Specifications

CHAPTER FIVE
MICROPHONES: USES
Once a microphone is selected for a given application,
it must be used properly to get the best possible results.
Again, there are two key areas: the interface of the
microphone with the sound source, and the interface
of the microphone with the sound system. The first area
involves primarily acoustic considerations for optimum
placement of one or more microphones. The second area
involves electrical and mechanical considerations for
optimum operation of microphones.
Microphone placement is a challenge that depends
on the acoustic nature of the sound source and the
acoustic characteristics of the microphone. Although this
may appear to be a very subjective process, a description
of some of the important acoustic interactions will lead to
a few simple rules for successful microphone placement.
Recall that sounds can be categorized as desired or
undesired and that the soundfield (total sound in a space)
is made up of both direct sound and ambient sound. The
level of direct sound decreases with distance (the inverse-
square law) while ambient sound stays at a constant level.
The critical distance is the distance from the sound source
at which the level of direct sound has fallen to the level of
the ambient sound. In practice, microphones must be
placed much closer than the critical distance to get an
acceptable ratio of direct-to-ambient sound.
This brings up the concept of distant pickup capability.
The proportion of direct vs. ambient sound picked up by a
microphone is a function not only of distance but of the
directional pattern of the microphone as well. For a given
ratio of direct-to-ambient sound, a unidirectional micro-
phone may be used at a greater distance from the direct
sound source than an omnidirectional type. This is called
the distance factor, and ranges from about 1.7 for a
cardioid, to 2.0 (twice the omnidirectional distance) for a
hypercardioid.
For instance, if an omnidirectional microphone picked
up an acceptable direct-to-ambient sound ratio at 2 feet
from the sound source, then a cardioid would have the
same ratio at about 3.4 feet, although the gain would have
to be increased electronically to achieve the same output
level. However, for a very weak source, or a very high
ambient sound level, the acceptable omnidirectional
location (again, less than the critical distance) could be as
little as 3 inches away, for example. In this case, even the
hypercardioid could only be used 6 inches away. This
ill-named concept of microphone reach is very subjective
and is dominated by the actual direct vs. ambient sound
level at the microphone position rather than by the direc-
tionality of the microphone. Even an omnidirectional would
have excellent reach, if no ambient sound were present!
In a sound reinforcement system, a second effect
occurs with greater source-to-microphone distance: in-
creased feedback potential. The farther the microphone is
placed from the desired sound source, the more gain, or
volume, is required to get adequate level. As the gain of a
sound system is increased, amplified sound produced by
the loudspeakers will be picked up by the microphone and
re-enter the system.
At some setting, this re-entrant sound will be amplified
to the same level as the original sound picked up by the
microphone and the system will begin to ring or resonate.
Higher settings (to compensate for greater microphone
distance, for instance) will result in the sustained howl or
squeal known as feedback. This condition depends not
only on the system gain, but on the source-to-microphone
distance, the microphone-to-loudspeaker distance,
and the directionality of both the microphone and the
loudspeaker.
16
MEETING FACILITIES
Audio Systems Guide for
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