From: kludge@grissom.larc.nasa.gov (Scott Dorsey)
Subject: Re: Pressure Zone Microphones
Organization: NASA Langley Research Center and Reptile Farm
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In article <1rjobdINNk1s@crcnis1.unl.edu> mpaul@unl.edu (marxhausen paul) writes:
>I remember, back before commercial mikes with the "PZM" label (for
>Pressure Zone Microphone) started showing up, I read an article,
>in some engineering journal, that described the principles involved.
>All the physics aside, from a practicle standpoint the author's said
>they built a prototype with nothing more than a small Sony lavelier
>mike laid up against a large aluminum disk.  In demonstrating it at
>a presentation to audio engineers, this was impressive enough that
>the people present were bugging them about where they could buy them.

Yup, it's not all that difficult to do.

>Anyway...I want to build some to use as "choir mikes" (wide coverage).
>I've had good luck using some small electret mike elements I got
>from Digi-Key - most of these are limited-bandwidth, peakey units
>but for a buck or two more I got the ones spec'd to go out to 20kHz
>with a bit flatter response.  

The Panasonic cartridges aren't bad, but they aren't spectacular.

>The distance of the mike opening from the flat plate is kind of
>critical.  I'm debating whether to mount it on it's side, which
>puts the opening a shade under 2mm from the plate, or mounting
>it with the opening actually pointing down into the plate at a
>small clearance.  I haven't dissected a unit like you can buy at
>Radio Shack to see how they do it.  Thoughts?

Don't mount it on the side.  Call up Crown, the company that has the
patent on the PZM, and ask them for information on construction and
use of the things.  You may have to determine the correct spacing
emperically with a noise source and a spectrum analyzer because the
design of the Panasonic cartridges uses a tuned cavity in front of
the electret element to increase the high frequency response, and you
are going to be altering the resistance through the cavity entrance.

Crown has a nice book on the subject, though it's unfortunately rather
short on mathematics.  There's a JAES article from many years back, too.

Still, for your application, you'll be a lot better off buying the cheap
Radio Shack models and using Phil Rastoczny's modifications to get a
clean balanced output.  Phil's mods seem to get reposted here on a regular
basis.
--scott
