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Microphone Science Project?

Discussion in 'Electronic Basics' started by Magoo, Dec 4, 2003.

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  1. Magoo

    Magoo Guest

    I'm looking for some info on building a microphone out of scrap materials
    as a science project. I've googled it but didn't find anything really good.

    Anyone know of a page that describes how to build microphone from scratch?
    Or maybe you have some tips of your own?
  2. A dynamic microphone is essentially a tiny speaker.
    a speaker has a permanent magnet in an iron core designed to produce a
    radial field (outward across a circle) in a circular air gap. The
    diaphragm that moves when air vibrations hit it has a small coil
    attached to it that sits in the air gap. When the wires move back and
    forth across the magnetic field, voltage is produced in the wire. If
    current is passes through the wire, the coil produces force on the
    There are also microphones based on other energy conversion

    For a simple home made version of the dynamic mic, it might be easier
    to mount a stationary coil and move the magnet in and out of it with
    the diaphragm. Not as high fidelity, because the magnet is heavy, but
    simpler to build.
  3. Rich Grise

    Rich Grise Guest

    Take a shoebox lid, affix two razor blades vertically an inch or
    so apart, and lay a pencil lead across them. When the box vibrates,
    it changes the resistance between the contacts, just like a carbon

    Also, just a piece of foil suspended between the poles of a horseshoe
    magnet, with contacts to sense current at right angles to the mag.
    field; this is called a "ribbon" mic.

    Good Luck!
  4. Magoo

    Magoo Guest

    Right. This is what I was looking for. I can't find info on how this
    works. I dont mean details on how to do it, I mean explanation of the

    So in your hint above, you run current through the razor blades, and as
    the shoe box vibrates, the pencil lead bounces? It's more or less a
    mechanical thing and carbon is used only because it's a conductor that
    happens to bounce well?

    I saw a page that explains how to build a working model of Bell's first
    telephone. It had a wire suspended from a diaphram over a copper cup
    containing vinegar. As the diaphram vibrated, the wire would contact
    the vinegar and current would be passed through the vinegar to the cup.
    So that's an entirely mechanical set up.
    If I understand things, this is a little bit different though, right?
    This method depends on the small amounts of current generated in the
    foil as it moves w/i an magnetic field. Is that right?

    Wouldn't you need some way to amplify the signal?
  5. Patina Creme

    Patina Creme Guest

    A condenser mic is basically a capacitor right? shouldn't I be able to
    build one at home? I have read a few pages on the net talking about buying
    a mic and using the capsule -im not too excited about that
    someone must have built the first one by hand.
    aren't the best capsules built by hand?
  6. You may well be able to build a decent condenser microphone. It is
    basically a capacitor with an air dielectric and one plate is a
    diaphragm with a bias voltage applied. In most modern ones, the bias
    is replaced with a slab of plastic that has an electric field trapped
    inside (called an electret) but you could also make one of those.
    Here is a site that hay help you with the basics, and give you the key
    words to use in Google to look further.
  7. Bob Masta

    Bob Masta Guest

    Electret mics are normally used instead of pure condenser mics,
    which require 200V bias and are typically >US$1000. The little
    electret capsules are only a buck of two and run on a few volts,
    having built-in FET preamps.

    However, it is possible to build your own electret, though I've never
    done it myself. The trick of an electret is a plastic film which
    forms the membrane and is permanently polarized with high voltage
    (200V, etc), sort of like an electrostatic equivalent of a permanent
    magnet. You do this by applying 200V across the film while it is
    cooking in an oven at just the right temperature, and cooling it
    with the voltage applied. Then when you remove the voltage,
    the plastic retains the charge because some polar molecules
    have rotated while the plastic was warm and soft, and can't rotate
    back when it is cool.

    Bob Masta

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis
  8. Bob Masta

    Bob Masta Guest

    For many years the standard microphone in telephony was a
    carbon button mic. It was essentially a little chamber of
    carbon dust between two conductors. When sound vibrated
    the dust, the resistance changed in time with the vibrations.
    The big advantage was really high output, since the voltage
    across the carbon was fairly high. (I'm not sure if they used
    the phone line 48V directly, but something like that.)

    Bob Masta

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis
  9. There was an article in Popular Electronics around 1968 about electret
    microphones, and there was a section on making your own electret microphone.
    Obviously, it was for the sake of understanding it, rather than making
    one that one would use for regular activity. The one part I do remember
    is their suggestion of using the high voltage out of a tv set to charge
    the microphone.

  10. Let's see if I can throw a few ideas your way. Already, two posters have
    mentioned the carbon rod on two razor blades and the condenser microphone
    methods. Actually, almost any taut foil next to a piece of Mylar with another
    foil on its backside would make a pretty decent microphone.
    How about using a small light source and reflecting it off a foil element
    and catching the resulting reflected beam with a photodiode or phototransistor?
    Then you get an optical microphone.
    Or, stretch a metal wire through a gap in a magnet- like a horseshoe type.
    Fasten a diaphragm to the wire and wrap a pickup coil around the magnet. The
    coil output will be the sound signal.
    Or, get some flexible circuit board stock, very thin stuff, and etch an
    extremely fine and convoluted pathway that covers its surface. Now, connect
    that to a Wheatstone bridge and you have a strain gauge that will turn the sound
    signal into a varying resistance.
    Just a couple of rough ideas.


    Chip Shults
    My robotics, space and CGI web page -
  11. Rich Grise

    Rich Grise Guest

    In a nutshell, yes, yes, yes, and yes. :)
    (more below)
    The current path through the razor blade, to the graphite lead, to
    the other blade, and out, has a certain resistance. And yes, the
    resistance changes as the lead bounces.
    It's a "poor" conductor, so it presents some resistance while
    just sitting there. The same with the carbon grains in a Bell
    phone - it's used because it's a "poor" conductor, so that
    the resistance will decrease when the pressure (sound wave)
    increases, pressing them together, and the resistance increases
    on the rarefaction wave.
    Yes - the carbon mic doesn't necessarily need an amp, because
    it's just a variable resistance, and can be just put in series
    with the speaker (which is the way old telephones work. after
    all); the ribbon mic is a tiny little generator, and that signal
    definitely needs to be amlpified.
  12. Patina Creme

    Patina Creme Guest

    Thanks for all of the input. I am going to build myself some microphones,
    from the carbon mic to the ribbon. There wouldn't happen to be a book out
    there on someones shelf titled "How to build microphones from A to Z"?

    this website is great. Thanks John Popelish.

    Thanks again everyone for the info,
  13. Rich Grise

    Rich Grise Guest

    see for my artists' conception of a ribbon mic.

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