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Re: Difference Tones

Since there are also so called first quadratic 2f1-f2 distortion-product
otoacoustic emissions being applied in audiology, one is tempted to
directly attribute any difference tone alias "terza suoni" or Tartini tone
just to the non-linear relationship between electrical and mechanical
quantities of the outer hair cells. However, this might be premature. Geoff
Wyvill describes in
http://www.cs.otago.ac.nz/ProjectList-2002.html why he has "good reason to
believe that the tones produced by Helmholtz are subtly different from the
Tartini tones associated with resonant strings or pipes and this is why
they occur only for certain notes."
He argues: "Helmholtz's tones are simply differences of frequencies from
two non-resonant sources. Using a computer, we can create the same sound as
a string or pipe, but from a speaker that has no particular natural
resonant tone. Thus we can reproduce the essential feature of Helmholtz's
siren without restricting the tone colour. We can also use Fourier analysis
on the computer to detect the tones independently of a human listener.
Develop the necessary computer programs and conduct the physical
experiments needed to settle these questions."

A modification of the Tartini tones is the likewise monaural "Heterodyne
Effect". As technicians found out, summation and difference tones will
emerge if one passes two pure tones together through a mixer. As examples
for f1 and f2 a South africa besed company
(http://www.soundharmonics.com/ahata/sh_main.asp) gave 100 and 104 Hz. This
is not convincing to me. There are certainly restrictions to the audibility
as resolved tones.

I do not deal with difference tones. Just being reminicent of Dove 1838, I
would like somebody to check in detail to what extent not non-linearity of
the cochlear amplifier is the physiological basis for the perception of
difference tones.

Incidentally, Hpersonic (http://www.acoustics.org/press/133rd/2pea.htmlHSS)
technology is using non-linearity of air to transmit a directable audible
sound via two ultrasound signals.


At 22:34 12.09.2002 +0800, Chen-gia TSAI wrote:
>In the CD of Ernst Terhardt's book 'Akustische Kommunikation' (1998,