Horn type directional microphone

dkazdan

Joined Mar 20, 2010
13
Reched said he would host some sound files, I'll send some to him when he replies next. We were out today recording with four different microphones: the horn, the parabola, an inexpensive shotgun microphone, and the internal microphones to our little Olympus digital recorder. We did some front/side/back comparisons to a standard sound source, and some bird recording. The results are pretty much as predicted:

The parabola and the horn are both very directional. The standard sound source was inaudible at 90 and 180 degrees, although on the sonogram a few harmonic lines were visible with the horn at 90.

The horn is lighter and an easier shape to handle while hiking. Its wider beamwidth makes it much easier to record a bird you can't see but permits more environmental noise. It has good low frequency response.

The parabola has a very tight focus and so very much reduces the noise of wind rustling trees and of moving water away from the bird. The bass rolloff is noticeable but doesn't interfere with the actual recording of most birdsong. It does make the recording sound environmentally less realistic. The tight focus makes it very difficult to record an out-of-sight bird that is calling only occasionally.

The inexpensive shotgun mic (sold for home video) is better than nothing. The Olympus recorder's internal microphone has some software fixes for tighter pattern which actually do help quite a lot. They don't have wide aperture, but they do remove side interference more than I might have expected.

The microphone element matters. The parabola had a Radio Shack computer microphone on it; I replaced it this evening with a different element and it made a huge difference. The new one is readily available from the DigiKey listings but has 20 dB better sensitivity than the RS one was rated for and 10 dB less noise. It matters.

The horn has a lot to recommend it over the parabola; in particular, it's light, has about the right directionality for most nature sound recording, and is all but free. Give it a try!

David
 

retched

Joined Dec 5, 2009
5,207
Here are the links to the recordings David was mentioning, and a copy from an email from him describing the files and whats in them:

I finally got these together. They're from the Squire Vallee-Vue Farm of Case Western Reserve University, east of Cleveland, Ohio, June 20, 2010.

I'll include only a few interesting tracks from the morning's recordings. This is not an "overly scientific" test for a number of reasons--I didn't take accurate distance measurements, and the two electret capsules are not identical. I'll guess that the parabola's is about 10 dB less sensitive than the one in the horn. The parabola is Edmund Scientific's 24 incher; the exponential horn is the one from sheet polyethylene that can be seen in a photo earlier in the thread, it's about a 11" square opening, 24" long.

Tracks 71 and 72 are in an open meadow with the recorder about 200 feet from a stream and an electronic musical instrument tuner about 100 feet from that. Looking at the sonograms, I can see that I was having a hard time holding the parabola accurately pointed at the sounds source. The focus is quite tight. The horn is looser, so the sound source can barely be heard at 90 degrees (the parabola's pickup at 90 is not evident on the sonogram, the horn's is). The horn gets much more of the stream noise when pointed away from the tuner than does the parabola.

60 (parabola) and 61 (horn) are a good pair from the same position, an eastern wood peewee at about 30 to 50 feet, about 30 degrees up. The red-eyed vireo singing is about 45 degrees off and is quite different with the two microphones. Wind noise in the background tree canopy is noticeably different between the two.

Track 53 is with the recorder's internal microphone, but with its software processing set on "zoom." Considering that this is a little handheld, it's really not bad (the recorder is an Olympus LS-10).

Track 56 is with an inexpensive hypercardioid microphone sold for camcorder use.
http://www.affectbusiness.com/record/53.mp3
http://www.affectbusiness.com/record/56.mp3
http://www.affectbusiness.com/record/60.mp3
http://www.affectbusiness.com/record/61.mp3
http://www.affectbusiness.com/record/71.mp3
http://www.affectbusiness.com/record/72.mp3
 

retched

Joined Dec 5, 2009
5,207
I was wondering if you could use some of that spray coating that is used on a cars undercarriage for road noise reduction.

That should help quite a bit to reduce the non-targeted sounds. If recording a bird at the 12 o'clock position and there happens to be a closer bird (or louder) at the 3 or 5 o'clock position, the dampening should keep the sound waves from transferring through the thin plastic to the mic.

Just an idea.
 

dkazdan

Joined Mar 20, 2010
13
Interesting thought--wouldn't have occurred to me.

Thanks for posting the files and the quoted email. News as it happens...

David
 

retched

Joined Dec 5, 2009
5,207
Well, I figured I would save your fingers from having to re-type it over again here to explain the files.

As for the spray, I think even light insulation or a foam of some sort would also do the trick. The undercarriage spray is a little on the heavy side...so It may or may not be feasible. Well it is feasible, but it may be a little heavy. We'll see.

I plan on making your model MARK-II once you have it figured out.
 

dkazdan

Joined Mar 20, 2010
13
Mark II is completed, sizes doubled, material is .007" polyethylene. It was much more difficult to build and looked wrong...then I realized it is wrong, and so is Mark I. The gore shape can't be exponential to make the horn itself exponential: The flare forces the horn shorter than the flat gore is, so the horn flares faster than exponential. Mark III will correct this...but I must say, Mark II performs quite well. With 24" square bell, it just barely gets out the door, it has about a 10 degree beamwidth, and it gets a lot of sound.

News as it happens...
 

dkazdan

Joined Mar 20, 2010
13
I'm not set up for formal testing--my last access to an anechoic chamber and related stuff was in Building 20 back in the day. The horn flares faster than it should, so by rights it should be oversensitive to bass. Still, it's directional, it's sensitive, birds spot it and fly as far as they can from it...seems like a well-functioning microphone!

I worked out what I think are the correct design equations given the error I made, and I'll try again. Doing it piecewise linear wasn't too hard, only cost my wife and me a few hours of arguing over the math. Figure out the differential equation for the curve was harder, and I knew I didn't have a prayer of integrating it. The Wolfram online integrator did it and was quite smug about it, reporting 0.03 seconds of computing time. Hmmph.

Maybe Saturday night I'll try recutting the gores, see if the shape really matters that much. I'll let everyone know.

I spotted this reference, I need to get to the engineering library to see if the issue is available (I can't find it online), or maybe someone here has old Institute of Radio Engineers journals?:

Clark, M.;
Bell Telephone Laboratories, Inc., Murray Hill, N.J.

This paper appears in: Audio, Transactions of the IRE Professional Group on
Issue Date: Jan 1954
Volume: 2 Issue:1
On page(s): 5 - 7
ISSN: 0000-0000
Date of Current Version: 29 January 2003
Abstract

An acoustic lens combined with a conical horn can be used to obtain a highly directional microphone without some of the disadvantages of the parabolic microphone. The directional characteristics can be calculated satisfactorily if one assumes that the horn provides uniform flooding of the lens aperture.


David
 
Here might be an odd request... I need to detect the presence of a fairly large underground tunnel ( big enough for a horse and wagon). I was thinking if I mounted a shotgun mike (which I have) in some sort of horn arrangement where I could arrange to have a reasonable air tight connection between the horn (pointed down) and the earth ( maybe via some sort of foam gasket) ... and then using as my "sound source" - hitting the ground sharply with a sledge hammer.... would I hear the difference between solid ground under the horn and a large tunnel under the horn?? Any thoughts? - any other (inexpensive) ideas?

Oh yes - why am I even doing this?? I am making a documentary and have to find tunnel that no one believes exists - but I have an "eyewitness" who claims to have been in it. I don't have the cash to rent/buy underground sonar equipment.
 

Bernard

Joined Aug 7, 2008
5,784
Might want to start your own thread, hchatfield. Back in 1961, a science class borrowed a 12CPS geophone, DC amplifier & pen recorder to demonstrate seismic activity using foot steps,stomps & book drops. You can make a geophone with a small speaker & weight attached to voice coil, maybe a $ .25 coin. Recorder to be determined. Just listening wont cut it.
 

bendavis2

Joined Aug 22, 2018
1
I know this thread is really old, but since people still may be using the information (and it came up for me while I was looking for cone mic plans), I wanted to add a link to an article that gives some pretty clear steps on how to build one of the cone type microphones that David mentioned several times. It also has some nice charts showing the different horn types and their projected acoustic resistances:

http://www.n5dux.com/ham/files/pdf/Super Directional Microphone.pdf

PS: David, if you ever did get your math right and have a simpler version of how to create the horn walls for those of us who are math incompetent, that would be great!

I'm not set up for formal testing--my last access to an anechoic chamber and related stuff was in Building 20 back in the day. The horn flares faster than it should, so by rights it should be oversensitive to bass. Still, it's directional, it's sensitive, birds spot it and fly as far as they can from it...seems like a well-functioning microphone!

I worked out what I think are the correct design equations given the error I made, and I'll try again. Doing it piecewise linear wasn't too hard, only cost my wife and me a few hours of arguing over the math. Figure out the differential equation for the curve was harder, and I knew I didn't have a prayer of integrating it. The Wolfram online integrator did it and was quite smug about it, reporting 0.03 seconds of computing time. Hmmph.

Maybe Saturday night I'll try recutting the gores, see if the shape really matters that much. I'll let everyone know.

I spotted this reference, I need to get to the engineering library to see if the issue is available (I can't find it online), or maybe someone here has old Institute of Radio Engineers journals?:

Clark, M.;
Bell Telephone Laboratories, Inc., Murray Hill, N.J.

This paper appears in: Audio, Transactions of the IRE Professional Group on
Issue Date: Jan 1954
Volume: 2 Issue:1
On page(s): 5 - 7
ISSN: 0000-0000
Date of Current Version: 29 January 2003
Abstract

An acoustic lens combined with a conical horn can be used to obtain a highly directional microphone without some of the disadvantages of the parabolic microphone. The directional characteristics can be calculated satisfactorily if one assumes that the horn provides uniform flooding of the lens aperture.


David
 
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