> I'm not sure what you mean by "extinction point." My dish has a 4"
> (102mm) focal length. Does that mean the lowest frequency it can
> resolve is 406.4mm, or about 838 Hz, no matter how big the dish
> diameter?
John,
This is only a first approximation but an omni mic in a parabola at
the focus will pick up a direct signal plus a reflected signal,
Assuming these are equal (which they aren't exactly) the direct signal
will be half a wavelength different from the reflected signal when
wavelength =3D 4 X focal length and they will cancel out at the "first
extinction frequency".
Half a wavelength =3D 2 X 103mm or 1 wavelength at 406mm equiv to
freq =3D 340,000mm per sec / 406mm =3D about 840Hz, as you said.
In practice, this won't be accurate, but there will be at least a
strong dip around this frequency. The next dip is at 1 1/2 wavelength,
2 1/2 and so on, with peaks at 1, 2, 3 wavelengths where the two
signals reinforce giving a "comb" frequency response.
This all gets very complex with directional mics and away from the
prime focus and with oblique sounds and this also applies to gunmics
off-axis.
You will get a comb response whenever the outputs of two
non-coincident mics are mixed with any rig.
A music effect which is produced by this type of comb filter is
"flanging" and this sounds like "eeeyyyaaaoooaaayyyeee" when it it's
varied. Try it with two mics pointing the same way. The flanging stops
when the capsules are side by side.
David
David Brinicombe
North Devon, UK
Cogito cogito ergo cogito sum - Ambrose Bierce
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