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Using a microphone to view sound waves on an oscilloscope

  1. Mar 21, 2012 #1

    I want to use and oscilloscope to demonstrate sound waves for our local school science day. I want to be able to allow kids to make different vocal sounds to see how the sound waves look like.

    I removed the microphone part from a headphone and used it in a circuit to see audio signals on an oscilloscope. The circuit I used is here.
    Please scroll down. I used the circuit with an amplifier and dual power supply. For power supply, I used two 9V batteries as suggested in the above website. I don't see anything interesting on the scope. I had a vibrating tuning fork next to the microphone. I expected a wave form on the oscilloscope, but nothing happened.

    Is there anything else I can do? Should I purchase a microphone? If so what type is better? If any one has done this before, and have a better circuit could you share it with me? Any hint or suggestion to get this work is much appreciated! The science day is fast approaching!! thank you.

  2. jcsd
  3. Mar 21, 2012 #2


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    You should not need any additional circuitry to see a signal on your oscope from the microphone. Just connect directly to the mic's leads. adjust the V/div until you get a suitable signal.
  4. Mar 21, 2012 #3
    I see two circuits on the ref'd page.

    The first is for a simple electret mic, which requires a DC power source to operate -- thus the resistors and 9v battery. It does need to be polarized correctly, usually the red wire to the + battery, and the signal you get will be in the milli-volts range so you need fairly high gain on your scope input. Try it without the 1K resistor across the mic as that is rather a large load in itself. If your mic is not an electret you don't need the battery so you could try just connecting the scope directly, again with high input gain.
    Tap on the mic or knock it on the table to get a fairly large signal which you might be able to see...

    The second is for a 741 opamp pre-amplifier. Since you say you used two 9v batteries, I guess this is the one you are asking about. If you're not getting a signal from your mic to start with there's nothing to amplify. It would be nice if "they" had included the schematic diagram so I can tell what it's supposed to be doing, but you should be able to disconnect the mic and just touch the input pin to get some noise on the output. Also try a resistor to each battery supply, or a 10K or so pot between them, as the input. You should see the output swing in the opposite direction which will tell you that the circuit is doing _something_ right.
  5. Mar 21, 2012 #4
    What do you see on the oscilloscope? "Nothing interesting" is a little too vague.
    It may be that either vertical or horizontal scale (or both) is not properly selected.
    What is your time scale?
  6. Apr 4, 2012 #5
    Sorry for the big delay. The problem is solved. I did not do anything differently. Just redid the wiring.

    As some one suggested here, I connected the microphone directly to the oscilloscope and all I see a a horizontal line. When I talk into it, I don't see any response even with V/div at its highest setting. I tried both mics. The mic that I broke off from the headphone and the electret mic that I bought.

    I don't know the inside working mechanism of the mic, but I was thinking that it might need some type of power to work. That is why I used the batteries. With the voltage divider circuit and the amplifier circuit, the system worked!

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