ear physics

Steve Ungstad steve_ungstad at agilent.com
Fri Oct 19 05:04:07 EST 2001


Guyz,

In your web page you show  Frequency distribution along the human
cochlea basilar membrane: passive tonotopy. The drawing spiral is
thinner at the base than at the apex. Is this accurate?

I've been looking for existing theories similar to my ideas. The most
important function of the ossicular chain is high frequency phase
inversion, where the mass of the malleus causes it to pivot about it's
center of mass. At lower frequencies, I suspect the whole Malleus moves
in the same direction, only providing 90 degrees phase shift with
respect to the sound reaching the round window. The point is that the
sound entering the two windows will be 180 degrees out of phase at
different points along the cochlea for different wavelengths. It should
be noted that this also requires that sound travel at two different
velocities in endolymph and perilymph, so that a phase difference
accumilates along the cochlea.

This requires that the sound travel the spiral at a relatively slow
velocity, sinse the cochlea has to be big enough to effect at least a 90
degree differencial delay for 20Hz. Surface waves are likely the means
to this slow propogation medium. Waves that travel on the interface
between a massive (~water) medium and a another that is very compiant
(~air) move very slowly. The velocity of waves in either medium alone is
much faster because they either lack the mass or the compliance. At the
interface between mediums, the mass of the heavy medium moves agains the
compliance of the lighter, producing a very slow wave propogation. The
slow propogation of sound allows for very small acoustic devices, like
the cochlea.

-There is likely a graduated low pass effect along the cochlea so that
secondary, "resonant" points are diminished wrt the initial point where
the phase differencial reaches 180 degrees.

-The depth of the middle ear will determine the upper frequency that is
audible. Assuming the delay to the round window, relative to that
through  the ossicular chain, is one half the wavelength of that upper
frequency.


-------------
Steve Ungstad

10/18/01





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