Re: Selective numbing

["Alain de Cheveigne'" <alain@xxxxxxxxxxxxxxxxxxx>]

>Although I don't have knowledge of formal studies on this (neither
>articles) maybe I can add some more flavour to the discussion presenting
>an effect I've been experiencing when exposed for some minutes to
>"buzzing" noises due to poor shielded microphones cables (bad ground or
>bad shield).
>
>After hearing this kind of noise for over 5 minutes (not loud, close in
>intensity to a normal human conversation) and turned off the mic (so
>stopping the noise) I noticed that all human voices I heard for some
>time were kind of "flangered", even my voice sounded as if flanger
>effect were being applied. It seems the noise had biased the hearing
>hardware so that incoming voice were just analysed with a different bias
>and producing a flanger effect. I didn't analyse the noise signal, but I
>suspect it is probably very rich in 60Hz (the AC power supply
>frequency).

I'm not sure what is the processing within a "flanger".  Is it a kind of
time-domain comb filtering (ie subtraction or addition of the signal to
itelf after a delay)?   If so, it might be worth considering that the
effect is be due to the lingering effects of a "harmonic cancellation
filter", such as I proposed in some recent modeling papers (JASA 93,
3271-3290, JASA 101, 2857-2865, JASA 103, 1261-1271).  The cancellation
filter performs a kind of delay-and-subtract processing of neural spike
trains within each channel coming from the auditory periphery.

I speculated that the auditory system might try systematically to suppress
parts of sound that have a regular steady-state structure, for example
harmonic.  Microphone hum is a good candidate, because it consists
presumably mostly of 60 Hz-spaced harmonics distributed across the
spectrum.  This spacing is too narrow for peripheral selectivity to be much
help in segregation, but time-domain filtering would be effective.

Supposing that harmonic cancellation were "left on" after prolonged
exposure to the hum, it might produce a "flanging" effect when the hum is
turned off.   Just a guess.  Sorry for the advertising...

Alain




------------------------------------------------------------------
Alain de Cheveigne'
Laboratoire de Linguistique Formelle, CNRS / Universite' Paris 7,
case 7003, 2 place Jussieu, 75251 Paris CEDEX 05, FRANCE.
phone:   +33 1 44273633, fax: +33 1 44277919
e-mail:  alain@linguist.jussieu.fr
http://www.linguist.jussieu.fr/~alain/
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