<html> Howard, That's a nicely put and interesting question.  I think the answer lies in the relative volume of the tones. When we are listening to beats, we are hearing two tones which are summing their waveforms, which makes the volume alternate between the combined volume (when the peaks of the waveforms coincide) and  the difference volume (when the peak of one coincides with the valley of the other).  This makes the tones themselves get louder and softer, and the beat is very easy to hear.  The pitches themselves are loud, so their variation in volume is fairly wide. If you speed up the beat until it is fast enough to become a pitch, you would then be producing a difference tone, but the volume of that tone is extremely low, nothing like the easy to hear variations in the actual string tones.  You can sometimes hear it when playing thirds in the mid-treble loudly, where you get kind of a low murmer or growl of a pitch.  Once the variation in volume of the piano tones becomes too fast to hear as a beat, those tones sound steady.  The beat is still there, but our ear perceives it as a pitch because the volume of the two tones is changing at a high frequency.  It isn't an actual string vibration, though, so nothing is radiating it into the air at a loud volume.  It is only a change in volume of the two notes, so it forms a relatively quiet tone. If we could hear difference tones at the same volumes as the partials themselves, boy the piano tone would sure change! I understand that large pipe organs are set up to produce "resultant" tones for deep bass, where say 32hz and 48hz and 64hz pipes are played together and you hear a low C 16hz.  When you add these tones up you get the first 4 harmonics of C-0, so you can hear and feel C-0 fairly strongly.  I have heard this effect on some well tuned pipe organs, and it is wonderful. Don Mannino RPT </html>