<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"> <HTML><HEAD> <META http-equiv=Content-Type content="text/html; = charset=iso-8859-1"> <META content="MSHTML 5.50.4134.100" name=GENERATOR> <STYLE></STYLE> </HEAD> <BODY bgColor=#ffffff> <DIV><FONT face=Arial size=2>Hi again...</FONT></DIV> <DIV><FONT face=Arial size=2></FONT> </DIV> <DIV><FONT size=3><<I have attached a small jpg to this = message.  You can see a small piece of the waveform of the file I made.  The left = part is the 20kHz tone, then the 20.5kHz tone is added.  The amplitude = modulation you can see (which is another way to look at beats), when put into the = air, is moving the air in a pattern of a 500Hz sine wave - just what one = hears.  It is my opinion that, even though the two high tones themselves are not = easily audible in this test, the amplitude modulation of the tone at 500Hz = becomes audible on its own.>></FONT></DIV> <DIV><FONT face=Arial size=2></FONT> </DIV> <DIV><FONT face=Arial size=2>Amplitude modulation is an entirely = different beast.  It looks *similar* and has similar properties, with one = very critical exception:  Amplitude modulation is a nonlinear process = that creates heterodyne bands (i.e. scattering energy to new = frequencies).  Beating from destructive/constructive interference is a linear process = that does not create spectral scattering.  Amplitude modulation is = multiplicative, not additive.  A classic example of amplitude modulation is = Anderson (from Jethro Tull) singing into his flute.  Another classic example is = "ooting," the practice of humming while whistling.  These sounds are very = "strange," and one can easily hear the spectral scatter.</FONT></DIV> <DIV><FONT face=Arial size=2></FONT> </DIV> <DIV><FONT face=Arial size=2>Peace,</FONT></DIV> <DIV><FONT face=Arial size=2>Sarah</FONT></DIV> <DIV> </DIV></BODY></HTML>