🚀 go-pugleaf

myers@hpfcdj.HP.COM (Bob Myers) writes:

>>>And  I've always wondered: can two ultrasonic sounds or an ultrasonic
>>>and audible sound beat to create an audible tone?
>>
>>	Obviously, yes.

>Wait just a minute, John.  As SOUND, the answer to this question is *no*.
>You will not, for example, hear a 1000 Hz tone when exposed to *sound sources*
>of, say, 20 kHz and 21 kHz.  The ear doesn't work that way.  (The
>phenonenon wherein one may tune a stringed instrument by listening for a
>"wavering" sound when one string approaches the frequency of the other is
>a case of interference producing a tremolo (varying intensity), not a vibrato
>(varying frequency).)  (And I hope I got the musicalese right - it's been a
>while!)

Excellent point!

>However, if such signals exist in an electronic circuit (as was the case in
>the instance you mention, with the ultrasonic pilot tone producing nasty
>results when mixed with 60 Hz), then these "beat frequencies" WILL very
>likely occur, as intermodulation occurs due to the non-linearities present
>in any amplifying device.


As Bob has pointed out, the key here is nonlinearities.  Interestingly,
the visual analog of this DOES work, i.e. the sum of two high frequency
gratings (which cannot be resolved) can produce a visible beat.
[To actually get these gratings on the retina, you have to use
laser interferometry, otherwise the optical point spread function of
the eye would blur them away].  This demonstrates not only the
neural nonlinearity, but also the fact that these high frequencies
ARE passed by the early stages of the visual process.
In order to hear beats between ultrasonic tones, it
would be necessary both to have a nonlinearity in the auditory
system, AND a cochlea etc. that would pass the ultrasonic tones
at least as far as the nonlinearity.  I suspect that the ultrasonic
tones don't make it as far as the nervous system for mechanical reasons.


-- 

	Jeff Mulligan (jbm@eos.arc.nasa.gov)
	NASA/Ames Research Ctr., Mail Stop 262-2, Moffett Field CA, 94035
	(415) 604-3745