> Looks like a standard tripple boundary doubled / corner reflector
> aka the pyramid:) - known art.
Mike,
I put the Brinibox forward as a _stereo project_ rather than a device.
It isn't a "boundary effect" system but a double horn amplifier. Known
technology yes, but what surprised me was that two simple pyramid
horns used together gave a good stereo image. And what could be
simpler in construction than two half boxes?
The other good news is that anyone can cheaply try the idea out.
My take on the PZM is that it is a miniature horn loading also using
the surface and cosine effects. Doesn't look like a horn but
acoustically that's what it is.
I've read all sorts of articles, some of which seem to deny the
existance of a "boundary effect". One article explained the use of
three reflectors saying that each reflector contributed 6dBs making a
total of 18dBs. Such an elementary boner put me off everything else in
that article.
As an ex pro film recordist, I do the experiment first and then work
out why later. An early discovery was the extra directionality you get
from placing a mic flat on a table. The theory behind this is the
surface effect.
With mics placed near a reflector, there are two distinct effects, the
cosine effect due to the longitudinal polarisation of sound waves, and
the surface effect which was once thought due to isothermal
propagation near an object. I have a simpler explanation and want to
experiment further when I get around to it. I think that thin
"absorbent" materials which are too this to have an absorbent effect
will alter the surface effect. I base this idea from using lapel mics
under tablecloths for recording table conversations while filming.
This amazed several of my assistants who didn't think it would work,
but the others used the principle themselves as recordists.
David
David Brinicombe
North Devon, UK
Cogito cogito ergo cogito sum - Ambrose Bierce
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