AW: Re: mechanical cochlear model

["reinifrosch@xxxxxxxxxx" <reinifrosch@xxxxxxxxxx>]

Dear Andrew,

Many others would be better qualified to comment on your posting,
but since they have not done so up to now, here goes:

The travelling cochlear wave is well established, I think. Even at the
"characteristic place", i.e., at the low-level "active peak", the travelling
wave has been experimentally demonstrated; see, e.g., Fig. 1 of
Ren et al. (2003), "Measurement of Basilar-Membrane Vibration Using a 
Scanning Laser Interferometer", Biophysics of the Cochlea (Titisee Proceedings),
World Scientific, New Jersey, pp. 211-219.

The mechanical-energy transfer from fluid to partition is efficient.
For instance, in slices dx near the base of a passive two-dimensional model 
as described by de Boer (1996) in Chapter 5 of "The Cochlea" (Springer, New York),
the pure-tone-wave mechanical energy is predominantly in the liquid; in a slice dx 
at the "passive peak", however, 30 percent of the energy is in the "upper-half-channel"
liquid, 40 percent in the partition, and 30 percent in the "lower-half-channel" liquid.

The construction of a corresponding mechanical model may indeed be difficult:
the direct longitudinal coupling between neighboring elements dx of the partition 
must be negligibly small, but the liquid must be prevented from penetrating through 
the partition.

Reinhart Frosch,
Dr. phil. nat.,
r. PSI and ETH Zurich,
Sommerhaldenstr. 5B,
CH-5200 Brugg.
Phone: 0041 56 441 77 72.
Mobile: 0041 79 754 30 32.
E-mail: reinifrosch@xxxxxxxxxx .

----Ursprüngliche Nachricht----
Von: andrew.bell@xxxxxxxxxx
Datum: 03.03.2010 00:00
An: <AUDITORY@xxxxxxxxxxxxxxx>
Betreff: Re: mechanical cochlear model

Dana is pointing to a demonstration in which a closed tube displays
resonance, whereas Christian would like to constuct a chamber in which
traveling waves are generated.

The pertinent question, of course, is: on which principle does the real
cochlea operate?

I have written a paper that compares traveling waves and resonance in the
cochlea, and it is available at

http://www.plosbiology.org/article/info:doi%2F10.1371%2Fjournal.pbio.0020337


I believe that resonance is the preferred principle because it is more
sensitive: it allows energy to build up over many cycles. The
Exploratorium's demo giving a startling example of what can happen.

The problem with the traveling wave is that it is relatively inefficient.
It is hard to transfer acoustic energy from the fluid to the partition. In
the physical models I have seen (including Békésy's), the model is
compromised by having fluid in the bottom half and air in the top - this
is sort of cheating in that it is not the situation in the real cochlea,
which is filled totally with liquid. Chris, I challenge you to produce a
traveling wave model with fluid in both upper and lower halves!

Andrew.

Andrew Bell
Research School of Biology
Australian National University
Canberra, ACT 0200