> Note that the 'gain' is actually an increase in the acoustic volume at
> the surface of the boundary, hence the name Pressure Zone Microphone
> of the original commercial implementation of this principal. So the
> microphone simply has a louder acoustic signal presented when the
> diaphragm is in the pressure zone. Thus less preamp gain is required,
> & less gain means lower noise in the resulting recording, even though
> the self noise of the system stays the same. I seem to recall that the
> pressure zone is about a quarter inch deep in the audible band.
Well, it's actually about thirty feet deep at the bottom end...
There really isn't a "pressure zone." Boundary mics get a 6 dB acoustic boost
without coloration (comb filtering) only when the wavelength is several times
the distance from the capsule to the surface. That's why the original Ed Long
experiments used an omni mic pointing face-down about a millimeter above a
boundary--to get it close enough so that the comb-filtering effects were up
above the audio band, and why typical PZMs today have an electret capsule
mounted on an arm facing the boundary with a tiny gap.
Here's an illustration of one of the experiments that led to the development of
PZMs:
http://i227.photobucket.com/albums/dd145/gadget69/RTAsetup003.jpg
"PZM" is Ed Long's trademark that he sold to Crown. The generic term is
"boundary mic."
The stereo boundary mic arrays that have been shown by experimenters here,
where an omni mic is strapped to a board, are very crude in comparison, and
would reveal awful comb-filtering from any particular direction. The fact that
such arrays can sound good is a testament to how forgiving nature recording can
be. Note that there are no frequency response charts for the MBHO boundary
mics. Also their description of the mic as "pressure gradient receiver" is
totally wrong.
This is why a Jecklin disk is padded--to kill the boundary effect at higher
frequencies.
For a quick tutorial see http://www.crownaudio.com/pdf/mics/136367.pdf
-Dan
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