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is it possible to make a directional mic out of an omni?

Discussion in 'Electronic Basics' started by tempus fugit, May 4, 2005.

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  1. tempus fugit

    tempus fugit Guest

    Hey all;

    I've got a couple mic elements that are omnidirectional. It would be better
    for me if they were directional (like a cardioid or something). How do they
    make the directional pattern? Is it something in the electronics, the
    casing, or the element itself? Is it possible to make the omni uni?

  2. Ban

    Ban Guest

    The directional capsules are different, they have a soundpath on the
    backside, where the omnidirectional ones have only a tiny capillary hole for
    air pressure changes.
    It is possible to convert an omni to directional by making an acoustical
    lens in front out of a bundle of plastic straws, John Woodgate had posted
    some pic not so long ago. You could also use a parabolic reflector with the
    mike mounted in the focus point.
    Another possibility is to electrically combine the signal of several
    capsules in form of a phased array. This is usually defined with the
    capsules equidistant in a line. Also you can make a nice stereo-mike this
    way with 9 or 13 capsules. I do not know how to calculate the positions and
    damping factors though, but I think K. Aylward has some software to do this,
    it should be similar for sound and antennas.
  3. Rich Grise

    Rich Grise Guest

    It's the casing, believe it or not. You mount the mic element at the end
    of a tube, and put holes in the sides of the tube, and baffles to increase
    the length of the acoustic path along the tube. That way, the sounds that
    come from the front, that miss the front of the mic element but travel
    alongside the tube, go through the holes in the tube, and back forward to
    the back of the element, arriving 180 degrees out of phase, reinforcing
    the signal from the front. Think of a transmission line 1/2 wavelength
    long. Sounds that come from the sides or the back arrive at the element
    out of phase, and so cancel out.

    It's theoretically possible to do this at home, but if your time has
    value, it'd be cheaper to buy one. :)

    Hope This Helps!
  4. Bob Masta

    Bob Masta Guest

    Following up on Rich's comments, you might want to Google
    for "Shotgun Microphone" if you are looking for super
    directionality and don't mind fiddling a bit. There used to
    be perennial articles in Popular Electronics and the like
    back in the '60s about how to build your own, but it all
    boiled down to just what Rich says: A pipe with holes in
    the sides. It won't be small, but it will be directional!
    You used to see these on TV shows where the host
    would point it at a member of the audience to pick up
    one voice from a crowded room.

    However, most small "unidirectional" mics are nowhere
    near this directional. They are simply cardioids: The
    response pattern looks like a figure 8 with the front lobe
    bigger than the back. Also note that the more directional
    a mic is, the more dips and peaks in its frequency response.
    If you are using it for music recording, it's often better to
    use an omni up close instead.

    Best regards,

    Bob Masta

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis
    Home of DaqGen, the FREEWARE signal generator
  5. tempus fugit

    tempus fugit Guest

    Thanks for the info guys. I guess I'll just leave it the way it is.
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