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Re: Wasn't v. Helmholtz right?
Andrew Bell wrote:
> -----Original Message-----
> From: Ben Hornsby [mailto:email@example.com]
> Sent: Friday, 16 June 2000 12:17
> To: Andrew Bell
> Subject: Re: Re: Wasn't v. Helmholtz right?
> I am a doctoral student in Audiology and have been following your discussion
> to some degree. One comment you made caught my eye:
> " Of course, at higher SPLs (above about 60 dB),
> vertical movement of the partition does begin, but only as a means of
> damping excessive motion."
> I noticed that other respondents to the discussion either agreed with this
> or didn't comment on it. I was under the impression (based on BM tuning
> curves) that a response on the BM could be observed at levels much lower
> than this. I believe I am missing something fundamental in the discussion.
> What do you base this observation on? Any comments are appreciated.
> Ben Hornsby
> Dear Ben:
> Thankyou for your interest and your perceptive question.
> Yes, you're right: BM responses can be observed right down to near zero.
> What I meant to convey by my statement was that whole-scale up and down
> movement of the cochlear partition (like that which is supposed to happen
> under the traveling wave theory) does not begin until about 60 dB SPL. Below
> this level, there is insufficient differential pressure developed across the
> partition for it to be pushed up and down. However, there is still
> sufficient common-mode pressure, a factor usually neglected in mathematical
> models, for the OHCs to detect and respond. Another way of saying this is
> that the acoustic impedance of the helicotrema is much lower than usually
> assumed, so that this hole effectively short-circuits the pressure
> difference across the partition. The common-mode pressure is simply the
> pressure built up uniformly throughout the cochlea by the inward movement of
> the oval window (like pushing in the loudspeaker cone on a sealed-box
> speaker enclosure).
I dont understand.
The normal assumption for the helicotrema is that its impedance
is typically treated as an inductor, which at low frequencies, has a
near-zero impedance. Are you suggesting it must be lower than one would get from
a large hole (i.e., the usual model)?
> Andrew Bell.
Jont B. Allen
AT&T Labs-Research, Shannon Laboratory, E161
180 Park Ave., Florham Park NJ, 07932-0971
973/360-8545voice, x7111fax, http://www.research.att.com/~jba