selecting an electric microphone for intercom

Discussion in 'General Electronics Chat' started by bug13, May 12, 2016.

  1. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
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    Hi guys

    I need to choose an electric microphone for an intercom. I usually google application notes for <component>. In this case, application notes for electric microphone. But I didn't find a good one.

    Can someone give me some pointer please?

    I am guessing the important specs I need is SNR and sensitive?

    Thanks guys!!
     
  2. #12

    Expert

    Nov 30, 2010
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    The important specs you need are directionality, like: https://en.wikipedia.org/wiki/Cardioid
    We often use two mics connected out of phase for near field communication. That configuration reduces background noise dramatically.
    Now that you have the signal to noise ratio you need, a j-fet amplifier is the way to go.
    ps, you can limit the frequency range by changing the cap at the top and the cap on the left. You don't want 40Hz to 20KHz for voice only. More like 300Hz to 3000 Hz.

    You can also go with a TL-071 amplifier.
     
  3. #12

    Expert

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  4. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
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    Hi #12,

    Thanks for you reply, I am not good at audio stuff, so please bear with me. Let me break it down one by one. How do you connect two mics out of phase?

    Note: this question is relative to my other question here.
     
  5. #12

    Expert

    Nov 30, 2010
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    Look for, "differential amplifier".
    Gotta go. Lunch is ready.
    BRB
     
  6. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
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    I assume you mean connect two cardioid type mic in right angle, and feed them into differential amplifier??? It starts to make sense for me.
     
  7. #12

    Expert

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    No, two mic elements right next to each other, facing the same way. Plug them into a differential amplifier and the only sound left is the difference between the two inputs. There is no such thing as a distant sound that is not equal across the distance between the mic elements. Only close up sounds will be amplified.
     
  8. #12

    Expert

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    Another idea about directionality. Imagine two speakers, side by side. They project sound with a definite preference for a vertical line of most loudness.
    Stack them one on top and they broadcast a louder beam in a horizontal plane.
    If your intercom is at face level, place the mics vertically to listen to a horizontal plane. If your intercom is mounted high, place the two mics beside each other to listen in a vertical plane.

    You're really getting an education today!
     
  9. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
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    Sorry I don't get it. I can understand that only the different between the two inputs is left. Buy why our voice is the different parts?

    The common signal (noise) here, are you referring to the noise in the power supply etc.. or the acoustic background noise? I can understand the electrical noise being the common part and ignore by the differential amp.
     
  10. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
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    Yes I am, if I am somewhere near you, I will buy you a coffee/beer/or whatever that float your boat so I can have a chat with you in person.
     
  11. #12

    Expert

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    I never said a word about the power supply. I am talking about using microphones to detect sound.
    The fact is, sounds that are close by will be amplified by two mics arranged in opposing polarity and distant sounds will arrive equally at both mics and be defeated by the differential connection.
     
  12. #12

    Expert

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    Sound System Engineering by Don and Carolyn Davis 1975
    Sams Publishing
    International Standard Book Number 0-672-21156-4

    No, I didn't go to school for this. I taught myself out of that book and worked with some bands in L.A.
     
  13. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
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    I can understand this explanation better than the other one. Thanks a lot.

    And what do you mean by arranged in opposing polarity? I though they sit right next to each other, and facing the same direction. Where is this opposing polarity thing from?
     
  14. #12

    Expert

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    ELECTRICAL polarity. Connect one mic to the inverting input of an op-amp and connect the other to the non-inverting input.
     
  15. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
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    Noted! And a second hand one only cost about US$5, nice!
     
  16. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
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    A picture worth a thousand words. Is that what you mean?

    8c8df14a-b5fa-4537-86f7-aa0e7b4245da.jpg
     
  17. #12

    Expert

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    Like this:
     
  18. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
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    Thanks a lot, that explains it very well.

    So in your experience, in a configuration like this, what is the maximum useful distance can it pick up?
     
  19. #12

    Expert

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    Only a few feet. It's about half dead (down 10 db) at 6 feet.
     
  20. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
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    I think this might be my last question.

    Because my mic is connected to my audio input up to 5-6 meters away. I am worried the noise pick up over the wire. So I want to pre-amplified the audio signal at the source (at my mic). How much gain do you recommend for me to amplify the signal?

    I am guessing as much as possible to increase SNR over the wire, but not too much to produce clipping??
     
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