Subject: Re: Wasn't v. Helmholtz right? From: Jont Allen <jba(at)RESEARCH.ATT.COM> Date: Mon, 19 Jun 2000 22:49:28 -0400Andrew Bell wrote: > > -----Original Message----- > From: Ben Hornsby [mailto:ben.hornsby(at)vanderbilt.edu] > Sent: Friday, 16 June 2000 12:17 > To: Andrew Bell > Subject: Re: Re: Wasn't v. Helmholtz right? > > Andrew, > > 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