Directionality
How does a mic respond to sound from different directions? The directional characteristic is defined as the variation of its output when the mic 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.
The two most common directional types are omnidirectional and unidirectional, with the “polar pattern” of the mic being the graphic representation of its directionality.
Omnidirectional mics exhibit the same output regardless of its orientation to the sound source, which will show on a polar graph as a smooth circle.
This indicates that the mic is equally sensitive to sound coming from all directions. Therefore, omnidirectional mics pick up sound from a wide area, but can’t be “aimed” to favor one sound over another.
Unidirectional mics, on the other hand, is most sensitive to sound coming from only one direction. On a polar graph, this will appear as a rounded but non-circular figure.
The most common type of unidirectional microphone is called a cardioid, because of its heart-shaped polar pattern.
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 mic from the sides (“off-axis”), and the direction of least sensitivity is toward the rear (the notch at the top of the “heart”).
For any mic, the direction of least sensitivity (minimum output) is called the null angle. For a cardioid pattern, this is at 180 degrees or directly behind the microphone.
Thus, a unidirectional microphone may be aimed at a desired, direct sound by orienting its axis toward the sound. It may also be aimed away from an undesired, direct sound by orienting its null angle toward the sound.
In addition, a unidirectional mic picks up less ambient sound than an omnidirectional, due to its overall lower sensitivity at the sides and rear. For example, a cardioid picks up only one-third as much ambient sound as an omnidirectional type.
Although the output of a unidirectional mic 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.
The total directional range for usable output is called the coverage angle or pickup arc: for a cardioid this is about 130 degrees.