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Old November 9th, 2009, 07:56 PM   #1
backinthesaddleagain
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Science Fair Project

Anyone ever build a parabolic microphone? I am building one with my son, but having no luck. Found plans in a science book. We have a 25" diameter plastic snow saucer as our reflector. I found a site on line that calculates how far the mic should be from the dish (based upon width and depth). I have a working mic and small amp. I am thinking maybe the plastic is too flexible and absorbing rather than reflecting sound. Was going to try to find a metal snow saucer, but its only November, and most are plastic these days.
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Old November 9th, 2009, 08:19 PM   #2
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Even a soft plastic like nylon should work. I've seen all sort of different designs, some work better than others. The snow saucer I assume is a saucer sled, and if this is true it is far too flat. You would be better off with a round tupperware bowl, better if it is not flat on the bottom.

A good big soup bowl might be OK too. Better if you get a sheet of acrylic and heat it until it droops into the bowl, after it cools heat the center again to provide a little more curve.

Alternate, two flat pieces of acrylic glued at say somewhere between 60 and 90 degrees, stand several yards away with a flashlight and shine it at the reflectors to help see if the mic is at the focal point.

If you really want to get into it, find the proper curve, cut it out of wood to make the "bowl" shape (OK to leave a bunch of space between the spokes) and then get the sheet of acrylic and heat until it forms the basic shape you laid out with the wood frame.

No matter what the math says, the ONLY way to focus one of these is to pick a distance and test it. Put a radio XX distance away, and move the mic in and out until you find the best compromise of gain versus frequency.

And yes I have worked with the really expensive CBS patented versions as well as the less expensive kind of consumer versions.
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Old November 9th, 2009, 08:26 PM   #3
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Thanks Greg. Went to the dollar store tonight in search of a bowl or wok, most had a flat bottom. Guessing a real wok doesn't have a flat bottom. Will find something with a deeper curve, as I have tried the snow saucer and moved the mike from 4" out to 20" and don't even pick up my tv 10' in front of me.
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Old November 9th, 2009, 08:50 PM   #4
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The ones we used to use were about 24 inches diameter and somewhere in the around 8 inches deep and they had a pretty high gain. The small consumer style were like 9 inches in diameter and only about 2 or 3 inches deep.

This is actually not too bad an idea, but I don't know how well the fabric will reflect the sound:
http://www.bambooturtle.us/ParabolicMicrophone.html
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Old November 10th, 2009, 05:57 AM   #5
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http://scientificsonline.com/product...cmss=parabolic
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Old November 10th, 2009, 06:36 AM   #6
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My roommate a few years back did something similar to this, he used an old satellite dish and converted it to a high gain antenna for wi-fi network connection. He used the dish and set a usb wi-fi stick at the focal point, we were able to pick up wireless network signals from about 200m away. we were getting ~80% signal strength where we could never see the network before.

Maybe you could use the same idea? just with the mic. the nice part about the satellite was that the focal point was easy to hit since the receiver was already there, we just replaced it with the
usb stick.
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Old November 10th, 2009, 07:04 AM   #7
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Something what will affect this is contained in the name. The dish should be PARABOLIC, not just a bowl.

It's not as important with sound as with light, but it does make some difference.

Look up plans for solar ovens. There are some out there that have pretty good parabolas made entirely of corrugated cardboard.

(Which could lead to another science fair project!)

Here's something your kids might find interesting. I once had the pleasure of talking to some very intelligent audio engineers (doctorate-level) who work at Bose and they told me something fascinating... if an object is smaller than the wavelength of a sound wave, then that object is effectively "invisible" to that sound.

Which is why, if you ever go to an outdoor festival where there's a band playing, you can hear the low frequencies from much farther away than the high ones. The highs are getting absorbed/reflected by things like trees and people, while the lows don't "see" those objects.

It's easy to calculate wavelength for various frequencies given the frequency and the speed of sound (about 1100 fps at sea level). IIRC and without doing the math, that gives a 40 hz wave (the low E on a bass guitar) a wavelength of 33 feet.

Anyway, this means that the bigger your dish, the more low frequencies you'll pick up.
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Old November 10th, 2009, 09:20 AM   #8
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Steal (I mean borrow) your neighbor's "Dish Network"(TV) Dish.
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Old November 10th, 2009, 12:10 PM   #9
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Especially here in Hope Valley where they mount them on trees or on stands on the ground!
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Old November 11th, 2009, 11:41 AM   #10
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It's going to be very windy for the next few days. Just say it blew into your yard and didn't know who's it was.
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