Difference tones

robert grimble <cthulhu@pipeline.com> wrote:
> Fred ---
>
> I understand the concept of the difference tones.
>
> However, the effect I am speaking about is a little more complex.
>
> If your ear would "hear" the difference tone of two notes, then a slight
> mistuning would not suppress the effect; i.e., if you played something
> that was almost a 2:3 fifth (for argument's sake, lets say it is a
> 2:3.001), you would hear a tone that is very close the the "1" -- 1.001 in
> that example.
>
> In practice, when the 2:3 interval is hit, you suddenly get another effect
> where what you perceive is a a very pure note below the two notes actually
> played.

This is still the difference tone - it just sounds 'very pure' because its
upper partials stop beating against the incident tones and their partials.

> As I say, it is a really startling effect, at least to me. I hade been
> listening to music and playing music for 30 years before I heard it
> demonstrated, although I had read about it, and my jaw just dropped. I
> think you need to have a really stable signal for it to work, a synthesizer
> or a well maintained pipe organ

A well-maintained pipe organ, if equal tempered, would have no pure
intervals.

> Bob G.

I would surmise that what you are describing is the unique psychoacoustic
effect that one experiences when the difference tone falls *exactly* into
a harmonic series which includes the sounded tones and all their harmonics,
the implied fundamental of which is within the frequency range of the ear
(above 20-25 Hz). All of the heard frequency components (incident and
ear-generated) are then "phase-locked", and the perception of consonance is
greatly heightened. This is exactly the 'drug' which all us JI junkies are
addicted to. Incidently, in the pursuit of my psychoacoustical studies, I
invented an electronic instrument which I call a "Harmonichord" which
simultaneously generates all the tones of the harmonic series up to the 32nd
harmonic _phase-locked_ to the fundamental. Any combination of harmonics can
be played by a two-dimensional touch-membrane keyboard. The fundamental is
tunable over a three-octave range up to about middle C. This is the instument
which I use to explore the phenomenon of difference tones.

Fred R.

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