Specifications
A particular method that is sometimes suggested for
overhead placement is a ceiling-mounted microphone,
usually a boundary microphone. This position should be
used with caution, for two reasons. First, it often places
the microphone too far from the desired sound source,
especially in the case of a high ceiling. Second,
the ceiling, in buildings of modern construction, is
often an extremely noisy location, due to air handling
noise, lighting fixtures, and building vibration. Remember
that a microphone does not reach out and capture
sound. It only responds to the sound that has travelled
to it. If the background noise is as loud or louder at the
microphone than the sound from the talker below, there
is no hope of picking up a usable sound from a ceiling-
mounted microphone.
Placement of audience microphones falls into the
category known as area coverage. Rather than one
microphone per sound source, the object is to pick up
multiple sound sources with one (or more) microphone(s).
Obviously, this introduces the possibility of interference
effects unless certain basic principles, such as the “3-to-1
rule”are followed.
For one microphone, picking up a typical audience,
the suggested placement is a few feet in front of, and a few
feet above, the heads of the first row. It should be centered
in front of the audience and aimed at the last row.
In this configuration, a cardioid microphone can cover up
to 20-30 talkers, arranged in a rectangular or wedge-
shaped section.
For larger audiences, it may be necessary to use
more than one microphone. Since the pickup angle of a
microphone is a function of its directionality (approximately
130 degrees for a cardioid), broader coverage requires
more distant placement. As audience size increases, it will
eventually violate the cardinal rule: place the microphone
as close as practical to the sound source.
In order to determine the placement of multiple
microphones for audience pickup, remember the following
rules: 1) the microphone-to-microphone distance should
be at least three times the source-to-microphone distance
(3-to-1 rule); 2) avoid picking up the same sound source
with more than one microphone, and 3) use the minimum
number of microphones.
For multiple microphones, the objective is to divide the
audience into sections that can each be covered by a single
microphone. If the audience has any existing physical
divisions (aisles or boxes), use these to define basic sections.
If the audience is a single large entity, and it becomes
necessary to choose sections based solely on the
coverage of the individual microphones, use the following
spacing: one microphone for each lateral section of
approximately 8 to 10 feet. If the audience is unusually
deep (more than 6 or 8 rows), it may be divided into two
vertical sections of several rows each, with aiming angles
adjusted accordingly. In any case, it is better to use too
few microphones than too many.
Once hanging microphones are positioned, and the
cables have been allowed to stretch out, they should be
secured to prevent turning or other movement by air
currents or temperature changes. Fine thread or fishing
line will accomplish this with minimum visual impact.
Use only high quality cables and connectors, particularly if
miniature types are specified.
Many older meeting facilities are very reverberant
spaces, which provide natural, acoustic reinforcement for
the audience, though sometimes at the expense of
speech intelligibility. In spaces like this, it is often very
difficult to install a successful sound system as the
acoustics of the space work against the system. Most
well-designed modern architecture has been engineered
for a less reverberant space, both for greater speech
intelligibility, and to accommodate modern forms of
multimedia presentations. This results in a greater
reliance on electronic reinforcement.
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MEETING FACILITIES
Audio Systems Guide for
Microphone positioning for audience pick-up