Microphones

Microphones are electromechanical instruments which convert pressure changes into electrical signals. Microphones can be divided into dynamic microphones and condenser microphones. In addition to these, we should include MEMS microphones, which are getting more common.

Condenser microphones are so named because they use a (variable) capacitance for the signal conversion – the word condenser being the old name for capacitors. The capacitance occurs between two plates where one (or both) is charged with a voltage. One of the plates is a movable membrane, which captures pressure changes in the air. As the plates move closer and further apart, the capacitance is varied. The variation is picked up and amplified to produce the audio signal – a varying voltage.

The electret microphone (technically Electret Condenser Microphone or ECM) is one kind of condenser microphone. The type is uses use a pre-charged membrane which vibrates in relation to a fixed plate – the membrane and plate forming two halves of a capacitor. The flexible membrane is made of a material which will store a voltage potential for several hundred years – the word for this kind of material is electret. A JFET transistor within the microphone capsule is needed to adjust the microphone's output impedance. Electret microphones therefore need to be powered. Often around 2-5V, but with current limiting resistors they can work at many voltages.
Electrets are cheap, compact, easy to use and have very good sound quality, especially considering the low price. The frequency range is wide, often 20-20Khz, and free from large resonances. The directional characteristic is commonly omni. Electret microphones are found in everything from toys to phones, field recorders, IoT devices, etc.

"Normal" condenser microphones work in much the same way, but instead of an electret material, two conductive capacitor plates are used. These plates do not retain a charge, so they need to be powered with a voltage, often 48V (so-called phantom voltage). They are mainly used in music equipment.

MEMS microphones are fabricated (etched) directly onto a semiconductor material. MEMS stands for Micro Electro-Mechanical System, the technology is otherwise used in e.g. accelerometers. Often, MEMS microphones have a built-in amplifier circuit. Many also have a built-in A/D converter and send serial data over e.g. I2S or with PDM (Pulse Density Modulation) instead of analog voltages.

MEMS microphones can be made very small and are relatively insensitive to electronic interference. They are often found in mobile phones and other hand-held products where lack of space is a factor. Since they can be manufactured with very tight tolerances, they also appear in matrices, in order to e.g. directional sound sources.

The most basic kind of microphone is the dynamic microphone. They work according to the same principle as speakers, but backwards: the diaphragm of the microphone is attached to a coil of copper wire. Near the coil is a fixed magnet. As the sound pressure changes, the diaphragm moves, and the coil and magnet form a varying voltage. That voltage is the audio signal. Dynamic microphones require no power to operate and are durable, but the construction is relatively large and heavy. Their dynamic sensitivity and frequency ranges are also limited compared to condenser microphones