SANctuary: How to make a Home made Directional Microphone
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How to make a Home made Directional Microphone
I saw an article in the 1992 edition of Radio Electronics' Electronics Experimenter's Handbook on this neat little device last weekend. It was the kind of thing which makes you slam your palm into your forehead and curse yourself for being so stupid as to not have thought of it already. Well, maybe it wouldn't do that to you, but it did it to me, cause I had put a lot of effort into making a true parabolic microphone and the results had sucked big time. A friend and I built this in about an hour, and it put his dad's $200 parabolic to shame. We were using a $5 mic he got from some mail order catalogue, and I'm sure that the results would be even better with a good one.
All you need is some cardboard, straping, duct, and electrical tape, a washer modeling clay, and a hand held microphone. You probably have the stuff to build this around the house. Basically, all it is is a rectangular cone with the microphone stuck in the small end. We made our cone 15" at the mouth and 2 feet long, with a 1" opening for the microphone. The article said they used a 12" opening, but it also said that rolloff was at about 5 degrees. Since this is a little more precise than would normally be wanted, we widened it and (I think) got better performance out of it. They also had an extension piece that widened the mouth to 2'X2', but it is too complicated and fragile to be really worth the trouble, so we just made ours wider to begin with. Here's how to build it:
Cut out 4 truncated triangles with the dimensions you want, and make the missing apex 1" wide. Then use straping tape on the inside and out to put it together into a cone. Make sure the corrugated ribs are running parallel to the mouth of the horn. The sides should curve inward slightly so they don't resonate easily. Then the washer (make sure you use a metal one) is glued in the base of the horn as a stop for the microphone. Use a little modeling clay to block the holes where the washer doesn't touch the inside corners of the horn, but don't overlap the inside edge of the washer with it, or you'll block the sound from the mic. The washer needs to have an opening at least 75% of the mic's top diameter, but get one with as much space as you can. Now wrap the little end of the cone with electrical tape and wrap the top of the microphone with duct tape until it fits the hole snugly. Don't worry about the holes, the area behind the top of the mic doesn't matter much. We also gave ours a coating of paint on the outside to make it a little more durable and screwed a cabinet handle into the cone about halfway up, but you don't really need those.
The result was REALLY ugly, but it did the job. Using a cheap earphone and a comically simple audio amplifier I through together in about ten minutes, it outperformed the parametric with it's big headset and fancy filters. We could have made it work even better by using two cones attached to each other so the angle changed, but it worked well the way it was.
You should use a handheld mic with a flat diaphram and no acoustic openings on the sides of the head, but I guess anything will work. Here's a feeble attempt at a diagram:
___ ___
I I \___
I I \___
I I ^ \___
15" I Iribs run <
I I Ithis way___ < Insert mic here
I I ___/
I I ___/
___ I___/
I I \___ is supposed to be smooth, with a
I------24"------I slight inward curve.
I I
Ok, so it's a little outta proportion. You get the idea.
Anyway, that about does it. If you like this, you might want to pick up the mag, cause it has some neat equations and charts that explain how it works. There are also a few other potentially devious projects in it, such as a "Radar detector tester." It broadcasts on police radar frequencies and sets off detectors. That'd be good for a few laughs on the Garden State. :)
Watch out for more by,
The Gaurdian
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