Audiogram defects (was Re: Cochlea -- quality of filtering)

Tony Woolf tony at howl.demon.co.uk
Fri Sep 22 13:57:50 EST 1995


In article <43s97f$1a9 at hollywood.cinenet.net>
           erc at cinenet.net "Eric Smith" writes:
            
> Thanks for all the answers (and references) to my question about the
> quality of filtering in the cochlea, both here and by email.  The
> general consensus seems to be that the quality of filtering is not very
> high.

I regret to say that, according to my information, the general consensus
seems to be out of date - see my post in the appropriate thread.  Will
anyone who wishes to contradict this, please give *modern* references.

> My next question is about the implications of this in cases where the
> hair cells are defective at some frequencies but not others.  It seems
> like the person would still hear tones of those frequencies, but they
> would sound like other frequencies, and would have a higher threshold.
> But an audiogram does not ask what the tones sound like, just whether
> they can be heard at all.  Thus, some people who are actually 100% deaf
> at some frequencies but have normal hearing at others, might have a
> very misleading audiogram.

Worse filtering seems (usually but not always) an early indicator
of damage or malfunction of the hair cells.  This can and does happen
in very narrow frequency bands.  I'm not sure what the practical
effect is, on a conventional audiogram, but this damage in narrow
bands is apparently undetectable by the subject in normal life.

Poor filtering is also an effect of TTS and lasts considerably longer
than the change in hearing threshold.  Exposure to noise prior to an
audiogram can therefore affect the filtering several weeks after the
noise occurred.

The worsened filtering can be detected by special techniques.  For
example, you can use noise with a notch of variable width to mask a
tone at the notch centre frequency.

I must emphasise that this is based on what I have read and on
conversations, not on practical experience.  It does seem to be
pretty solid information and I can give plenty more references.

-- 
Tony Woolf  (tony at howl.demon.co.uk)



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