Path: utzoo!utgpu!news-server.csri.toronto.edu!clyde.concordia.ca!uunet!wuarchive!sdd.hp.com!ucsd!ucbvax!hplabs!hpfcso!hpfcdj!myers
From: myers@hpfcdj.HP.COM (Bob Myers)
Newsgroups: sci.electronics
Subject: Re: ..but what about _output_ filtering for D/A's?
Message-ID: <17660121@hpfcdj.HP.COM>
Date: 17 Oct 90 18:50:27 GMT
References: <1319@beguine.UUCP>
Organization: Hewlett Packard -- Fort Collins, CO
Lines: 24

>>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!)

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.


Bob Myers  KC0EW   HP Graphics Tech. Div.|  Opinions expressed here are not
                   Ft. Collins, Colorado |  those of my employer or any other
myers@fc.hp.com                          |  sentient life-form on this planet.