Generally, a microphone is a transducer as it converts sound waves (acoustic energy) into electrical energy. The very first microphone was purely mechanical in nature. A metal diaphragm is connected to a needle, which “draws” a pattern on a metallic foil. When the air pressure changes due to a person’s voice, the diaphragm vibrates and moves the needle. The needle then scratches the foil with the vibration pattern. The sound is recreated when the needle is made to run over the foil again. The vibration pattern being followed by the needle makes the diaphragm move and reproduces the sound.
Microphones now work the same way but does the process electronically. Instead of a scratched foil with the vibration patterns, the change in air pressure is now converted to an electrical signal. The diaphragms can be of any material such as plastic, paper or aluminum. Diaphragms differ in producing sound which gave rise to different classifications of microphones.
This kind of microphone is the simplest of all. It uses carbon dust hence the name carbon microphone. A diaphragm made up of plastic or thin sheet of metal on one side compresses the carbon dust and changes the resistance when reached by sound waves. Letting electricity pass through the carbon lets the varying resistance change the amount of current flowing through it. The first telephones, even some recent ones, use this technology.
This classification makes use of the electromagnet technology. When the sound waves reach the diaphragm and cause it to vibrate, it also moves a magnet or a coil of wire which produces a small current.
A condenser microphone is like a capacitor with a back-plate moving when hit by sound waves. This movement changes the distance between the plate and the diaphragm, which also alters the capacitance. This fluctuation in capacitance produces electric current which is amplified to a measurable level. A small battery is needed for this type of microphone that serves the voltage of the capacitor.
With this type of microphone, a thin ribbon is used as the diaphragm. This ribbon is surrounded by magnetic field. The ribbon moves when hit by sound waves, and this changes the measure of the current flowing through it.
The diaphragm of a crystal microphone is attached to a crystal which is known to change its electrical properties when the shape is changed. This change in the properties of the crystal produces electrical signals.
Microphones also differ in their directional property. This property refers to the microphone’s sensitivity to the direction of sound source. In terms of directional property, microphones can be any of the following:
a. Omni-directional – a microphone that picks up sound from all directions evenly.
b. Uni-directional – a kind of microphone that picks up sound mostly from just a single direction such as cardioid and hypercardioid microphones.
c. Bi-directional – microphone of this kind picks up sound in opposite directions.
Essentially, microphones work because of the diaphragm that is common to all kinds of microphones. The diaphragms move due to the picked up sound waves and produces the signal depending on the technology being used.