One of the most important characteristics of a microphone is its' pickup pattern, or directionality, which is way it responds to sound waves arriving from different directions. There are three basic patterns, omni-, bi-, and uni-directional.
Directional characteristics of a microphone are shown in polar coordinates on a graph called a polar plot. These measurements are taken by placing a mic on a turntable which rotates 360 degrees, with the diaphragm in the exact center of rotation. A loudspeaker is placed at some distance from the microphone and fed a fixed frequency steady signal. As the mic is rotated, a graphic level recorder registers the output level either in voltage or decibels relative to a fixed reference level. The graph of output level versus angle represents the directionality.
The amplitude (p) of any polar response may be described in general form by the polar equation:
p = A + BcosΘ (where A + B = 1)
The specific values of A and B determine the actual shape of the response pattern.
For omni-directional mics A = 1, therefore B = 0. Solving for p we see that the amplitude is constant regardless of the angle (p = A). Thus omni mics, as the name implies, pick up sound somewhat equally from all directions. Omni mics are classified as pressure mics, since sound pressure alone conveys no information about angular direction. A sound-pressure device simply reads pressure at a point in space without regard to orientation.
Omni mics have excellent low frequency response, are less susceptible to breath and wind noise, and have smoother frequency response than unidirectional mics.
Bi-directional (Figure Eight)
For bi-directional mics A = 0, therefore B = 1. Solving for p we see that the amplitude is proportional to the cosine function, which is 1 at 0 and 180 degrees, and 0 at 90 and 270 degrees. Thus bi-directional mics pick up sound from the front and rear, and reject sounds coming from the sides. These mics are classified as velocity mics, because the pattern responds to the air particle velocity information.
Bi-directional mics are used in recording and sound reinforcement applications where their pattern provides pick up for two adjacent sources.
When a pressure element is used in conjunction with a velocity element, a uni-directional pattern is derived. From the above equation, when A = B = .5, p solves as follows: 0 degrees p = 1, 90 and 270 degrees p = .5, and at 180 degrees p = 0. This results in a heart-shaped response pattern, thus the name cardioid, where sound is best picked up from the front (primary axis) and mostly rejected from the rear.
Cardioid mics are undoubtedly the most popular of all patterns, and are widely used in sound reinforcement and recording. Their response is not as smooth as an omni and they are more sensitive to wind and breath noise. An inherent characteristic of uni-directional microphones is the proximity effect, which is an increase in the low frequency response when the mic is very close to the source.
"Sound Reinforcement Handbook" Gary Davis & Ralph Jones 1989
"The Microphone Handbook" John Eargle