John,
The problem with frequencies around 500Hz is that a parabola will not
focus these frequencies well and the increase in level will be low. A
530mm 21 inch parabola is smaller that the wavelength at 500Hz, so the
forward gain will be small.
The fundamental parameter for a parabola is its focal point which is
level with where the profile gets to 45 degrees. This is the optimum
point for high frequencies but you could try other distances for the
seals.
The "extinction point" is theoretically where twice the focal length
is half the wavelength. At 334 Hz the wavelength is 1 metre, and the
"extinction" focal length 250 mm or about 10". Note the word
"theoretically".
For a parabola to work, you need to use a wide pickup angle mic or you
will be wasting much of the reflector surface. Using two mics when the
capsules are separated will introduce other cancellation frequencies.
However the response of a gunmic at 500Hz will not be very narrow
either and there will be a lot of other pickup. Try my trick of
putting the ME66 on or near the ground. The other option is if you can
find a large natural baffle like a cliff or a building to reduce the
unwanted noises. Ideally you want a parabola ten times the size. :-)
An open boat perhaps, but that would be better used to get closer to
the seals.
However, as ever, try different variations and see what happens.
David
David Brinicombe
North Devon, UK
Cogito cogito ergo cogito sum - Ambrose Bierce
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