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Re: DIY Parabolic Dishes.

Subject: Re: DIY Parabolic Dishes.
From: "Walter Knapp" waltknapp
Date: Sun Oct 1, 2006 10:03 am (PDT)
Posted by: "Rich Peet"

> As you may recall I use a 32" dish with the focal point being close to
> the rim.
> Do not worry so much about standing waves.
> If cavity resonance is noticable to you than you can easily dampen
> that.  My windscreens dampen it well.
> Ideal in my eyes is setting the focus at the rim or just outside.
> On occation I do use a mic above the dish as a noise cancellation
> device and if standing waves were a large problem with my dish I would
> hear it.

If I remember right in one of the previous discussions you pointed out
the notch in gain of your dish as a limitation on low frequencies of
dish size rather than the notch that's characteristic of the standing
wave issue with your dish design. If the particular frequency range of
the notch is unimportant to what you record, then yes you can ignore it.
For 1:1 dishes that notch in gain is somewhere in the 250-500 Hz range
or close to that. As you move the focus outside of the dish the notch
spreads primarily upward in frequency as well as lowering gain more
within the effected range.

Since quite a few species of frogs call within the range in question I
cannot ignore the effect. I at one point did record with a long focal
length parabolic, but not anymore.

> The focal length a dish is designed for does change the flavor of the
> recorded sound. Randy Little who did the math on that, I recall, also
> ended up recommending a 1:1 ratio of focus to depth.  Klas does not
> agree and that is simply another flavor.

Sten Wahlstrom's 1985 paper has a pretty complete set of math on
parabolic behavior and replaced Randy's earlier effort. It seems
Wahlstrom's formula were computer designed from data on existing
parabolic mics. Wahlstrom notes that 1:1 is used primarily for
convenience and tradition. Though his math indicates that a focal length
1/4 the dish depth is somewhat better. He says the difference is only a
few dB of gain, so probably not worth worrying about.

The shielding effect of the dish can give you several dB of extra
clarity if the mic is down inside vs 1:1. That, of course assumes
unwanted noise is coming evenly from all sides. It's probably part of
the change in flavor.

I think that the math is still shaky enough that the bottom line is that
optimizing parabolic design still contains a lot of trial and error.

In the case of Klas' parabolic it does have the advantage of being about
the best available commercial parabolic for nature recording. Not just a
different flavor, a better one. Like all parabolics it's a compromise


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