How Microphones Work

Converting sound into electrical signals

By on May 8th, 2008 12:40 GMT
Often referred to as mike or mic, the microphone first became commercially practical with the invention of the carbon microphone by Thomas Alva Edison in October 1876. Back then microphones were called transmitters. Practically, the microphone is an electronic device capable of capturing minute air pressure waves or sounds and convert them either in an electrical current or an electrical parameter such as electrical resistance, capacitance or inductance.

According to the electrical parameter used, microphones can be classified in carbon microphones, dynamic microphones, ribbon microphones, condenser microphones and crystal microphones. Some of the first microphones ever constructed consisted of metallic membranes connected to a metal needle scratching against thin moving metal foil. The metallic membrane captured the sound pressure and converted it into linear mechanical movement that was transmitted to the needle. This enabled the needle to punch patterns into the thin metal foil accordingly to the sound coming from the surrounding medium.

The reverse process, or reading and playing the registered sound, implies a similar approach. However, this time the needle presses against the recording material and converts the patterns into linear mechanical movement. This movement can then be amplified and converted back into sound through a number of processes.

Types of microphones

Carbon microphones are amongst the oldest, simplest and most used types of microphones even to this day. They work by converting air pressure variations into electrical resistance. The membrane collecting the sound waves presses against a carbon dust material that varies its electrical resistance in the process. By running electric current through the carbon dust, one can obtain an electrical current variation that is amplified and recorded.

Dynamic microphones on the other hand harness the electromagnetic effects determined by the movement of a magnet inside a conductive wire coil. The vibrations of the magnet are basically converted into tiny electrical currents that are amplified and recorded.

Ribbon microphones work on a principle rather similar to that of the dynamic microphones, but instead of vibrating a microphone inside a coil, a thin ribbon is suspended in a magnetic field. The vibration of the ribbon translates into inductance variations inside the coil generating the magnetic field.

Condenser microphones rely on the properties of capacitors. However, the plates of the capacitor are no longer immobile and are free to move in relation to each other according to the air pressure changes. This generates a variation in the capacity of the device, which can be converted into electric signals.

Crystal microphones are based on the piezoelectric effect. Piezoelectric materials have the ability of directly converting electric energy into mechanical movement and vice versa. The most common piezoelectric material occurring naturally on Earth is quartz, which is often used to make crystal microphones.