Re: What do we hear in High frequency hearing

[David Mountain <dcm@xxxxxx>]

Many years ago, when I had more hair cells, I found that I could hear
steady-state tones up to 25 kHz and had a roommate that could hear to 28
kHz. The perception was similiar to any other very high frequency tone.
No doubt there is considerable individual variability in the
high-frequency limit for normal human hearing that depends on a
combination of our genetics, our age and how gently we have treated our
ears.
 Those of us who work in cochlear mechanics generally believe that
the high-frequency limit of the audiogram is due to the characteristic
frequency at the very basal end of the cochlea (Ruggero and Temchin,
2002).  In the case of hearing ultrasound via bone conduction, it would
seem that the acoustic signal would have to be bypassing the usual
cochlear filtering process and stimulating the hair cells via a
nonconventional path.  Otherwise, the very steep high-frequency slope
associated with cochlear tuning would have severely attenuated the
stimulus.

As for 96 kHz sample rates, remember that sampling theory only says that
it is POSSIBLE to reconstruct signals that are sampled above the Nyquist
rate, it doesn't tell us HOW.  Accurately reconstructing signals that are
sampled near the Nyquist rate using analog filtering on the output of a
digital-to-analog filter is a non-trivial task.  Oversampling the signal
makes reconstruction much easier.  I would guess that the subtle
perceptual differences between conventionally sampled audio and
oversampled audio probably have nothing to do with hearing over 25 kHz but
rather with the accuracy of the reconstruction of the signal below 25 kHz.

References

Ruggero MA, Temchin AN. The roles of the external, middle, and inner ears
in determining the bandwidth of hearing. Proc Natl Acad Sci U S A. 2002
Oct 1;99(20):13206-10.


--------------------------------------------------------------------

David C. Mountain, Ph.D.
Professor of Biomedical Engineering
Boston University
44 Cummington St.
Boston, MA 02215

Email:   dcm@bu.edu
Website: http://earlab.bu.edu/dcm/
Phone:   (617) 353-4343
FAX:     (617) 353-6766
Office:  ERB 413