"Bob Cain" wrote in message
...


Porky wrote:

"Vladan" wrote in message
...

After so many words spent on the effect in regard to speaker, when are
you to start examining it in regard to (dynamic) microphone.

And don't forget the air. Molecules are moving. There must be some
dopler there, too.

In a microphone, the diaphragm excursion is so small that any possible
Doppler shift would be negligable, even assuming that there is Doppler
shift
in a mic.

Nonetheless, even if the mic can measure pressure or
velocity at a point with zero excursion there is still a
non-linear relationship between the motion of a tiny
zero-mass test particle normally at rest at that point and
the pressure/velocity measured there. Remaining tubular for
the nonce, if we had a mixture of a 40 Hz and a 2 kHz wave
each at 94 dB SPL and if the motion of the test particle is
that of the signal then the pressure at its rest position
will show about 0.024% IM distortion relative to the HF
fundamental and sidebands about -75 dB down from it.

If we get into Doppler shift due to motion in air molecules, I suspect
we're getting down to "the bumble bee doesn't really fly because the math
says it can't" point.

The thing that concerns me, now that I can put numbers to at
least the tube conditions and see that they can get big, is
that if conditions are such that in the very near field of a
speaker a signifigant percentage of its LF velocity is
coupled to the air then the Doppler effect can get _very_
signifigant because of the large excursions needed to offset
the poor far field coupling. My hope is that a retarded
wave cancels most of that up close so that it won't grow to
too great a signifigance.

The degree of near field LF coupling for a given far field
measure from a speaker in an enclosure is the next thing I
want to know.

If the equations show that all that much Doppler distortion in a speaker,
why can't we hear it?
By Occam's Rasor, either our hearing mechanism has built in compensation,
so Doppler distortion doesn't matter, or the math is wrong and it needs to
be revised. That isn't to say that it doesn't happen in the
piston-in-an-infinite-tube model, it just means that the speaker/room model
is a totally different animal.