speaker used as "microphone".

Discussion in 'General Electronics Chat' started by secretagentman, Aug 17, 2012.

  1. secretagentman

    Thread Starter New Member

    Aug 14, 2012
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    Greetings to ALL. I posted some things about this experiment/project I am doing on the Physics part of the forum and forgot one question that I thought would probably be easily answered here, so if this breaks any rules I apologize in advance. I am new to forums, and to "computing" in general. My question is : in using a speaker as a crude microphone, would a low impedance rating(say-4 Ohms) provide a greater or lesser output
    voltage than a higher impedance(say-16 Ohms). Moving coil phono cartridges with higher output seem to always have higher D.C. resistance. I know that resistance and impedance are two different "animals", but that seems to be the only such spec they use. At least I could find no manufacturers which specified impedance. I appreciate any facts foremost, but advice and opinions in a positive light (even if to correct) are always welcome as well. Thanks in advance, for your time, patience, and tolerance. secretagentman.
     
  2. Wendy

    Moderator

    Mar 24, 2008
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    They are not very high fidelity, but magnetic speakers are used all the time for microphones, in things like intercoms, for example.

    I can't really answer the question about signal levels, never thought much about it.
     
  3. steveb

    Senior Member

    Jul 3, 2008
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    It's hard to compare using impedance as the only spec. All things being equal, lower impedance would provide a better component to interface into an audio amplifier. However, both 4 and 16 ohms is relatively low impedance when compared to the input impedance of an amplifier. Generally, you want low impedance sources driving into high impedance inputs (for voltage driven audio stages), so this will be good. However, if the 16 ohm speaker is able to generate more voltage for the same sound input, then that will likely give you more overall output once connected to an amplifier. The detriments of the higher impedance are small and easily overcome by the greater voltage generated.

    Very high impedance sources (mics or phonos etc.) are more challenging to interface into an amplifier, but obviously it can be done. Once impedance gets to high levels, then the actual value of impedance is much more important to consider. There is not only the issue of attenuation of the signal but frequency response and equalization.
     
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  4. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Many years ago, I used a common base amplifier for a speaker/mic amplifier. It is a good impedance match to the speaker. Googling brings up such an amplifier. My only issue with the one I found is that they use an 8.2Ω emitter bias resistor, which will waste a lot of speaker/mic power. For decent linearity, the DC emitter current should be about an order of magnitude higher than the peak signal current from the speaker (I don't know what that number is). Some negative feedback would improve linearity at the expense of gain.
     
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  5. #12

    Expert

    Nov 30, 2010
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    I just weeded out the confusing parts.

    I'm sure you know that using a speaker for a microphone will have poor quality in both distortion and frequency range. Getting a usable signal is not difficult. Getting an excellent signal is near impossible.
     
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  6. steveb

    Senior Member

    Jul 3, 2008
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    I agree.

    Another confusing thing about my post is that the usual model of a voltage source driving voltage amplifiers does not necessarily apply in this case of using a speaker as a mic. This is clear with RonH's example of the speaker driving a CB amplifier with relatively low input impedance. The impedance interactions and frequency response issues are not trivial here.
     
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  7. #12

    Expert

    Nov 30, 2010
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    I just grabbed a 16 ohm, 6x9 speaker and shouted at it. 40 mv p-p into a 1 meg scope probe and 20 mv p-p into a 1k resistor in parallel with the scope probe.
     
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  8. Ron H

    AAC Fanatic!

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    If you can get even more current at lower load resistance, a common base amp might work well. Another advantage is better damping.
     
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  9. #12

    Expert

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    Is there a hidden meaasge in there?
    Do you want me to try 100 ohms? or 10 ohms?
     
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  10. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Sure, both, if you can measure the voltage.:p
     
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  11. #12

    Expert

    Nov 30, 2010
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    I got out 2 POS speakers, aimed them at each other and found a resonance at about 90 Hz.
    I set up .25V p-p at 1 megohm and found no attenuation while reducing the shunt as low as 1k
    at 100 ohms, .16 V p-p
    at 10 ohms, .088 V p-p
    at 5 ohms, .07 V p-p

    at this point I quit because the resistance of the jumper wires and alligators became significant and I was too lazy to solder everything together.
     
  12. secretagentman

    Thread Starter New Member

    Aug 14, 2012
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    I have to admit that I might stand a better chance with this project if I can find an elementary electronics forum, or "buckle down" with a course of formal study. I appreciate everyone's replies. I only understood the half of it. That is my fault, not yours.Relying on my track record, give me a dead amp or electric guitar, and chances are quite good that I'll fix it. Want to modify said guitar or amp,I have many mods up my sleeve. Need an effects pedal, I've built every type of distortion pedal imaginable, plus compressors, noise gates, flangers, phasers,buffers,ring modulators,envelope followers,talk boxes, and battery powered amps to boot. I maintain a large collection of schematics and even more books which allow me to do this. I was once an electronics assembler(avionics), I guess that's what I still am. I apologize if I wasted anyone's time. Thanks, and best wishes to ALL. secretagentman.
     
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