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Re: mechanical cochlear model

Dear Jan and list:

Your intuitions are close to the mark, I think. The problem with the
traveling wave model is that the maths tends to obscure rather than
clarify what is going on.

The TW model describes what's happening in passive structures at high
sound pressure levels. But for active systems and low SPLs, it doesn't
seem to work so well. A live cochlea functions very differently to a dead

I hope some of the references I provided help you find a believable
explanation of what is going on. The cochlea is tiny and hidden, and we
need logic and intuition - and the remarkable window provided by
otoacoustic emissions - to work out the truth of the matter.


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

On Thu, March 4, 2010 8:29 pm, Jan Schnupp wrote:
> Dear Andrew and List
> here you have been putting your finger on something that I have never
> properly understood (and which I secretly suspected the large majority of
> hearing researchers haven't really understood either. You off us two
> analogies: a piano being thumped, and a surface wave on a pond. To
> my mind it seems that the cochlea *must* be a lot closer to the first of
> those model than the second one: Firstly it seems to me that the cochlea,
> being completely filled with fluid and encased in a rigid shell, has no
> surface along which a wave could propagate, and you'd have a hard time
> making a propagating pond surface wave that has an amplitude envelope like
> that seen on the BM. The BM is really very unlike the surface of any pond
> I have ever seen. Secondly, pressure
> waves propagate very fast in fluid (1200 m/s or so), and the cochlea is
> rather small, so when the stapes "thumps" the cochlea, the resulting
> pressure gradients ought to be available to set things in motion almost
> instantaneously and simultaneously along the whole length of the BM. Or am
> I wrong about this? The only way I could ever
> understand the travelling wave is by realizing that simultaneously
> activated filters tuned to different frequencies will go out of phase to
> produce, as you say, something that looks like a travelling wave. Is that
> not good enough? I have great difficulty believing that the lion's share
> of mechanical energy travels along the cochlea by virtue of the a more
> basal part of the BM dragging up and then pushing down a more apical part,
> given how hard the fluids above and below the BM would oppose such a
> movement. I can more easily imagine the mechanical energy mostly
> propagating though the fluid column and dragging the BM along, so we end
> up with spring (BM segments) mass (fluid column) resonators. So to my
> mind, there is a lot more thumped piano in the cochlea than there is pond
> surface, but I don't claim to be an expert on cochlear mechanics, and if I
> have got it all wrong then I'd really like to know, ideally with an
> *accessible* reference, why I am wrong.
> I do have to give lectures on the cochlea and would hate to spread
> misunderstanding, but I don't think I can get up in front of my class and
> tell them "imagine the travelling wave like the ripples on a pond". If the
> students are like me (always a dangerous assumption, I know) then they'll
> find that analogy more confusing than enlightening.
> Many thanks in advance for any insights, and best regards,
> Jan

> Dr Jan Schnupp
> University of Oxford
> Dept. of Physiology, Anatomy and Genetics
> Sherrington Building - Parks Road
> Oxford OX1 3PT - UK
> +44-1865-272513
> www.oxfordhearing.com