--- In Walter Knapp <>
wrote:
>
> 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.
YES, 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
> problems.
>
> 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
> 0.1Hz has a huge wavelength.
WHOA! Really think about it. The microphone diaphragm is not a
rigid structure intended to reflect sound pressure waves - it is a
compliant structure meant to be moved by sound pressure waves.
>
> 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.
>
> Walt
>
Deep dish, shallow dish, IMHO not nearly as relevant as choice of
microphone and factors of convenience. A shallow dish (with focal
point beyond the rim) is suited to use of a cardioid microphone;
otherwise, an omni should be used. A "1:1" dish (with focal point at
the plane of the rim) makes it very easy to assure proper positioning
of the microphone diaphragm, and fairly convenient to build a
suitable mounting structure. A deep dish protects the microphone
more from wind and may be easiest to build a mike mount, but may be
more prone to cavity resonance.
Good recording,
Randy
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