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Re: DIY Parabolic Dishes.

Subject: Re: DIY Parabolic Dishes.
From: "Walter Knapp" waltknapp
Date: Sat Sep 30, 2006 6:36 pm (PDT)
Posted by: "John Hartog"

> Besides standing waves or cavity resonance, and considering a focus
> less than the height of the dish to isolate from background noise, the
> reason for having the focal point close to the height of the dish
> seems for the sake of efficiency of space and materials.

This is the primary reason why such dishes are most common.

   The wider
> the dish the lower the amplified frequencies =96 right?

Not really so. This idea comes from folks who equate soundwaves with
radio waves. Radio waves oscillate transversely, thus a antenna must
match this transverse size. However sound waves are movements of
particles along the axis of transmission. The wavelength is along the
direction of transmission, not transverse. Thus even a tiny dish will
have gain at low frequencies and the zero gain point does not change a
lot with size, if any. It is this area that contains the most
differences between theory and practical field use and causes the most

If you think about it the exact same interaction of sound waves occurs
with microphone diaphragms, and microphone diaphragms are tiny compared
to parabolic reflectors. With a diaphragm that's about 1/2" in diameter
a MKH 110-1 microphone is specd to pick up sound down to 0.1Hz. And
Message: 0.
Subject: 1Hz has a huge wavelength.

Gain at the low end is somewhat better as dish size goes up, this we do
know from field experience. For this reason 20" is considered a lower
limit for dish size for audible sound. The Telinga, which is this size
can produce considerable gain on bullfrog calls, which go down close to
the lower end of our hearing. A bigger dish will produce more gain, but
it does not go up all that rapidly. Issues of practical field use
quickly overwhelm whatever theory is saying.

> Throwing these practicalities aside and considering only acoustical
> properties, what would be the ideal shape of a parabolic reflector for
> nature recording?  I'm imagining an eight-foot dish a couple feet deep
> for recording from shore a back set of breakers in a twenty-foot
> swell, or maybe a slightly smaller one for recording from the lookout
> by the jetty the birds and other activity in the channel near the
> Columbia River bar.

Dig around in Wahlstrom's paper. I don't think there is a ideal
parabolic, though some are definitely better than others. If you have
enough porters to carry it, the eight-foot one might be fun, but if you
are carrying it a 20" Telinga will get you far more recordings.


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