"Bob Cain" wrote in message


I've got an argument that so far has withstood some scrutiny
which shows that Doppler distortion in a myth.

What would refute it and point out any flaw in the reasoning
would be the dynamical expression for the time varying
function of the pressure wave in an infinite tube with an
ideal piston as a function of an arbitrary, time varying
function of the force applied to that piston.

Fool that I am, I'm kinda stuck down here in the real world. Forget the
math, forget the long-winded discussions, the question that interests me
most is whether or not there's Doppler distortion where it really matters -
in the sound field in front of the speaker.

A couple of us have been pursuing the measurement route, and as often the
case the results are practically speaking, not all that startling.

First off, we've found that actually measuring Doppler distortion is not all
that easy. This is complexified by the fact that speakers have lots of
distortion of many kinds, and at most loudspeaker Doppler distortion is
relatively small. BTW, along the way, I've found reason to doubt a lot of
published jitter measurements. They don't seem to distinguish AM from FM.

But bottom line, we think we are measuring some Doppler distortion.
However, there's so many other kinds of distortion of a similar nature
happening at the same time, that it's practically a non-issue.

People who like looking at raw evidence can visit:
http://www.pcavtech.com/techtalk/doppler/ .

It might be most informative to compare these two sets of graphs and data:

"Triple Tone Lab Measurements - 316 millivolts RMS" and "Triple Tone Lab
Measurements - 10 Volts RMS"

Triple Tone Lab Measurements - 316 millivolts RMS shows a speaker operating
at a 1 meter SPL of about 78 dB. The background noise in the room actually
masks harmonics generated by the 50 Hz fundamental. Most visible distortion
products are 60 dB or more down (0.1%). Of course 0.1% is an absolutely
rediculously huge amount of distortion compared to the 0.003% THD that some
on RAP seem to want to worry about.

Triple Tone Lab Measurements - 10 Volts RMS shows a speaker operating at a
1 meter SPL of about 105 dB SPL. Don't be confused by the 1 meter SPL of 105
dB, out in the room the SPL is loud enough, but fairly modest (under 100 dB)
by modern standards.

Frankly, with 10 volts applied to its voice coil, this speaker has been
turned just about every which way but loose. 50 Hz THD is some place around
10%. IM is around 3%. It sounds pretty badly stressed, in real life.

The driver under test is a Peerless 6.5" speaker that is similar to what you
might find in one of the better near-field monitors. I would imagine that a
speaker like this would be rated for a maximum 1 meter SPL in excess of 110
dB. You can imagine how distorted it is under those conditions!