Specifications

The solution to this problem is to use the three-to-one
rule: for multiple microphones, the microphone-to-
microphone distance should be at least three times the
source-to-microphone distance. For example, when using
individual microphones for a city council, if a talker’s
microphone is one foot away, then the next nearest
microphone should be at least three feet away from the
first. This insures that direct sound from the talker will not
be strong enough to cause noticeable interference when
picked up by the more distant microphones. As the
source-to-microphone distance increases, the distance to
adjacent microphones must also be increased.
An implication of the three-to-one rule is the following:
avoid picking up the same sound source with more
than one microphone. Microphones should be placed
and aimed to minimize areas of overlapping coverage.
This is important for a number of meeting facility sound
applications. For audience pickup applications, each
section or area should be covered by only one microphone.
For lectern applications, only one microphone should be
used. When a person wearing a lavalier microphone also
speaks into a fixed microphone, one of the microphones
should be turned down.
The third form of interference, reflection pickup, may
occur whenever there are nearby sound-reflecting surfaces.
This is often true in meeting facility settings: hardwood or
stone floors, brick or glass walls, wood or plaster ceilings,
and solid lecterns and desks. Recall that reflected sound
is always delayed relative to the direct sound. When the
delayed, reflected sound mixes with the direct sound at the
microphone, comb filtering is again the result.
The first solution is to increase the direct sound level,
by placing the microphone as close as practical to the
sound source, so that the direct sound is much stronger
than the reflected sound. Interference effects only become
noticeable when the reflected sound is comparable in level
to the direct sound. Close placement may not be possible
in the case of area coverage, or moving sound sources.
The second solution is to decrease the reflected
sound level. The microphone may be moved away from
the reflective surface, or reoriented for minimum pickup of
sound from that direction. The acoustically reflective
surface may possibly be moved away, reoriented, or treated
with some sound-absorbent material. However, this is often
not feasible, for economic or aesthetic reasons.
The third alternative is to minimize the delay. Since the
delay is due to the difference in the paths of the direct and
reflected sound, this can be accomplished by moving the
microphone so close to the reflective surface, that the
direct sound and the reflected sound have nearly the same
path. This raises the frequency at which comb filtering
begins. If the microphone can be brought very close to the
surface (within one-quarter inch), any comb filtering will
occur above the audible range.
18
MEETING FACILITIES
Audio Systems Guide for
3 to 1 rule
Reflection pickup