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Audio, how to make microphone unidirectional?

  1. Apr 14, 2010 #1
    hi
    the question is about how can I arrange a microphone such that it has a "line of sight" kind of sensitivity, only in one direction?

    What Ive tried so far is to place a 5/8" dia tube towards the direction to be measured and the mic is at the other end of the tube. Ive found in listening to a running engine this helps to isolate where noise is coming from, but its not working for this.

    And Ive tried a "slit" where a 1/4" vertical slit is made in a wood panel, and this also did not give the selectivity needed.

    Im considering trying to take some home insulation material/fiberglass R13 and try to insulate everything but an opening.

    But wanted to ask, what are the physics involved? (I dont want to reinvent the wheel) I want the microphone response to be near zero except for the exact place its pointed at. the measurements will be taken in a straight line from 5 feet to 25 feet away from the microphone
    thanks!
     
  2. jcsd
  3. Apr 14, 2010 #2

    berkeman

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    Hint -- they use them on the sidelines of NFL football games in the US....
     
  4. Apr 14, 2010 #3
    you can buy a directional microphone at most electronics stores, which is a lot easier than trying to make your own.
     
  5. Apr 14, 2010 #4
    OK so its a parabolic mic setup that should work; anyone know a link to the equations for how to calculate the needed size/focal length ?

    that reminds me, there is an old dish network dish sitting around somewhere, but would it have the curve needed?
     
  6. Apr 14, 2010 #5

    berkeman

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    That should work for starters. The bigger the parabola, the bigger the gain.

    [PLAIN]http://scienceblogs.com/startswithabang/upload/2009/04/weekend_diversion_do_tinfoil_h/tinfoil_hat_antenna.jpg [Broken]
     
    Last edited by a moderator: May 4, 2017
  7. Apr 14, 2010 #6

    berkeman

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    And without the tinfoil hat:
     

    Attached Files:

  8. Apr 14, 2010 #7
     
    Last edited by a moderator: May 4, 2017
  9. Apr 14, 2010 #8
    This looks like an excellent setup. But in this case its too complex, Id like to calculate the values for the parabolic antenna, make some fixture with cheap items (because having no work for almost 5 months there is no r/d budget) and something to adjust the audio focal point

    or maybe a box within a box within a box each one with only a narrow slit for an opening and stuffed with insulation. but how to calculate the best sizes?
     
  10. Apr 14, 2010 #9

    berkeman

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    It was one of the first hits for a google image search on top hat antenna.

    Did you get a chance to try the satellite antenna? It seems like it would work pretty well.

    How many of these things do you need to build?
     
  11. Apr 14, 2010 #10
    havent tried the dish yet; there needs to be 3 of these that respond about the same (at least 2 to start with) for one thing; and then another one just needs to be pocket sized
     
  12. Apr 15, 2010 #11
    The physics involved is that directional microphones are basically antenna theory using sound waves! As others have noted one way is to use a reflecting dish. These are like a telescope and focus the sound waves onto a normal microphone as an image just as a reflecting telescope does. These are often used for recording birds and work very well. I bring up birds because dish type directional mics have a low frequency cut-off determined by the diameter of the dish. Hence they work best with high pitched sounds (like birds) unless very large.

    Another sonic "antenna" is the phased array. Imagine many individual micro phones strung out in a line perpendicular to the source. Sound from that source now arrives at each mic at the same time so the signals all add giving a nice output. But if the sound source is off to the side each mic picks up the sound with a different phase and they tend to cancel and you get much less.

    A variation of this is called the "shotgun" mic. It is a large bundle of tubes of random lengths that are attached to the front of a single microphone. It's the same idea as the phased array only now it is all in phase when sound is coming in on the axis of the pipe bundle. Google "shotgun mic". These are common for video cameras and the like for sound. I've seen do it yourself built-it articles for making one around. I'll be you could find some on the Internet. They are cheap. Just a mic and a bunch of tubing.

    Good Luck.
     
  13. Apr 15, 2010 #12
    Ok thanks thats helpful, and this idea, the shotgun mic is something i DID try, just thought up the idea myself. it was not selective enough. The parabola for recording birds? good idea.

    I think the other idea is to take some junk CDs, line up the holes but space the disks some increasing distance apart and wrap the outside of the discs with heavy R13 insulation; maybe metal discs are needed and or the internal hole diameter needs to change also.

    There was one of those round cylinder boxes of oatmeal and an old CD disc on top of it that was about the right size, hey a disc! or what about a bunch of discs! if that doesnt work then the next thing is to put a microscope glass slide at the end of the above disc array, bounce a laser off the glass and onto a QPD, thru a lens somehow so the spot isnt so tiny, then detect the signal from there
     
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