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Evanescent waves in the cochlea.

Dear colleagues,


Better late than never: evanescent (standing) liquid-sound-pressure waves appear to play a fairly important part in the cochlea. In my paper "Old and New Cochlear Maps", Canadian Acoustics Vol. 37, No. 3 (2009) 174-175, I introduced a "BMR map" defined to yield, for a given distance-from-base, the resonance frequency of the local basilar-membrane (BM) resonator, calculated under the assumption that the liquid "above" and "below" the cochlear partition is absent. As explained in the just mentioned paper, at that resonance frequency the imaginary part of the local BM impedance vanishes. With liquid, however, evanescent waves generated by the oscillating BM decrease the local BM resonance frequency, by typically half an octave, so that the local with-liquid resonance frequency is about equal to the "characteristic frequency" (CF), i.e., the frequency which in a healthy cochlea yields, at the considered distance-from base, the maximal travelling-wave BM excitation by low-sound-level sine-tones. For non-human mammals, the well-known Greenwood maps [JASA 87 (1990) 2592-2605] in my opinion yield approximately that characteristic frequency.


Conjecture: spontaneous oto-acoustic emissions are caused by with-liquid BMR oscillations; at given (assumed) frequency, these oscillations occur near the "characteristic place", so that the emissions can be transported to the stapes by slow retrograde travelling surface waves. I plan to add a chapter on these evanescent-wave effects to my (almost finished) book "Introduction to Cochlear Waves". The following observation is probably relevant: the tapping of a full or submerged wineglass yields a short click if the water is disturbed. A nice tone of a duration of a second or so results only if the water is quiet.



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 .