The unidirectional microphone is most sensitive to sound arriving from one particular direction and is less sensitive at other directions. The most common type is a cardioid (heart-shaped) response. This has the most sensitivity at 0 degrees (on-axis) and is least sensitive at 180 degrees (off-axis).
The effective coverage or pickup angle of a cardioid is about 130 degrees, that is up to about 65 degrees off axis at the front of the microphone. In addition, the cardioid mic picks up only about one-third as much ambient sound as an omni. Unidirectional microphones isolate the desired on-axis sound from both unwanted off-axis sound and from ambient noise.
For example, the use of a cardioid microphone for a guitar amplifier which is near the drum set is one way to reduce bleed-through of drums into the reinforced guitar sound. Unidirectional microphones have several variations on the cardioid pattern. Two of these are the supercardioid and hypercardioid.
Both patterns offer narrower front pickup angles than the cardioid (115 degrees for the supercardioid and 105 degrees for the hypercardioid) and also greater rejection of ambient sound. While the cardioid is least sensitive at the rear (180 degrees off-axis) the least sensitive direction is at 126 degrees off-axis for the supercardioid and 110 degrees for the hypercardioid.
When placed properly they can provide more focused pickup and less ambient noise than the cardioid pattern, but they have some pickup directly at the rear, called a rear lobe. The rejection at the rear is -12 dB for the supercardioid and only -6 dB for the hypercardioid. A good cardioid type has at least 15-20 dB of rear rejection.
The bidirectional microphone has maximum sensitivity at both 0 degrees (front) and at 180 degrees (back). It has the least amount of output at 90 degree angles (sides). The coverage or pickup angle is only about 90 degrees at both the front and the rear. It has the same amount of ambient pickup as the cardioid. This mic could be used for picking up two opposing sound sources, such as a vocal duet. Though rarely found in sound reinforcement they are used in certain stereo techniques, such as M-S (mid-side).
Ambient Sound Rejection
Since unidirectional microphones are less sensitive to off-axis sound than omnidirectional types they pick up less overall ambient or stage sound. Unidirectional mics should be used to control ambient noise pickup to get a cleaner mix.