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## Re: DIY Parabolic Dishes.

 Subject: Re: DIY Parabolic Dishes. "Walter Knapp" waltknapp 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 parabolic. Walt
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