I've always been interested in building some kind of outdoor sensitive microphone pickup. I recall decades ago projects for 'shotgun mics' as well as parabolic mics in popular electronics mag. I'm pretty sure I can handle the electronics part, preamp, low noise op-amp possibly with some active filtering and I've even seen some nice audio AGC chips on E-bay that might work well.

What I'm looking for is ideas on the best mic elements and mounting methods. Would like something that would pick-up birds and other wildlife in my back yard up to say 100 feet or so.

So anyone have success with sensitive outdoor microphone design?

Lefty

 

Just about any hard parabolic surface will do. Although not ideal, spherical surfaces also work. Common sources might be "security camera domes". Put the microphone at the 'focal point', mounted on a wire tripod (coat hanger or piano wire). Microphone can be almost anything that's fairly sensitive, like a cheap surplus electret. To isolate from vibration, cut a hole for the microphone in some foam rubber and stuff the mic into it. Mount the foam rubber on the wires.

 

That 'shot-gun' mike reference brought back some memories!

Long ago, I built one that consisted opf a series of 'tuned' pipes, arranged cylindrically, with a plastic funnel glued to the end that had all the tube ends flush. A microphone was placed in the funnel pipe, flush with the cone if you understand what I mean. The whole thing was then mounted on a pistol grip. I also put a tripod mounting plate on the base of the pistol grip.

I used it for many years with great success (I used to be actively involved in wildlife film-making). If I can find the details, I'll pass them on to you. I found it to be mush more sensitive than a parabolic, easier to use because the overall diameter was much less than a parabolic, and extremely directional. Mounted side by side with the camera, the mic picked up what the camers was aimed at with excellent fidelity.

 

Are the pipes tuned like pan flute pipes via length or in their diameter? I'd be interested in even a crude drawing. The basic idea of the physical construction could help people here re-create them as finding the values for tuneing a pipe aren't overly difficult. Even a rough guess of the physical dimensions would be useful.

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The pipes were all the same diameter, around 6mm if I remember, and were tuned by length, similar to pan pipes, guitar frets or pipe organ. They were arranged around the longest pipe forming a cylinder, with a 'spiral' of pipes of diminishing lengths.

I know I have the original details somewhere, I'll try to dig them out over this weekend and post the details.

 

Believe it or not, I have found the ORIGINAL article, dated 1965!!!

When I built mine, I 'metricated' the dimensions to suit, but the priciple remains the same.

I also used a small button-mic insert in the first one I built (electret) but later graduated to a "real" mic insert which I salvaged from a hand-held mic with a damaged casing.

I built a small mic preamp into the microphone assembly, powered from a 9V battery.

It worked for me for many years.

Have fun - it's s great idea, and is a sure-fire conversation starter when you are spotted using it! Some park officials took a suspicious interest in my device when I first unleashed it on an unsuspecting park administration in an East African game reserve, but when it was demonstrated to them, they were in awe and followed me around for a day or two, fascinated by the possiblilities. I built them one for them to play with!!

Attached Files

ShotGunMike.pdf (531.1 KB, 48 views)

 

Thanks a lot for finding that historic project article. Many have read it in the past but I never found anyone that actually tried to build one.

I wonder about the mic element used. They used a large diameter crystal element and the article says it's important to mount it as close to the tube elements (without actually touching) as possible. With todays micro diameter mike elements is that going to be a problem? I would think the smaller the mic element the further away from the tube end it needs to see to be able to 'capture' the sound coming from individual tubes, esp along the outer diameter or the tube assembly. I guess expermination is the best way to see. Now to see where I can find 60 foot of 3/8 tubing cheap

Lefty

 

When I made my first one, with a button electret mic, I mounted it in the narrow exit pipe from the funnel section. (Hope you can picture that)

The quality was excellent, but the frequency response was a bit poor.

When I mounted the "real" mic capsule, I once again mounted it a close to the exit of the funnel as I could.

I would imagine that the old crystal mic used in the original had a large pickup area, so it made sense to bring it close to the ends of the pipes.

Acoustically, I have no idea what effect the funnel wals have on sound waves. They are going to bounce around a bit, but I doubt that too much phase error creeps in. As I mentioned, mine has worked flawlessly, so I guess there's both cancellation and reinforcement going on, with the nett result being relatively decent fidelity.

Working at often large distances from animal subjects, I must say that even if there was a bit of minor phasing, it was better than making silent movies!!

I wish you luck in finding the materials and look forward to seeing your project when it's finished. I'll dig mine out of storage and post some pics.

If you want the schematic of the mic preamp, I'll be happy to post it as well.

 

Here is a link to a Parabolic Mic on my Website.
http://www3.telus.net/chemelec/Proje...abolic-Mic.htm

Personally, I find parabolic Mics to be More Directional than Shotgun mics.

I also have a supply of Crystal Mic's that are about 1.25 Inch diameter, should you need one.

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Thanks again for your feedback and information. First thing is to obtain material as the mic part is easy to experment with after. I'm thinking that plastic tubing would be much cheaper, easier to work with and certainly lighter, what are thoughts on that change of material?

Also what bandwidth do you think is optimum for wildlife. I know human speech is mostly contained in the 300hz to 3khz band and S/N ratio improves with narrowing of bandwidth so 20hz-20khz seems a waste and only increases noise, no?

Lefty

 

Depends on the fidelty you want and exactly what you're listening for. Bullfrogs lower frequencies are only a few hundred hertz. While a bat's screech is up past 50khz. Noise isn't too much of a consideration asuming you still get a decent S/N with the target audio. Any repeating noise signal can be filtered out with software after the fact as long as you have a good solid reading of just the noise level.

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Well I'm pretty sure I couldn't hear that bat's screech even if the mic could Not planning on using any DSP software or any digital for that matter, just good old analog electronics.

Thanks
Lefty

 

Lefty, there is much free software out there and inexpensive commerical stuff as well that can modify a passable mic into a studio quality audio sample with minimal effort. Sticking with straight old analog electronics is intentionally trying to be old school. While not inherantly bad ignoring the usefulness of advanced post production is almost a crime.

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I would really recommend a Parabolic Reflector over a Shotgun mic. Yes its Bulkier and heavier, But definately Better.

And Whatever you build, definately go for a Good Wide Bandwidth.

Than use a Filter to Pick the Frequency for what your trying to listen to.

For the Shotgun Mic, Plastic Straws are not that good and if you change Diameters, you need to adjust the Lengths accordingly for getting the proper frequencys.
The Best Tubing is This Walled Aluminum tubing from Hobby Shops, But DEFINATELY NOT CHEAP.

As to the Digital Filtering, Can't say I totally agree.

Getting More Involved in this, You can add a Cancelling Mic on the back of the reflector or shotgun mic, Connected Out of Phase to help cancel reflected sounds.

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I would totally disagree - start off with the best quality you can, then improve if required - you can't really improve poor quality, only try and make it less poor (which doesn't make it right!).

This is why you record un-processed sound when recording, you can then add processing (if required) during mixing down, but you can't remove it if you recorded the processed sound initially. This is VERY basic recording technique!.

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