nth
order harmonics and every other harmonic
below it."


"For multi-tones an nth order nonlinearity
causes sidebands at ±n times the modulation
frequency and every other value of n below
it, as well as harmonics (as above)."

"The masking effect of the human ear will
tend to make higher order nonlinearities
more audible than lower order ones."

"Nonlinear by-products that increase with
level can be completely masked if the order
of the nonlinearity is low."

"Nonlinearities that occur at low signal levels
will be more audible than those that occur at
higher signal levels."

This conclusion is of particular significance for audio IMO.


Note in paper II that there is Table 1 which shows the Gedlee number (their metric for distortion audibility) and the different test tones that they used and their distortion contents. What is particularly interesting is that for 9 and 10 the distortion values are quite low but the Gedlee numbers are very high, which means that they were perceived as highly distorted even when they were in fact not. Conversely, some highly distorted signals (13,16,20,21) were not perceived as such.

However, when the Gedlee number is above 10 the correlation becomes much worse meaning there seems to be a limitation to this correlation approach.


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