3.5 Microphones

Microphones are used to convert acoustical energy into electrical energy. The microphone serves as an example of the idea that a specific purpose can be accomplished using many different physical principles.

Carbon:

Carbon microphones are made by encasing lightly packed carbon granules in an enclosure. Electrical contacts are placed on opposite sides of the enclosure. When an acoustical pressure is exterted on the carbon granules, the granules are pressed closer together which decreases the measured resistance. This is a very low quality acoustic transducer, but has been used in telephone handsets even through the current day.

Capacitor (condenser):

Capacitor microphones are made by forming a capacitor between a stationary metal plate, and a light metallic diaphragm. When an acoustical pressure impinges on the diaphragm, the diaphragm moves and causes the distance between it and the stationary plate to change. As mentioned in a previous section, this will change the capacitance of the device. In order to measure the capacitance, one must apply a charge to the device. When this is done, the change in capacitance will result in a change in the voltage measured across the device since

Electret and Piezoelectric:

Electret microphones are capacitor microphones which use an electret material between the plates of the capacitor. As mentioned earlier, electrets are materials with a permanent polarization, and hence surface charge. A benefit to using electret microphones is that they do not need any external circuitry to create the charge, and hence are much easier to use. Many high quality, low cost electret microphones are available currently.

As discussed previously, piezoelectric crystals are crystalline structures which are similar to electrets in that they exhibit a permanent polarization of the individual cells. It is possible to use piezo sensors as microphones as well. Since they are in the form of a thin film, they are very useful if one is interested in detecting surface vibrations of an object.

Magnetic (moving coil):

Moving coil, or dynamic microphones are based upon the principle of magnetic induction. When an electrical conductor is moved through an electric field, a voltage is produced. This voltage is proportional to the velocity of the conductor. A moving coil microphone is made by attaching a coil of wire to a light diaphragm which moves in response to acoustical pressure. The coil of wire is immersed in a magnetic field, hence the movement of the coil in the magnetic field will create a voltage which is proportional to the acoustical pressure.