Ben Bradley wrote:



If you made a transducer that increases and decreases the air
pressure without using physical movement, there would be no doppler
distortion.

Impossible on first principles of acoustics. Increasing and
decreasing the air pressure results in totally predictable
changes in the velocity of the air. The are simply
proportional through the (real) characteristic impedence of air.

If the SPL is high enough, yes, nonlinearities occur in the
air and the above isn't true but you have to get pretty
darned high for that to have any signifigance. At the
levels we listen to, air is highly linear.

My argument is simply that if you can reproduce velocity of
air then by the above, the pressure has no choice but to
remain in phase and proportional if it remains in the linear
regime. If you can measure it you can reproduce it by
moving a piston with the measured velocity. Exactly. The
resulting pressure wave contains no distortion.

The above argument stands whether we are talking about
reproducing pressure or velocity because in air they are in
phase and proportional in a plane wave and deviations from
planarity only have linear consequences.


And a heads up, "Porky" over there is quite similar in demeanor to
"Phil Allison" here on RAP.

Actually, Porky has been nothing but congenial and careful
of late. It was on that tentative basis that I chose to
address his post.


Bob
--

"Things should be described as simply as possible, but no
simpler."

A. Einstein