Dan Dugan wrote:
>>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.
In the case of the MKH110 or less so the MKH20, there is a open gap
between the diaphragm and the rest of the boundary. Yes, you do seal the
join, but not at the boundary surface. In both Lang's and my design the
seal is behind the boundary surface with the thickness of the boundary
as the gap depth. Plus there is a additional gap inside the mic housing
that goes deep into the mic. I'm not sure how much the boundary is
contributing, and how much is just the central barrier and mounting angle.
A lot of my purpose in building this was to get more familiar with what
was and was not critical. Then from there I can go on to other design
shapes I have in mind with less alternatives to build.
I did find that placing a cap on the mic with the small gap made little
difference to the sound. That was one of the questions I had. If nothing
else, getting that cap installed and adjusted was annoying, so it's nice
it came out like it did.
I reserve PZM more or less for a Crown style design with the mic over
the boundary, and call the rest just boundary mics. Not sure if that is
the best way to go about naming these devices, but it kind of looks like
the way Crown views it.
I'm already into round two of the design. I'm making a 2nd modified SASS
that I will use with the pair of MKH40's I have, (and the pair of
MKH20's I just won on auction). I'm working on making this design much
easier to install and remove the mics. Less parts involved, simpler to
machine, easier to line everything up.
The MKH110's have developed a annoying whine. I'm going to have to make
the power circuit more elaborate I think. I'm still tracking it down. It
first turned up with the thunder recording. You can hear it in what I
put up, the mp3 encoder made it worse.
Walt
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