<html> Hey again, There is definitely something to what Sarah and Bob Scott are saying.  The character of the "resultant" tone changes dramatically depending on how the sounds are played back. In the high-end Sennheisers plugged directly from the Soundblaster Live card, the 500 hz tone sounded very soft and very pure (sounds like a sine wave).  When sent out digitally to my home receiver (which means the sound card is converting the digital output to the more standard 44.1kHz sample rate) and played through a very high quality home stereo, the 500 Hz tone was quite lout and more square sounding - harsh and distorted.  In the headphones the two high tones were just audible, in the home speakers only noise was audible. When played through my laptop computer speakers, the 500 Hz tone was also very loud and distorted.  When played through my home computer speakers, there was no 500 Hz sound at all (just a little noise), I think because they can not produce the 20kHz tone at all. Well, this was a fun experiment.  Mick, I had Cool Edit create the tones directly - that is the cleanest source of pure sine waves I have, and they worked very well.  That way the sound card wasn't relied upon to generate the tone, only to play it. Don Mannino </html>