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Re: DIY Parabolic Dishes.

Subject: Re: DIY Parabolic Dishes.
From: "Walter Knapp" waltknapp
Date: Wed Oct 4, 2006 10:30 am (PDT)
Posted by: "John Hartog"

> I received the Wahlstrom paper from Walt and found it informative and
> helpful towards making a decision about focal length.  I'm thinking
> now I'll try a focus of 1/4 dish depth as the basis for my design.

Should be interesting, I've never seen that deep a dish except light
reflectors. I'd expect it would be harder to get good stereo from such a
dish, may end up only good for mono.

> Wahlstrom mentions filtering "-6 dB per octave above 1000 Hz to
> compensate for the reflector response =85"  Is that close to what you
> would suggest?

Before setting a specific eq for frequency you should probably look at
the frequency shift in sound with distance which is opposite to the
increasing gain with frequency of the parabolic. Then figure out what
distance you will be recording at and calculate how much eq of the
parabolic will be necessary for that distance. (of course calculated and
actual response of a parabolic can be quite different which makes it

Of course in deciding a eq you are deciding a apparent distance for your
subject in the final playback. If you completely compensate the
parabolic to a flat response then you will have left the frequency shift
with distance intact and the recorded caller will sound distant. Usually
people prefer the caller sounds close. Which will be best done by
fiddling very little with the parabolic's gain profile.

Most testing of parabolics is done at close distances and thus does not
really get into the dropoff in frequency with distance. If you are using
a parabolic primarily for it's tight pattern at very close distances
then you probably will need to do some eq. Of course that depends on
what you are recording, a narrow frequency band of a single species of
caller is not going to change much with eq. You have to have a wide
range of frequencies represented for eq to be at all worth it. And then
it depends on the final sound you want.

Bottom line is that a flat response out of a parabolic may not be the
most desirable way to go. Especially if you intend distant callers
recorded with it to sound closer.

Klas has a discussion of some of this in his white papers.


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