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Re: active travelling waves, DPOAEs and the Cochlea Amp.
At 12:20 AM +1000 10/5/07, Matt Flax wrote:
My question to the list is :
Can anyone please explain how this travelling wave point of view can
generate a DPOAE when the DP is the 2f2-f1 DP ?
In this case, the DP sits between the oval window and the probe
frequencies f1 and f2.
If you can explain that, then please also explain what happens to the
apical going travelling wave ???
Distortion-product oto-acoustic emissions originate where the
distortion is, which is the region where both f1 and f2 have strong
responses. Odd-order nonlinearities (OHC approximating saturation)
will generate 2f1-f2 and 2f2-f1 about equally, and they will
propagate in both directions, probably preferentially toward the
apex. To me the question is not why we get 2f2-f1, but why it is as
much weaker than it is. As you point out, this may involve
reflection of some 2f1-f2 from the place where it is localized more
apically. Or as Erik Larsen points out, it may be that most of the
distortion is happening at place where 2f2-f1 doesn't propagate very
well. But the fact that you get some 2f2-f1 is in no way surprising.
The apical-going part of 2f2-f1 is quickly attentuated, since it's
past its best place. The part going toward the base, if not
attentuated too much, will quickly arrive at a region where it will
be amplifies on the way out.
I do not have a computational model that will reproduce such
bidirectional nonlinear effects, but I suspect it's not hard. Does
anyone have one? Maybe I'll give it a try some time.