> > It seems that in order to capture the amplification benefits of
the
> > reflected energy, the microphone's element must be nearly (to a
> > minuscule dimension for high frequencies on a wavelength basis) at
> > the interface in order to benefit and avoid comb filtering
boundary
> > effects. I'm not sure a microphone assembly with embedded capsule
> > allow the reflective surface to get close enough to the energy
> > coupling region of the capsule. What do you suppose the test
setup
> > consists of to look for comb filtering in the frequency response?
Is
> > it just a matter of getting a calibrated sound source and
sweeping a
> > tone? Probably exploring the spacial relationships also.
> >
> > One thing about the SASS concept that sort of confuses me,
regarding
> > reflected energy amplification, is that how can it be so small
> > relative to a parabola, when we need large parabolas in order to
> > capture low frequencies effectively?
>
> PZM and boundary effects are a different world and don't use
reflections
> the same way. Many of your questions will be covered in some detail
in
> Crown's Boundary Microphone Application Guide, the essential
starting
> point if you want to get to understand this stuff:
> http://www.crownaudio.com/pdf/mics/127089.pdf
>
Further to that point, the biggest virtue of PZM/boundary technique is
that it places the mic element AT (or very very near) a boundary, thus
there's no reflections from that boundary to cloud the image. There's
a theoretical 6dB gain at the boundary itself.
> I have the same doubts about using complete microphone assemblies
in the
> SASS enclosure. I do know that Lang's setup does work very well,
I've
> looked at it and listened to the results. The question is more can
it be
> made even better?
With boundary techniques, the idea is to pick up pressure, not
velocity, and any directionality comes from the boundary. So one only
needs omnidirectional mic elements.
PZM's are a patented approach to boundary miking where a tiny mic
element is so close to the boundary that the gap does not produce
comb-filtering in the audible spectrum. But you can also achieve the
benefits of boundary miking by positioning the mic in a hole in the
boundary so that the mic element is in the same plane as the
boundary... no reflections. So a nice quiet omni like the MKH 20 works
fine installed into a SASS baffle, when the condenser membrane is
flush with the boundary surface.
Parabolic mics are reflectors; they reflect and focus the soundwave
from a bigger area down to the smaller surface area of the mic. The
PZM directional arrays are more like horns; they concentrate pressure
from a bigger area at the mouth of the horn and create a bigger
pressure difference at the mic.
Some interesting notes on PZMs and making arrays at
http://www.vectorbd.com/bfd/broadcas/boundry.txt
.... but I couldn't find the images online. You can probably request
this literature from Crown.
It's easy to make experimental arrays like these - you can use any
flat workable material like masonite, lexan, acrylic, thin plywood...
and any decent omni, even electrets can be used.
I made a "SASS"-like baffle out of polyurethane packing foam - it's
very dense and firm but resilient, and I used the venerable
Radio-Shack PZM mics in it, and the image is quite good, so don't be
afraid to go this route either.
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