WHAT?/ AND YOU MAKE KIDS TOO?

im leaving this planet.

---breath--- ---breathe--- ok.

dkasdan, I am interested in hearing (pun intended ) more about the horn mic design you have been using.

Dont let BMorse see it or his 2 year old will probably build it better than me and use a laser range finder and gps location systems with an optical star map backup.
(love you Bmorse)

 

Plans, Pictures etc please. I made some of the horn mics using cardboard, then I graduated to some melamine? covered wall covering stuff the "slick" side in, then used some lumberyard aluminum extrusions for the corners, but I had to use some gussets in the inside which I am sure altered the characteristics of the horn. I used a ratshack three wire electret for the pickup, performance was OK but now a days digital recorders are cheap enough to try this thing again.

 

I have to say WOW.

It is RARE that a year old thread comes back to life. neat.

So, Earthshaker77, You want to give this a go? Again?

 

Bring It On

 

OK, you guys sent me off to my filing cabinet...the article I came across originally was not Popular Electronics (my mistake), it was Radio Electronics for July, 1990, Dale B. Blackwell "Super Directional Microphone" on pp. 41-44, continuation p. 52. He did it in corrugated cardboard and has a pretty good acoustic discussion of the type. There is surprisingly little else out there that discusses them; the Olson book on acoustic engineering does not (although it does talk about the inverse, exponential radiators), and neither does my college acoustics text, Acoustics by Leo L. Beranek (of Bolt, Beranek, and Newman fame) (about the same vintage as the Olson, 1954).

I will take some photos of mine, but not tonight. It was not made to be extremely directional, as it was made for a specific project that needed a 45 degree (or so) acceptance angle. The materials available cheaply for a first experiment sized out just about right. I did take it on a hike in a quiet woods in Tennessee a few weeks ago, and got some good, straightforward bird recordings. Is there a way to post audio files on this system?

On the workbench now is a larger version, still for a 45 degree acceptance angle, targeting more acoustic amplification (wider aperture). The plastic is too floppy to support the size, so I'll have to use stiffening ribs.

Overall, the homemade exponential horn microphone compares well with the homemade parabolic microphone, is lighter and cheapter but clumsier to handle and carry. It is easier to pack flat, though. Check out the for-real professional parabolas, especially Telinga--those things aren't cheap, and I'm not sure what exactly the money buys.

Another old article, this one available on line, is about parallel tuned-tube microphones. I'm not going to try it and basically don't recommend it because it's clumsy and I've decided I like wide-aperture microphones (parabolic and exponential) for the bird recording, but here it is:

http://72.52.208.92/~gbpprorg/mil/mic/ShotGunMike.pdf

More later...

David
Cleveland, Ohio
Amateur radio AD8Y

 

Nice. I like the shotgun snooper.

Cant wait to see your design for your directional.

As for sound files, you may want to find a free on-line host and upload your sound files there. You can then link them here.
You can also check with your internet provider to see if they offer you web space included with your monthly subscription . Most do. For instance comcast.net/~johndoe.

Having storage space online is a very good idea. If anything happens to your computers hard drive, you will want your files safely backed up online. I use a online backup service, my free webspace, as well as my own server box at home.

 

Here's one photo of the thing, let's see how that works before I post more and give a description:

 

What are you using for a mic? Are you just testing with a standard PC mic? Something more exotic?

 

Looks like a standard PC mic.......

B. Morse

 

What are you using for a mic? Are you just testing with a standard PC mic? Something more exotic?

It's the cable and housing from a standard PC mic, but I replaced the element with a higher sensitivity, lower noise one. Among the educations from this project has been about electret elements: There's a very wide variety of them available, and they're all fairly cheap (<$5.00, even for exotic ones) so it's worth experimenting.

 

does anyone have a copy of the original article? I remember there were some equations involved, a way to tailor the response.

 

The basic design is take four pieces of material, cut them into exponential shapes of appropriate size, and stick them together into a square cross section. Cut me a little slack for the flat exponential on the material getting distorted in projection, but the error is small enough that there won't be any real standing waves in the horn (that's the problem with a conical horn).

The narrow end needs to be the size of the microphone. The wide end and the length are as you choose for the acceptance angle and aperture. The sheet plastic cutting boards I bought were 15" by 24", so I used those dimensions for the opening and the length, respectively. My electret element was 3/8".

Draw a line down the long axis of the material and mark off widths on some given station interval, I used 1", with the half-width given as

y=1/2 Wm exp(lh/a)

where Wm is the width of the microphone end,
lh is the length of the horn,
a is the spacial constant lh/(ln(Wh/Wm))
where Wh is the width of the horn end (and ln is natural log).

Make four of these, then pierce holes at intervals along the edges (again, I used the 1" stations) and lace the four pieces together (I used monofilament fishing line). The whole shape came together quite nicely. Run a bead of hot glue along the edge for the seal, use expanding glue to fasten the electret element in, and start recording. For $15 or so in material and an hour of basement work, this thing is light and works very well.

Let me know what clarifications are needed...

David

 

Tried something interesting this evening--compared an exponential horn to a parabola. I used a standard sound source (music tuner at about 1000 Hz, out in the back yard where reflections shouldn't be prominent), standard distance (about 10 yards), the sound meter on my digital recorder, and my two homemade microphone rigs. One is a 24" diameter parabola, the other is the 10: square, 15" long exponential horn. My prediction was that the horn would be about six decibels down from the parabola by aperture, and somewhat less directional. Measurement was 7 dB down, 3 dB down angles about 10 degrees vs. 30. Worked out.

My wife and I are going to cut the plastic this evening for another horn and pack it for a trip we're taking soon. We will lace and duct tape it together when we get to destination, do our recording, then break it down to pack for the trip back. It should be light and compact for travel--and again, total investment of about $15.

News as it happens--has anyone else tried this?

David

 

Good work, now what you and your lovely wife could do is make some recordings, and post them somewhere the rest of us can hear the results.
Perhaps a horn microphone blog
Thanks for the update, Ryan

 

OK--give us a couple of weeks for some interesting bird recordings, and I'll find a place to post the sound files.

Last weekend's adventure was recording a bobolink, but that was with the parabola. Good recording if you ignore the whine of a model airplane in the background. The airspace was busy that morning.

David

 

When you get the sound files, PM me. Ill host them for you.

 

Thanks. We're back from the trip. The Arizona desert did not have as much bird recording to do as I had hoped, but I have a few files. I'll do some editing and compressions and let you know.

The horn did seem to work well, and it packed nice and flat in the suitcase.

David

 

Well, like I said, Ill host them so we can all hear them, if needed.

 

Here are two recordings, although not from the Arizona trip. Take a look at the bird recording site xeno-canto,

www.xeno-canto.org

Search on worm-eating warbler and black-and-white warbler, look at the ones posted by Laura Gooch (currently the last ones on each list, Laura uploaded them this morning). These were birds in a park in Sewanee, Tennessee. The setting was away from urban noise but not silent. The birds were about 100' away and recorded with the polyethylene horn.

Sonograms are a standard technique in bird recording. There's a surprising amount of research done in local dialects to follow population genetics and isolations, for example.

More later...

David

 

This will get you to the page where the sound is located.

worm-eating warbler:

http://www.xeno-canto.org/america/species.php?query=sp:3708.00

and black-and-white warbler:
http://www.xeno-canto.org/america/browse.php?query=black-and-white+warbler

There is a little play button below the sound image on the first page.

On the second link, it is a little more self explanatory.

As for the quality, its GREAT. Especially @ 100ft.

The hiss is very low. Good compression.