Referring to the article in digest 951116 by Richard Weisenberger 
(forwarded from piporg-l group) --

If a side-by-side comparison concert were to be arranged the 
electronic organ, with it's audible distortion, would be noticeably 
inferior. (Fortunately for churches and manufacturers, such comparison 
situations are rare.)

The knotty problem in high-power audio systems remains in the 
loudspeakers: they are still the main cause of intermodulation 
distortion.  At low power levels one can get a pretty clean sound 
nowadays from the variety of consumer loudspeakers.  It's fun to 
compare them in a hi-fi shop, while playing your own favorite 
(hence well-known) piano CD.

A thoughtful old-timer engineer explained why multiple-port speakers 
(with woofer, squawker & tweeter) were developed:

"Imagine that you are hearing two adjacent loudspeakers.  One (the 
woofer) radiates a low frequency sine wave and the other (the tweeter) 
a high frequency sine wave, at equal sound pressure [equal perceived 
intensity].  Both radiators are stationary except for the sine wave 
motion of the loudspeaker cone.

"Now imagine that the two sine wave signals have been combined, to 
radiate from only one loudspeaker.  Low frequencies demand long 
excursions of the cone, but the high frequency barely wiggles it.  
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."

And the result is a multitude of new, unwanted frequencies, clustered 
about the high frequency sine wave.  This is called intermodulation 
(IM) distortion, and it can be _very_ unpleasant.  Remember, in the 
50's, how the finest of the high fidelity phonographs were demonstrated 
using recorded bells?  The ear is very discerning about these familiar 
percussion sounds, and the smallest IM distortion in the system becomes 
a literal pain-in-the-ear!

One realization of the "ideal loudspeaker" might be a disk or a cone, 
such that the active ring of vibration moves inward with increasing 
frequency, thus precluding Doppler modulation of the high frequencies 
by the lows.  If this approach were feasible we wouldn't need 
loudspeaker arrays. ...

-- Robbie Rhodes