Directional Characteristic of a Microphone

The directional characteristic of a microphone is defined as the variation of its output when it is oriented at different angles to the direction of the sound. It determines how best to place the microphone relative to the sound source(s) in order to enhance pickup of desired sound and to minimize pickup of undesired sound.
Many people have the misconception that microphones only pick up sound from sources they are pointed at, much as a camera only photographs what is in front of the lens. This would be a nice feature if we could get it, but the truth is we can only approximate that action, and at the expense of other desirable qualities.

The polar pattern of a microphone is the graphical representation of the output produced vs. the angle of the sound source. The output is represented by the radius of the curve at the incident angle. The two most common directional types are omnidirectional and unidirectional.

Omnidirectional Microphone

A omnidirectional is simplest microphone design that will pick up all sound, exhibits the same output regardless of its orientation to the sound source. This indicates that the microphone is equally sensitive to sound coming from all directions.
An omnidirectional microphone can therefore pick up sound from a wide area, but cannot be “aimed” to favor one sound over another. Generally have good to outstanding frequency response.

Unidirectional Microphone

A unidirectional microphone is most sensitive to sound coming from only one direction. On a polar graph, this will appear as a rounded but noncircular figure.

Unidirectional microphone may be aimed at a desired, direct sound by orienting its axis toward the sound until the output of a unidirectional microphone is maximum for sound arriving at an angle of 0 degrees, or on-axis. It falls off only slightly for sound arriving from within a certain angle off-axis. For any microphone, the direction of least sensitivity (minimum output) is called the null angle. it may also be aimed away from an undesired, direct sound by orienting its null angle toward the sound. The total directional range for usable output is called the coverage angle or pickup area.

In addition, a unidirectional microphone picks up less ambient sound than an omnidirectional, due to its overall lower sensitivity at the sides and rear. The most common type of unidirectional microphone is called a cardioid, because of its heartshaped polar pattern.

Cardioid Microphone

A cardioid type is most sensitive to sound coming from in front of the microphone (the bottom of the “heart”). On the polar graph this is at 0 degrees, or “on-axis”. It is less sensitive to sound reaching the microphone from the sides (“off-axis”), and the direction of least sensitivity is toward the rear (the notch at the top of the “heart”). For a cardioid pattern, this is at 180 degrees or directly behind the microphone. Coverage angle for a cardioid microphone is about 130 degrees. A cardioid picks up only one-third as much ambient sound as an omnidirectional type.

Supercardioid and the Hypercardioid Microphone

Compared to a cardioid type, supercardioid and the hypercardioid have a progressively narrower coverage angle: 115 degrees for a supercardioid and 105 degrees for a hypercardioid. However, unlike the cardioid, they have some pickup directly behind the microphone. This is indicated in their polar patterns by a rounded projection, called a lobe, toward the rear of the microphone. The direction of least sensitivity (null angle) for these types is about 125 degrees for the supercardioid and 110 degrees for the hypercardioid. In general, any directional pattern that has a narrower front coverage angle than a cardioid will have some rear pickup and a different null angle. The supercardioid has the maximum ratio of on-axis pickup to ambient pickup, while the hypercardioid has the least overall pickup of ambient sound (only onequarter as much as an omni). These can be useful types for certain situations, such as more distant pickup or in higher ambient noise levels, but they must be placed more carefully than a cardioid to get best performance. The Hypercardioid pattern is very popular, as it gives a better overall rejection and flatter frequency response at the cost of a small back pickup lobe.

A "shotgun" mic carries these techniques to extremes by mounting the diaphragm in the middle of a pipe. The shotgun is extremely sensitive along the main axis, but posseses pronounced extra lobes which vary drastically with frequency. In fact, the frequency response of this mic is so bad it is usually electronically restricted to the voice range, where it is used to record dialogue for film and video.
Bi-directional Microphone
bidirectional type. As the name implies, it is equally sensitive to sound from two directions: directly in front of the microphone and directly behind it. Its polar graph consists of a front pickup area and an identical rear lobe and resembles a “figure 8” pattern. Although the front coverage angle of a bidirectional microphone is only 90 degrees, it has equal rear coverage. The null angle is at 90 degrees, which is directly at the side of the microphone. While the bidirectional microphone is not used by itself in any typical house of worship sound application, it is occasionally used in combination with other types for stereo sound reproduction.