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April 7, 2011

To: Andrew Bell, Michael Rapson and the cochlear-amplifier@xxxxxxxxxxxxxxxxxxxxxxxxxxxx

In reading the many comments on the properties of the cochlear-neural processing of sounds I note some concepts that do not seem to be given much attention in the discussions, though generally there is acceptance of the presence of some active amplifying processes in the sensory system.    I offer consideration of a few points that may aid integrating the various observations and theories towards understanding better, the whole-animal response as part of the concerns.

+ The response of both ears via the CNS should be fully considered in evaluating the role played by the various elements of the peripheral sensors, which should also include the whole body sensing of very long wave length acoustic energy.

+Note for example, that the ISO-220 (2003) revised Equal Loudness Curves reveal an extraordinary rise in low frequency SPL required for equal loudness, over the entire spectrum below 1 Khz.      A look at those curves also shows that the threshold across the range below 1 kHz is correspondingly extremely elevated.   These 2003 curves show that:  @ 31.5 Hz free field Equal Loudness sound of  20 phon requires 77dB more SPL than the 1 kHz  SPL;  40 phon  requires 34dB  more SPL and 80 phon  requires     30 dB more SPL //    FURTHER note that

 at 63 Hz the increases for equal loudness are somewhat smaller; 20 phon requires   48 dB more SPL than the 1 kHz SPL;  40 phon requires 33 dB more SPL; 80 phon requires 18 dB more SPL .

This loss of sensitivity at the longer wavelengths corresponds with the relative increased level of what might be termed background low frequency noise level from natural low frequency energy (even in a natural environment outside of populated areas).

The role of noise – particularly 1/f noise as a stimulating component of sensors (dither?) should also be considered as part of the system analysis and of course in model development.

+ Interestingly, a whole animal body responds to acoustic and ground or contact-borne long wavelength vibration. This all suggests that the role of sensory processes should be considered from a perspective of the entire information receiving system – i.e. whole animal response. 

It’s not just “Das Ohr” but the whole system response to radiation of “acoustic” vibratory energy and the information it carries. 

Regards to all,

Mitchell Cotter