Microphone types

A microphone is a device that can pick up sound (pressure variations) in a similar way as the human ear.
The human hearing mechanism requires that the variations occur at least 20 times a second, but not more frequent than 20 000 times a second (often written as 20 kHz). Microphones can have wider frequency range than this, specially at low frequencies.

Microphones are made with different responses, and used for different measurement purposes.
A "Pressure Field" microphone is made to pick up the sound pressure, but since the microphone has a physical size, there is also some increase in the sound pressure just in front of the microphone. A "Diffuse Field" microphone (also often called "Random Response" microphone) try to sense the sound from all directions equally loud. A "Free Field" microphone is very similar to the other microphones except that it is made to compensate for the increased pressure (beacuse of its presens) and thereby measures the sound as if it was not there. This is the microphone type that is most frequently in use.
Actually, in the real world we do not have either a pressure, diffuse or a free sound field. It is always some kind of mixture.

It is typical to make microphones with known diameters. One inch, 1/2 inch and 1/4 inch sizes are the most common. The physical size of the microphone have influence on the frequency range and self noise level.

The microphone standard is IEC 61094.

Pressure variations with frequencies lower than 20 Hz should also be regarded as sound. They are normally referred to as infrasound. Sounds with frequencies higher than 20 kHz lie above the normally audible region and are referred to as ultrasound. But even these frequencies have influence on the hearing ability and the human body.