Microphone

Microphones are devices that are called "transducers"; that is to say, microphones convert sound waves into electrical signals. In a way, microphones work in the opposite manner that loudspeakers work -- loudspeakers convert electrical audio signals into sound waves that can be heard by human ears.

Sound recording involves a conversion between acoustic and electrical signals. The sensor that makes that conversion is called a microphone (also called a mic or a mike, for short), or more closely a capsule, or a pick-up. The microphone contains two major elements, one that senses the sound (membrane) and one that transforms that sound into usable electrical signals.

Omnidirectional (pressure type)

A rigid closed volume („box”) with a membrane on one of its surfaces (with a little valve to balance constant pressure) can respond to pressure changes. Since pressure is not a directional parameter, these types of microphones sense sound from all directions, thus called omnidirectional. They are in theory perfectly omnidirectional, but in practice higher frequencies show directional differences, because the size of the microphone case is more comparable to the wavelength and so is an obstacle to the wave causing interference and shadowing.

Figure-eight (pressure gradient or particle velocity type)

In its simplest description, a pick-up pattern of a figure-eight microphone detects sounds directly in front of the microphone, and directly behind the microphone, but not to the sides of the microphone. If you were to plot the sensitivity of such a microphone, you would get the most sensitivity directly in front and behind the microphone, but that sensitivity would taper off to zero at the sides. A graph of such a response pattern would be two solid circles, one atop of the other, in so-called figure-eight fashion.

When the membrane is fixed in a way that both of its surfaces are open to the air, it responds to particle velocity according to the direction of the change of the pressure, thus it is called a pressure gradient type or particle velocity type microphone. Since both surfaces of the membrane are open, a pressure change affecting both sides of the membrane similarly will not make the membrane move, but a change in the particle velocity will. The membrane is most responsive to a direct incidence. This means that a velocity vector having the same angle as the normal vector of the membrane surface excites it the most. Consequently the membrane is entirely non-responsive to a particle velocity component with an angle parallel to the surface, since the component matching the surface’s normal vector is zero in that case, so such microphones have a figure-of-eight directional response. In practice this means that sounds coming from the side are not audible for this type of microphones.