Subject: Re: What do we hear in High frequency hearing From: David Mountain <dcm(at)BU.EDU> Date: Thu, 7 Nov 2002 16:41:30 -0500Many 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(at)bu.edu Website: http://earlab.bu.edu/dcm/ Phone: (617) 353-4343 FAX: (617) 353-6766 Office: ERB 413