Tuesday, November 22, 2011

Experimental Parabolic Microphone

I've always wondered how well a parabolic mic works. Here is my experimental setup for testing such DIY parabolic microphone. It is a great weekend project and will let me experiment with high-gain / low-noise audio amplifiers. The heavily wooded area off my back yard is plentiful of singing birds.

Photographic tripod is used as mount for the parabolic dish. I had to construct a simple mounting bracket and a "focusing" contraption that will let me use different size and shape microphone elements.

I got the actual parabolic reflector from "the place where you can find anything" - eBay. The parabolic reflector is made out of polyethylene plastic. The diameter is 21 inches with focus point 4 inches from the bottom. It came as one solid reflector - I had to drill the mounting hole. I had a few ideas for mounting the dish - I wanted to be compact and simple so I decided on a single hole in the center.

The microphone mounting frame is made from semi-rigid coax (RG-402) and small PCB board for attaching the mic element. I used a threaded cable feed-thru to both - attach the dish to the bracket and mount the RG-402 frame with the microphone. A "sandwich" of metal and rubber washers - including two large and thick rubber washers provides "shock-mount" for the dish. The mic frame is fed through the threaded feed-thru using silicone cemented inserts.

The "focusing" rig lets me move the microphone element to the exact focus point of the dish. The focusing range is about 2 inches. Normally, the mic can be fixed in the focus of the parabola, but I am planing to experiment with different mic elements and they all vary in size and shape so I wanted to be able to adjust the mic frame with no hassle. Two spring-tensioned wing nuts fine-tune the mic frame.

Small plastic container is holding the battery pack, microphone preamp and 900 MHz FM transmitter (a.k.a Baby Monitor - For the initial testing and to validate the concept I just modified a baby monitor and then used my IC-R20 scanner to listen and record). Currently, I am working on my low-noise/high-gain preamp. The radio-channel link sort of works but the noise levels are way too high. Proper mic pre-amp and better (broadcast grade) FM transmitter are planned for the next stage.

Hand-held mode. Large plastic handle salvaged from old angle-cutter provides comfortable grip.
A word of warning - the surface reflectivity of the polyethylene dish is high enough to produce smoke from the mic's wind guard while I was playing with the dish and decided to verify the focus by pointing it at the sun. It took less than a second! I was able to act quickly and saved the guard from catching on fire :) (stupid move but the damn thing looks transparent :-)


Grrxyn said...

That is a really cleaver use of semi-rigid coax, if I ever seen one!

Unknown said...

Just curious about how your parabolic mic works. Did you finish your testing? I also wanted to ask you if you remember the vendor on ebay. I have been looking but have not found one that looks like yours.
Do you remember your approx cost? Telinga, foldable available online but they're expensive.


Unknown said...

Please let me know where you found your parabolic dish if you remember and your cost.



Andrey E. Stoev said...

The Parabolic Mic works better than I expected. The performance is somewhat poor on lower frequencies but at higher, above 1 kHz is pretty good. This was expected because of the size of the dish. I am feeding the output of the preamp to a multi-band equalizer to bring up the lows and I am getting very good results.
The dish was purchased in Aug 2011 from eBay member "sdill471" for $38.50. He is still selling them as far as I can see - last one sold on Dec 19, 2013 - Auction# 231114759030. Hope this helps!

Unknown said...

nice work. I also have ordered such a dish. What microphone are you using?

Studio l'Oreille Gauche said...

Nice work.

I beleive there are two factors acting to cut off bass frequencies:
1) phase cancelation: try changing the mic position (closer/farther from the reflection point) to check if bass cancellation occurs (very common problem in audio)
2) The plastic mass may be too low to reflect bass frequencies.

Just my two cents.