Raimund Specht wrote:
>
>
> 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.
Much of what you mention is much the same on or off axis. Including the
citations you gave, which are, as near as I can tell, only the on axis
for a particular tested parabola, or a limited number of them. Certainly
unwise to believe that all reflectors behave the same. Syd's question
went to the changes as you move off axis. Which, of course is not all
that simple either. These differences are a separate problem from on
axis design, though some of it is related.
A difference that you did not note is the difference between the
relatively flat larger parabolas, which are mostly what's been tested in
the publications, and the much deeper style, of which the Telinga is a
example. The homemade I replaced with the Telinga was of the flat style,
with the focus way out in front of it. I had to baffle it's mic to keep
down local pickup. When I went to the Telinga there were a number of
differences that seemed to be due to this curve difference. I'm
beginning to think it may be better to use deep parabolas regardless of
size. I need to find time to examine that question more.
In the case of the Telinga we have to keep track of the mic elements
too. The entire system version uses mics that have characteristics quite
different from just putting a single regular mic, or even a pair of them
out there. As I noted, the DAT Stereo element is both multicapsule and a
boundary mic (PZM style). It is the interaction of all this that gives
the Telinga system like I use it's properties.
I'm not inclined to think of parabolics as all having one sound or
characteristic specs except maybe in the broadest of generalizations.
Like other mics there is considerable variation.
Walt
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