Specifications

It should be noted that this discussion of directionality
assumes that the polar pattern for a microphone is
uniform, that is, the same shape at all frequencies.
In practice, this is not always achieved. Most microphones
maintain their nominal polar pattern over only a limited
range of frequencies. This is the reason that published
polar patterns include curves measured at several
frequencies. High quality, well-designed microphones are
distinguished by the uniformity of their polar pattern over a
wide frequency range and by the similarity of the pattern to
the theoretical ideal.
There are a few operational differences between
omnidirectional and unidirectional microphones. A useful
feature of most unidirectional types is proximity effect.
This refers to the increased low frequency response of a
unidirectional microphone when it is placed closer than
one or two feet to the sound source. Proximity effect
becomes most noticeable at very short distances where
typically there is a substantial bass boost at less than two
inches. Proximity effect adds fullness and warmth to the
sound, but can also “muddy” the sound in speech
applications. Omnidirectional microphones do not exhibit
proximity effect. Omnidirectional microphones also are less
sensitive to wind noise and to handling noise. Most
professional unidirectional types have effective built-in
windscreens and shock mounts to compensate.
Selecting an omnidirectional or unidirectional
microphone again depends on the sound source and
the destination of the audio signal. For recording (but
not sound reinforcement) of meeting participants, an
omnidirectional microphone may be used to pick up sound
from all directions rather than emphasizing individual
voices. However, as part of a sound reinforcement or P.A.
system, an omnidirectional microphone may be more
prone to feedback because it cannot be aimed away from
the loudspeakers.
A unidirectional model can not only help to isolate one
talker from other nearby talkers, but can also partially reject
background noise. In addition, a unidirectional microphone,
properly placed, minimizes feedback, allowing higher sound
reinforcement levels. For these reasons, unidirectional
microphones outnumber omnidirectional microphones in
most meeting facility sound applications.
4) Electrical output: How does the microphone
output match the sound system input?
The electrical output of a microphone is characterized
by its sensitivity, its impedance, and by its wiring
scheme. The same characteristics are used to describe
microphone inputs in sound systems. This determines
the proper electrical match of a microphone to a given
sound system.
The sensitivity of a microphone is defined as its
electrical output level for a certain input sound level.
The greater the sensitivity, the higher the electrical output
will be for the same sound level. The sensitivity should be
within a range that will deliver a reasonable signal level to
the sound system input: not so high that it will overload
the input and not so low that electrical noise is noticeable.
It should be noted that for weak or distant sound, a
microphone of high sensitivity is desirable, while loud or
closeup sound can be picked up well by lower sensitivity
microphones.
Impedance is, approximately, the output electrical
resistance of the microphone: 150-600 ohms for low
impedance (low Z), 10,000 ohms or more for high
impedance (high Z). While the majority of microphones
fall into one of these two divisions, there are some that
have switchable impedance selection. In any case, the
choice of impedance is determined by two factors: 1) the
length of cable needed to go from the microphone
to the mixer input, and 2) the rated impedance of the
mixer input.
The maximum length of cable that may be used with
a high impedance microphone should be limited to
no more than 20 feet. For longer cable lengths, the
high frequency response of the microphone will be
progressively diminished. Low impedance microphones,
on the other hand, may be used with cables of 1000
feet or more with no loss of quality, and are therefore
recommended for most meeting facility applications.
13
MEETING FACILITIES
Audio Systems Guide for
Comparison of microphone patterns