Inside the Oktava 319 condenser microphoneIn a condenser microphone, also known as a capacitor microphone, the diaphragm acts as one plate of a capacitor, and the vibrations produce changes in the distance between the plates. There are two methods of extracting an audio output from the transducer thus formed: DC-biased and RF (or HF) condenser microphones. With a DC-biased microphone, the plates are biased with a fixed charge (Q). The voltage maintained across the capacitor plates changes with the vibrations in the air, according to the capacitance equation (C = Q / V), where Q = charge in coulombs, C = capacitance in farads and V = potential difference in volts. The capacitance of the plates is inversely proportional to the distance between them for a parallel-plate capacitor.

A nearly constant charge is maintained on the capacitor. As the capacitance changes, the charge across the capacitor does change very slightly, but at audible frequencies it is sensibly constant. The capacitance of the capsule and the value of the bias resistor form a filter which is highpass for the audio signal, and lowpass for the bias voltage. Note that the time constant of an RC circuit equals the product of the resistance and capacitance. Within the time-frame of the capacitance change (on the order of 100 μs), the charge thus appears practically constant and the voltage across the capacitor changes instantaneously to reflect the change in capacitance. The voltage across the capacitor varies above and below the bias voltage. The voltage difference between the bias and the capacitor is seen across the series resistor. The voltage across the resistor is amplified for performance or recording....

The MEMS (MicroElectrical-Mechanical System) microphone is also called a microphone chip or silicon microphone. The pressure-sensitive diaphragm is etched directly into a silicon chip by MEMS techniques, and is usually accompanied with integrated preamplifier. Most MEMS microphones are variants of the condenser microphone design. Often MEMS microphones have built in analog-to-digital converter (ADC) circuits on the same CMOS chip making the chip a digital microphone and so more readily integrated with modern digital products. Major manufacturers producing MEMS silicon microphones are Wolfson Microelectronics (WM7xxx), Analog Devices, Akustica (AKU200x), Infineon (SMM310 product), Knowles Electronics, Memstech (MSMx)and Sonion MEMS....

Early microphones did not produce intelligible speech, until Alexander Graham Bell made improvements including a variable resistance microphone/transmitter. Bell's liquid transmitter consisted of a metal cup filled with water with a small amount of sulfuric acid added. A sound wave caused the diaphragm to move, forcing a needle to move up and down in the water. The electrical resistance between the wire and the cup was then inversely proportional to the size of the water meniscus around the submerged needle. Elisha Gray filed a caveat for a version using a brass rod instead of the needle. Other minor variations and improvements were made to the liquid microphone by Majoranna, Chambers, Vanni, Sykes, and Elisha Gray, and one version was patented by Reginald Fessenden in 1903.

These were the first working microphones, but they were not practical for commercial application. The famous first phone conversation between Bell and Watson took place using a liquid microphone....

Laser microphones are often portrayed in movies as spy gadgets. A laser beam is aimed at the surface of a window or other plane surface that is affected by sound. The slight vibrations of this surface displace the returned beam, causing it to trace the sound wave. The vibrating laser spot is then converted back to sound. In a more robust and expensive implementation, the returned light is split and fed to an interferometer, which detects frequency changes due to the Doppler effect. The former implementation is a fun tabletop experiment; the latter requires an extremely stable laser and precise optics....

Competing with carbon microphones were magnetic microphones that we know today as dynamic mics. A sound moves a small magnetic coil in and out of a magnetic field. This is just like a speaker except the moving coil makes electric current instead of an electric current making a coil move. The weight and size of the coil is the limiting factor in the quality of the music. A small coil makes for a much better quality microphone, but at a cost of ten times that of the carbon mic.

Variations on the dynamic mic are the Shure Controlled Reluctance element that works by moving an iron pin in and out of a magnetic field changing the shape of the field and causes electrons to move in a coil. Later, the controlled magnetic element seems to be the same idea with a marketing change. They probably changed the name because the word reluctance is a technical word that describes the change of the magnetic field, but has negative connotations in day-today speech....

Condenser microphones make use of a thin piece of metal foil that is stretched above another piece of metal, as the foil moves it changes the capacitance of the system. This can be detected and turned into a current. Condenser mics are cheap and easy to make and have a good response.

Electret mics are similar to condenser mics in that they create a capacitance between two pieces of metal foil, but there is a polarizing voltage, or fixed-charge, applied to the back plate when it is made which is permanent. Electret microphones can be tiny and are used in cell phones....