Subkick Microphone Attenuator

Discussion in 'The Projects Forum' started by nickarnal, May 6, 2012.

  1. nickarnal

    Thread Starter New Member

    Nov 28, 2011
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    Hello.

    I am building a low frequency microphone out of a 4-ohm speaker by running it in reverse. I plan to hook it up to an XLR jack. From what I have gathered online, you hook up the plus/minus leads of the speaker to pins 2 and 3 on the XLR cable and leave pin 1 open. Does that sound right?

    Also, I found on a website that you can make a U-pad (the first figure) to attenuate the signal. Now, how would I find the impedance of the microphone/pad circuit? The link says that it is just the value of R2 but that does not make sense to me... wouldn't it be the impedance of the microphone/speaker (4-ohms) + R1 combined in parallel with R2?

    [​IMG]


    Thanks!
     
  2. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    Almost, you should connect pin 1 to shield, that is assuming you´re using symmetrical cable for pins 2 and 3 which should be the norm for microphones.

    Since this is meant to mic very low frequencies, you don´t want to load the speaker too much. That means that r1+r2 should be (by my guess) at least 20-50 times larger than the 4Ω. I suggest not going over 600Ω total as that might be more susceptible to noise.

    But then again, if the speaker is under-damped so it has too pronounced resonance on some frequency, you can try damping it more by putting different resistors in the 2-20Ω range in parallel with the speaker itself and see if it the sound gets better.
     
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  3. Audioguru

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    Cheap intercoms (used at gas stations) use the speaker as a microphone and the sound is horrible. An electret mic sounds MUCH better.

    A speaker used as a mic or a real mic has a very low output level. I do not know why you want to attenuate it.
     
  4. kubeek

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    Try using some quality 10" speaker to closely mic a kick drum or a bass amp, you will see how high the output actually gets. Also with good large speakers the quality in the sub-bass to start of mid range is actually pretty good. After that it naturally rolls off, but we have standard mics for that.
     
  5. Audioguru

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    Oh, the speaker as a microphone is for a drum. I think it will resonate like crazy, but maybe that is what you want, BOOM BOOM.
     
  6. kubeek

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  7. nickarnal

    Thread Starter New Member

    Nov 28, 2011
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    I am not sure I understand what you mean by connect pin 1 to shield. I am going to have the +/- wires of the speaker wired to a female XLR.

    And also, how would the speaker feel a load if its being used as a mic? From what I have read, I understand that the mic/attenuator circuit needs to be low impedance compared to the preamp so that the preamp bridges the mic and receives most of the load. But I am not sure I understand why the speaker needs to be low impedance compared to the attenuator.
     
  8. kubeek

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    If you got a metal enclosure the copper braid of the mic cable should be connected to it.

    [/QUOTE]And also, how would the speaker feel a load if its being used as a mic? From what I have read, I understand that the mic/attenuator circuit needs to be low impedance compared to the preamp so that the preamp bridges the mic and receives most of the load. But I am not sure I understand why the speaker needs to be low impedance compared to the attenuator.[/QUOTE]If the total resistance of the attenuator is too low, then it will be like an allmost short across the speakers terminals. This will caues the diaphragm to almost stop moving. On the other side, if the load resistance is not enough, the diaphragm will move too freely and will resonate. This all also depends on the shape and volume of the enclosure.
     
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  9. Audioguru

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    The cable to ANY microphone should be shielded audio cable to block pickup of mains hum. The shield is connected to ground at the amplifier and is connected to pin 1 (X) of the XLR connector.
     
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  10. nickarnal

    Thread Starter New Member

    Nov 28, 2011
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    Ok so I am going to make R1-440 ohms and R2-33 ohms which gives me a 17dB attenuation and a total impedance of 30.7 ohms. Does that sound alright?
     
  11. Audioguru

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    17dB of attenuation is 1/7th the output. That is a lot of attenuation for a microphone.
    Depending on the Qt and enclosure for the speaker as a microphone it might resonate badly.
    The speaker is not loaded with only 30.7 ohms. Instead it is loaded with 440 + 33= 473 ohms which is almost infinity to a 4 ohm speaker. It will sound very boomy and ring like a bell at its resonant frequency.

    Most audio amplifiers have an output impedance of 0.02 ohms or less so they squash the resonance of a speaker. Then the speaker performs exactly what the amplifier tells it to do without resonating.
     
    Last edited: May 7, 2012
  12. nickarnal

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    Nov 28, 2011
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    How come you just add the resistances to find the total impedance? I was taught in Circuits 1 to find the thevenin equivalent resistance by shorting any voltage sources (the speaker as a mic) which leaves 4 ohms and R1 in parallel with R2.
     
  13. Audioguru

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    R1 and R2 are in series with each other. Their total resistance is parallel to the speaker/mic.
     
  14. nickarnal

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    Nov 28, 2011
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    Ok I think I understand. I guess I was trying to find the impedance of the mic/pad combo, where I used a thevenin equivalent looking into the two terminals on the right.

    Now Audioguru, can you explain why the speaker being loaded affects the sound? I understand if the speaker is outputting sound, but I don't understand if it is being used as a microphone. Also, what would be the ideal load on the speaker?
     
  15. Audioguru

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    A speaker has a strong low frequency resonance like a bongo drum. It causes a boomy sound when used as a microphone. A resistor load on it reduces the resonance but also reduces the output level for all frequencies.

    A speaker also has a few higher frequency resonances that causes the sound to be shrieky. A resistor load also reduces these resonances.

    When a speaker is used with a modern amplifier (not vacuum tubes with an output transformer) then the extremely low output impedance of the amplifier (0.02 ohms or less) damps the resonances. Modern amplifiers have a good "damping ratio".
     
  16. kubeek

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    Here is an equivalent circuit of my 8Ω speaker. If you load the speaker parallel to meter V1 it will affect the preqquency response of it so it will also change the sound you record.

    The actual best value of the load is completely dependant on the speaker and enclosure, so I think it will be best to try different loads and see what gets you the best sound.
     
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