Iron Mike

Q. In my job, I work with microphones a lot. How does a microphone work?

A. There are three types: dynamic, crystal and electret. In the dynamic type, there is a thin diaphragm which vibrates along with the sound waves in the air. The diaphragm is attached to a coil suspended in the field of a permanent magnet. When the coil vibrates, a voltage is induced in it that varies along with the sound vibrations.

This voltage is the output signal of the microphone. Dynamic mikes are designed to operate into a low impedance, such as 600 ohms. Typical signal levels are about 50mV.

Crystal microphones use a piezoelectric crystal in place of the coil and magnet. In these types of crystals, mechanical pressure produces a voltage. Crystal mikes are of poorer quality than dynamic mikes, and are used in applications where an inexpensive microphone is acceptable.

They operate into a high impedance, usually 100 K to 1 Megohm, and have a high output voltage level, typically 1 volt.

Electret microphones use a capacitor with a special dielectric which holds a charge indefinitely. The plates of the capacitor vibrate along with the sound vibrations, which changes the value of the capacitor. This makes the voltage across it vary, and it is this voltage that becomes the microphone's output signal.

These microphones have excellent frequency response, and are generally used where small size and a high-quality signal are important. Electret mikes usually require a small battery. They are designed to operate into a high impedance.

An older type of microphone used a container full of carbon particles. The diaphragm was attached to the container so that the vibrations would change the pressure on the carbon particles. Thus their resistance varied along with the sound vibrations. The resistance element was connected in series with a fixed resistor and driven by a source of DC. Thus, the voltage across the resistance element served as the microphone's output signal.

Carbon mikes tended to be unreliable. The carbon particles would become compacted, and the mike had to be tapped a few times to loosen them. They have become obsolete, and are rarely used nowadays.

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