Marty Michener wrote:
> To put a barrier in a parabola I would say is worse than useless:
> it prevents each half from providing directional sound focusing
> to each mic, so makes the sound amplification less by 3 dB
> at each mic. Why do it? I have read the explanation of
> the PZM effect, and it is all new to me, so I don't pretend to
> understand it. If what you say about it is all true, then
> you are also correct that the sound at which you aim the
> axis would be recorded mono in both mics, and the off
> axis sound would be blocked differentially to the two mics.
> I am not convinced yet that this would amount to a stereo
> recording. It would be nothing like what I am showing in
> my diagram and what I can provide you with hours of from
> my old recordings.
The Telinga stereo mic in it's original form is a boundary mic of the
PZM type. The barrier is in there to provide the boundary. Loose 3 dB
because you have the barrier, gain 4 dB because it's a boundary mic.
Sounds like a wash to me. And may explain my practical (not theoretical)
experience with the Telinga. The Dual Science element (no central
barrier) and the DAT Stereo Mic (central barrier to provide boundary
effect) have, as far as I can tell about the same sensitivity.
There is a difference in the sound from the two mics. For a caller
centered exactly on axis, the Dual Science will give a crisper sound
than the DAT Stereo. As soon as you get off axis slightly it's not so
different.
I also doubt that it's stereo is the same as your x/y setup. But, it
does produce a fairly decent and representative stereo field. I would
say from wider than the 6 degrees you have. Though I've not bothered to
test it.
I'd have to agree, if using non-boundary mic setups the barrier may be
unnecessary, certainly can be avoided. I find it a little surprising
that 2" is enough for two omni's. The parabolic can be thought of as a
lens all right, but it's not typically a very sharp focus over the whole
frequency range. And most lighter dishes flex, so are not necessarily a
perfect parabolic.
Care to analyze a M/S setup? No barrier, point toward the dish. If done
with MKH mics there is about 1 1/4" separation between the diaphragm of
the Mid mic and the Side mic (if using Rycote dual clips or equivalent).
The question is should the Mid mic be exactly centered and the Side mic
above (or below) that? Or should some compromise location be chosen?
Look at the M/S MKH-30/60 to see the relationship of the mics.
http://frogrecordist.home.mindspring.com/docs/ms_mkh30+60.html
For a stereo parabolic the more interesting question on barriers is if
you baffle the mics. With omni's you are not only picking up from the
focus of the parabolic, but direct from the local environment. It might
be better (or at least different) to either baffle if possible or use
directional mics who's pattern is right to have them picking up entirely
or mostly from the parabolic.
Walt
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