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Re: DIY Parabolic Dishes.

Subject: Re: DIY Parabolic Dishes.
From: "Walter Knapp" waltknapp
Date: Sat Sep 30, 2006 6:07 pm (PDT)
Posted by: "John Hartog"

> I must be nuts to bring this up again, but I still have some burning
> unanswered questions -so here is one.

Yes, you are nuts. The level of misinformation and misunderstanding on
this topic is very high. And a lot of folks with locked in ideas. I hate
getting involved in it anymore.

> Regarding the focal point (and I know it is not really a point), what
> is the ideal distance inside the rim.  I am hearing different opinions.
> One from a Cornell page suggests if the mic is positioned inside the
> dish there would be a distortion from cavity resonance (or something
> like that) off the sides of the dish.

Theory says the optimum location is a focal distance that's 1/4 the dish
depth. That's 1/4 from deepest part of the dish. Though any position
inside the dish is only slightly different. Practical dishes are usually
not as deep as a l/a=3D1/4 dish. The telinga works well with a focal
length somewhat inside the dish.

For there to be cavity resonance the sound has to keep bouncing around
in the cavity. For parabolics it's more like bounced back out. Maybe
they are talking about flex of the reflector material from the sound
hitting it?

> Conversely, a few of this group suggest a standing wave at the rim
> opening would cause distortion, so the mic is best put inside the dish
> somewhere below the level of the rim.
> Is the standing wave only exactly at the rim level or does it extend
> somewhat into and out-of the dish?  If so how far each way?  Would the
> optimal position of of the focus be at a certain ratio of the dish
> depth?

To quote Sten Wahlstrom from his paper "The Parabolic Reflector as an
Acoustical Amplifier":
"It is the interaction between the direct sound and the reflected sound
that causes a standing-wave pattern in front of the reflector when these
components are nearly equal in strength."
His paper has formula, graphs and so on that make it pretty clear.

Looking at all that the amount of interference from the standing wave
increases the farther out the focal length relative to dish depth. At
least up to a focal length of 4X dish depth. It does even slightly
effect the 1/4 dish depth focal length reflectors, more so focal length
and dish depth the same, and fairly extreme if the focal length is
longer than dish depth. What it tends to cause is a notch in the dish
gain somewhere in the 250-1000hZ range. With the longer focal lengths
the gain in that region may even be negative.

Note this standing wave problem is not unique to parabolic reflectors.
Avoiding it is what dictates the diaphragm location in boundary mics and
also how they get a little extra gain, and it should always be
considered anytime you have a reflective surface near a mic. The effects
are frequency dependent.

I've put up a pdf of that paper several times for folks to get, but it's
not up now. If you did not get it when I had it up let me know and I'll
see if I can get it to you. Wahlstrom's paper is fairly old now, but
I've found that what he has agrees well with actual field use of a


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