From: "Greg Clark"
> Remember that more gain is not the only goal. As the dish size increases=
> you also improve the frequency response of the dish. That is another reas=
on
> to try a microwave aluminium dish first. Making large accurate parabolas=
> (bigger than 24 inches) with the focus at the plane of the rim of the dis=
h,
> is difficult.
As dish size increases you do increase gain. I'm not sure the relative
frequency response curve changes that much if the dishes are of the same
depth vs dia. Gain is increasing with increasing frequency in all size
dishes, though theory and actual measured gain can differ quite a bit.
Focus at the plane of the rim is not the optimum. Optimum gain occurs at
something like 1/4 the depth of the dish for focus. But practical dishes
have focus fairly close to the plane of the rim. You have some low
frequency falloff in a notch due to interference if focus is at rim
depth, and even less if you stay inside. Part of the Telinga's success
is that it's focus is somewhat inside the dish. The big no no is having
the mic out beyond the rim, lots of interference from the standing wave
in front of the dish leads to quite a notch in gain down in lower
frequencies. At worst it can be negative gain for a certain frequency
range under 1khz. In addition to this, there is some improvement in the
directivity of the mic as it's location goes deeper into the dish. The
dish acts as a barrier to sound from behind.
It was all discussed excessively a while back in this group. Some of
that discussion was even correct, though not all. Read "The Parabolic
Reflector as an Acoustical Amplifier" by Sten Wahlstrom (J. Audio Eng.
Soc., Vol. 33, No. 6, 1985 June) for a good nature recording oriented
look at the issues as known then. There is considerable confusion on
using parabolics for recording sound. Partially because even experts
make the error of treating sound as electromagnetic waves and applying
formulas derived from that. Sound is the movement of atomic particles
along the direction of travel. It's also worth reading the white papers
at the Telinga site for other perspectives.
Walt
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