[sdiy] Phase shifts and instantaneous frequency

[Ian Fritz]

You can *easily* demonstrate to yourself that differences in phase are, in 
fact, audible.  To do this, make a signal with two sine waves at 100 and 
201 Hz.  This is a signal with a slowly varying phase difference between 
the two components.  If the phase differences don't matter, then you will 
hear a steady tone.  If they do matter, then you will hear beats.  In fact, 
you *do* hear beats.  Yes, you can easily detect phase differences between 
two tones.  (I learned about this from one of the websites I ran across 
during our previous discussion of this topic.)

What's going on?  The beats are a result of nonlinear mixing in the 
ear!  They do not occur at low volume levels, or if the signals are 
presented separately to each ear.  You can easily demonstrate this to 
yourself, also.  The nonlinear mixing produces a difference tone at 101 Hz, 
which beats against the 100 Hz signal.

Try it!

   Ian