>What PZM is about is a mic element mounted facing a boundary and close
>enough to it that direct and reflected sound arrives together. This
>increases the gain of the mic, and helps to flatten the frequency
>response. The original SASS has two tiny mics mounted in holders facing
>the boundary planes. The holders actually form the gap in that case. The
>gap width determines the upper frequency for the setup.
The original PZM experiments, done by Ed Long, used a conventional
omni condensor mike mounted with the nose almost touching the
reflecting plane. Then Wahrenbrock (sp?) turned it into a product
with a little Knowles electret mounted on an arm. Subsequently Crown
took that design over. The upside-down mounting isn't necessary or
even desirable.* Subsequent boundary mike designs have used a
forward-facing diaphraghm level with the surface. That's the point,
getting the diaphragm to merge with the surface so the surface
reflection is used without having any distance difference between
direct and reflected waves.
-Dan Dugan
* Well, I suppose it smooths out that high-frequency directivity lobe
that most omnis have, but it introduces problems of its own.
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