--- In Walter Knapp <>
wrote:
> There are a number of things going on in a parabolic as you go off
axis.
> For one, there are now phase differences from the differences in
> distance from source from the near and far side of the reflector.
So,
> you will get some comb filtering that's less present in the on
axis
> situation. The comb filtering will effect the higher frequencies
to a
> greater extent than the lower ones. And that's probably most of
what you
> are hearing.
Yes, there are many other physical effects taking place in a real-
world reflector. Unfortunately it does not only reflect sounds. It
may also absorb a fraction of the incoming energy and the dish may
start to vibrate (at high frequencies we can not perceive) to some
extent. These two effects should also depend on the frequency of the
incoming sounds and they will therefore influence the overall
frequency response of the parabolic microphone. This can be seen
from the citated plots I posted recently
(www.avisoft-saslab.com/parabolas.htm).
There are significant differences between a parabola made of plastic
and one made of aluminum. Even within different parabolas made of
plastic might be differences (with respect to stiffness and surface
properties).
So I would agree with Walt, that there is not only the acoustic wave
theory to consider.
Raimund
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