i am no mathematician, but i have variations of a five
capsule array (the largest spacing beteen capsules is
2 1/2 inches, the smallest is 1/2 inch) for 20 years
and have often hung one in front of a crudely made
parabolic dish (reflectors from building luminaires,
mostly, but the odd pot as well).
low mic noise is important, but not so important as it
would be for a naked mic.
if one does not look for the full 20 30 db gain a
parabola is capable of, but is satisfied with about 10
(because the reflector is not a parabola, in fact) one
can get fairly flat frequency response (used it to
record interviews in the desert). of course a capsule
mounted on flat plastic plate of about 6 inches has a
boundary gain of about 6 db at speech frequencies, and
the five capsules give me a noise reduction of about 6
db. that helps too.
I suspect there are boundary effects on a parabolic
reflector that come into play at lower frequencies,
which is why we still have gain below the half
wavelength point. 1/6th wavelength is the figure
quoted in crown's pzm manual.
p.s. i once mounted my array microphone in a
re-entrant horn which was lying around. sounded
terrible, huge gain!
umashankar
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