Subscriber <Linnstaedt@aol.com> sent me this note:

> Dear Robbie, thanks for your very informative article.
>
> > This is equivalent to attaching the high-frequency tweeter to the
> > _cone_ of the woofer, and the result is that the high-frequency sine
> > wave is frequency modulated by the low-frequency motion of the woofer.
> > It's a simple Doppler effect."
>
> In all of this, I am puzzled that we don't experience IM distortion
> in our hearing.  Each ear uses a single diaphragm (eardrum) which
> transmits its vibrations via bones of the inner ear.  Seems that
> the mass of the moving bones should create their own distortion
> through the effects of inertia.  Would love to study this situation
> more.  What are your thoughts?

You're quite right, the ear itself does create distortion.  When you
ride the Merry-Go-Round horse past the real, _live_ WurliTzer 150 or
165 band organ, the sound is quite loud, unpleasantly so.  But at a
greater distance it's jolly and exciting and a pleasure to hear.
I don't know the exact mechanisms, but the ear gets more-and-more
non-linear as the sound level gets greater, and that generates
harmonic distortion products and intermodulation distortion.

Incidentally, for years hi-fi audio amplifiers for years have touted
their ever-lower distortion specifications for Total Harmonic Distortion
(%THD) and Intermodulation Distortion (IMD).  But manufacturers seldom
mention  loudspeaker distortion characteristics.  They're too ashamed!

-- Robbie Rhodes