[GreenKeys] Incredible . . .

[Craig Sawyers]

> Sound and the perception of sound is a very complex subject.
>
> If you really want to go crazy take a real piece of gear like a
> Teletype machine and do a full spectrum audio snapshot. Then mask off
> the result with the various common filters.

The ear has its own filtering mechanism, interestingly enough.  The three
bones in the inner ear (whose function is essentially a mechanical impedance
matcher between the air and liquid) have two tiny muscles attached - the
tensor timpani and the stapedius muscles.  These are acutally actuated under
different circumstances, and essentially lock the mechanism to attenuate the
sound.  This generally occurs above 70dB, and when you speak (like sidetone
suppression in telephones).

But for repetitive percussive sounds, like a pile driver, the brain locks
into the repetition frequency of the thuds and *only* tightens the muscles a
fraction of a second before the next thud - then relaxes again so that you
can hear quiet noises in between the thuds.

Since the clattering of a teletype has a particularly percussive structure,
those tiny muscles are probably constantly twitching (their response time is
40ms), so changing the frequency structure of the sound might have quite
unpredictable psycho-acoustic consequences.

Damned cunning....

Craig