Horn type directional microphone

Discussion in 'The Projects Forum' started by Earthshaker77, Nov 8, 2008.

  1. Earthshaker77

    Thread Starter Member

    Nov 8, 2008
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    I remember reading an article the discussed how to build a horn type directional microphone. I built a version of it some time ago from that article, does anyone remember that article?, does anyone have a copy?
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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    No, but I have seen catalogs offering microphones mounted at the focus of a parabolic dish for better directivity and sensitivity.
     
  3. Earthshaker77

    Thread Starter Member

    Nov 8, 2008
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    I know about those types and in the article I mentioned, they discussed advantages and disadvantages of both types, as I recall the horn performed better because it was more directional, with the parabolic the mic can pickup things from the sides in higher noise environments, where the horn doesn't exhibit that characteristic, but the horn by its design needs to be made for a certain frequency range for the best results, and that was discussed in the article as well.
     
  4. Audioguru

    New Member

    Dec 20, 2007
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    As I said on the other site, pros use a a shotgun mic.
     
  5. Earthshaker77

    Thread Starter Member

    Nov 8, 2008
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    And it is 200 bucks, about 180 bucks more than the horn would cost.
    I just read your other post, excuse me for posting on more than forum, I really did not know was a bad thing to do. Different forums in most cases might mean different people that have different resources. Just so you know there is another one over at epanorama, but it hasn't appeared in the forum yet.
    So to sum up I AM NOT looking for a shotgun mic. I AM looking for some plans for another directional mic design. Besides I never mentioned anything about what the pros use. I just want a simple thing that works for recording ducks and geese at a local lake.
    Here is a pic of one I made several years ago, from a tractor radar ground speed indicator.
    http://img.photobucket.com/albums/v48/Earthshaker/horn.jpg
    [​IMG][​IMG]This thing actually worked but because of its small size, the frequency response was higher than desired. Originally the electronics were under the plastic cover seen on the top, so I modded it so the electronics were remote.
    [​IMG]
     
    Last edited: Nov 8, 2008
  6. Audioguru

    New Member

    Dec 20, 2007
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    A horn with a square shape and straight sides does not work well because sounds bounce from one side to the other and become delayed and cause cancellation of some frequencies. 'Pro" horn speakers have curved sides to try to reduce the effect of sounds bouncing between the walls but still have many resonances and nulls at different frequencies.

    Have you tried using a megaphone as a directional microphone? Their horn makes people sound like ducks so maybe it will make the ducks sound like ....
     
  7. Earthshaker77

    Thread Starter Member

    Nov 8, 2008
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    Actually I have tried a pa speaker horn as a microphone the speaker (driver) was blown anyway, so I modded it to fit a electret mic. Because of the design of the original speaker, all I had to do was yank the guts out remove the voice coil assembly and install the mic element in the center plastic insides, kind of hard do describe but if you have even taken a 5" CB type horn speaker apart you know what I am talking about. The re-entrant horn mic actually worked ok it was quite directional, but the freq response because of the metal horn assembly made it sound real tinny.
     
  8. SgtWookie

    Expert

    Jul 17, 2007
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    If you use a "shotgun" mic, it's going to be "tinny", and have a relatively narrow frequency response.

    Try using an unwanted satellite dish with an omnidirectional mic. Shield the mic from unwanted input from the sides, perhaps by using an audio insulated cone (picture lots of fiberglass). Place the mic where the original feedhorn was to retain the focus of the antenna.
     
  9. awright

    Well-Known Member

    Jul 5, 2006
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    Anyone interested in making directional microphones should obtain a copy of Harry F. Olson's book on acoustical engineering. It is a compendium of discussions of the characteristics of many acoustical devices from musical instruments and table radios to "perfect" horns and parabolic reflectors. Sorry, I do not remember the exact title and many of my books are in storage (probably composting by now). The book I am referring to was originally published quite a long time ago, possiblys before WWII, but I can't swear to that. I am sure that it is quite old as it was an old classic when I studied acoustics in the early '60s.

    For a parabolic reflector OR a horn, directivity is directly related to aperture dimensions - a larger aperture gives more directivity at a given frequency. If the dimensions of the aperture are small relative to the wavelength of the sound, the directivity is negligible. If the dimensions of the aperture are large relative to the wavelength, the directivity is high.

    Using a horn does not provide an escape from this fundamental relationship. Using a shotgun mic can get you away from the requirement for a large aperture because you are exploiting reinforcement and cancellation of sound along the length of the array, rather than across the aperture of the device.

    A 1 meter diameter parabolic reflector mic I built many decades ago had excellent directivity at high frequencies, but negligible directivity below about 1 kHz, exactly as theory would predict. This was a precise parabolic shape as it was formed by spinning overnight at a constant speed a 1 meter diameter bowl of epoxy as it cured.

    The performance of a horn is also related to the mathematics of the expansion of the horn (conical, exponential, hyperbolic, parabolic, etc.) and the rate of expansion. Earthshaker77, the "tinny" sound you got out of the reentrant horn mic could well be the result of the dimensions of the horn and the departure of the flare rate from the ideal, rather than from the material of the horn. A horn of small dimensions will act as a high pass filter at mid-audio frequencies resulting in a "tinny" sound.

    Intuition does not always provide very good guidance in the area of directional mics. Many people think that a horn eliminates side lobes in the directivity characteristic because the mic is hidden down in the throat of the horn and can't "see" off axis. This is a fallacy since sound does not travel only in straight lines but, rather, diffracts around edges in a predictable but complex manner related to wavelength and dimensions. Even an ideal imaginary circular piston has side lobes. Olson provide many directivity diagrams for various acoustical devices at various frequencies.

    Not trying to discourage experimentation - just trying to point out that a lot of the math of horns and other directional devices is available in the literature and can provide useful guidance to the experimenter.

    awright
     
    Last edited: Nov 9, 2008
  10. Wendy

    Moderator

    Mar 24, 2008
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    One of the books I read a long time ago (I think by Forrest Mimms) mentioned a simple unit made of straws cut to various lengths (resonant to acoustic frequencies) and bundled in front of a mic. Is this a shotgun mike, and does it apply to this conversation?
     
  11. Earthshaker77

    Thread Starter Member

    Nov 8, 2008
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    I thought about that a few years ago, I had a small dish and it was complete, I had taken all the guts out of the "feedhorn" and after a slight struggle, I got the antenna out of it, the remaining opening was almost the perfect size for a 3/8" diameter mic element, never finished it but I have a friend that has a few small dishes laying around. I wonder if a unidirectional element would work better?
     
  12. Wendy

    Moderator

    Mar 24, 2008
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    How do the new unidirectional speakers work? Most speakers are transducers, so they might hold clue, but I've yet to read anything on how they work.
     
  13. Earthshaker77

    Thread Starter Member

    Nov 8, 2008
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    I read a similar article that used 5 or 6-1/2" diameter PVC pipe sections, but of course I can't find it again. If I can find an real low cost alternative, that will work, so your input to the conversation is welcome :)
     
  14. Audioguru

    New Member

    Dec 20, 2007
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    A pro shotgun mic is very long and has a good low frequency response. It has a switch to reduce low frequencies.
    A shotgun mic for a video camera is a dinky little thing that sounds as tinny as its size and price.

    Chemelec (look for his website full of projects) has a little speaker as a mic with many metal tubes in front of it. Each tube has a different length and resonates at a different frequency. It is supposed to be directional but I think the boomy resonance of a little speaker will sound horrible as a mic, like a kid's walkie-talkie.

    Home Depot stores play music over their little re-entrant horn speakers outside. Boy oh boy does it sound tinny.
     
  15. Earthshaker77

    Thread Starter Member

    Nov 8, 2008
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    Thanks all for the replies, I have decided to just give up on building something. Maybe someday, I will get "r" done.
     
  16. 3dpan

    New Member

    Nov 15, 2008
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    Earthshaker,

    If you're still thinking about recording your ducks etc, I've been experimenting with using a standard umbrella as a "parabolic dish" with a cheap stereo lapel mic to pick up the sound of bird calls.
    Works surprisingly well, and has encouraged me to make a more permanent parabola.
    Cheers,
    Alec
     
  17. Trike

    New Member

    Mar 29, 2009
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    I saw that article years ago it was in one of the electronic project mags, build an Exponential Microphone. Check the web for the exact formula and calculate your curve, connect the points in a smooth curve and shape simple cardboard to the derived curve. I did that, and used some cut cardboard ribs to hold the shape. Painted everything with fiberglass resin and once it set (45 minutes or so) I had a light weight horn assembly that brought all the sound down the horn right to the mic element.
    You can extend the curve of the horn to make the throat area as large as you like. I found that an electret mic worked best. I also found that if you added a chamber in the rear of the horn, the freq. response improved. What I used on the old mike was a paper roll from a toilet paper roll and put one end to the horn exit (small end) and added a sliding cardboard bulkhead to mount the mic element on. I used a bit of foam to glue to the bulkhead and then hot glued the mic to the foam for isolation. by sliding the bulkhead wall in the tube, you can somewhat tailor the freq response, quite fun experiment. For a week end project, some cardboard cutting and fitting and a bit of experimenting I had a neat horn mic that did service for a buddy's garage band recording for the lead singer. Plot your curve from the middle point of the microphone so that the beginning of the curve takes place right at the edge of the mic element's outside diameter. Have fun, its a great project to play with.
     
  18. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    That is "Kind of" how a shotgun mike works, except rather than tuned straws, it uses the wavelength of the sound in a linear fashioin along the length.
     
  19. dkazdan

    New Member

    Mar 20, 2010
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    The article was in Popular Electronics about 20 years ago. It described an exponential horn, square cross-section, as an alternative to parabolic dish microphones. The author built his of corrugated cardboard. I made mine from thin, flexible sheet plastic (disposable cutting boards from a five-pack) and use it for nature sound recording. It works very well, and is all but free. The major advantage over a parabolic microphone of similar diameter is in low frequency response.

    Gain scales approximately as aperture (diameter). Directivity scales as ratio of length to aperture. Mine is 11" long and 15" square (that's the size the cutting boards came in) and uses an electret microphone element. I also built a 24" parabolic reflector with a commercial parabola (the stamped steel one from Edmund, good burning glass!), it works as expected, narrow beamwidth, about a 500 Hz rolloff.

    I'm making more exponential horns for bird sound recording on trips, as they can be packed flat and laced up on location. I recommend the design.

    David

     
  20. BMorse

    Senior Member

    Sep 26, 2009
    2,675
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    This is funny to me since my 12 year old just put together a parabolic mic assembly last night out of some parts we had laying around the shop, only using a 3 transistor amplifier for the mic, and the sounds he is picking up is very good and very directional.....

    Parabolic Mic.jpg

    B. Morse
     
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