Specifications

A cardioid type is most sensitive to sound coming from
in front of the microphone (the bottom of the heart shape).
On the polar graph this is at 0 degrees, or on axis. A
cardioid microphone is less sensitive to sound coming from
the sides (off axis), and least sensitive to sound from the
rear (the notch at the top of the heart shape). For any
microphone, 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 the
desired sound source by orienting its axis toward the
sound. It may also be aimed away from undesired sound
sources by orienting its null angle toward these sounds.
In addition, a unidirectional microphone picks up less
unwanted 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 microphone is
maximum for sound arriving at an angle of 0 degrees
(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 microphone, this angle is about
130 degrees.
Two related types of unidirectional microphones are the
supercardioid and the hypercardioid. Compared to a
cardioid type, these 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 microphone 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 direction of least pickup.
The significance of these two polar patterns is their
greater rejection of ambient sound in favor of on-axis
sound: the supercardioid has the maximum ratio of on-
axis pickup to ambient pickup, while the hypercardioid
has the overall least pickup of ambient sound (only one-
quarter 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.
And these microphone types do not necessarily provide
better feedback rejection than a cardioid.
Other types of unidirectional microphones include the
shotgun and parabolic reflector. These have extremely
narrow pickup patterns and are used for distant pickup or
in very high ambient noise situations. These specialized
microphones are utilized for broadcast and feature film
production, not for public address systems. Their sound
quality and mechanical limitations make them generally
unsuitable for typical meeting facility use.
One additional directional microphone is the 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 meeting facility applications,
it is occasionally used in combination with other microphone
types for stereo sound recording.
Supercardioid pattern Hypercardioid pattern
Bidirectional pattern
Audio Systems Guide for
MEETING FACILITIES
12