Specifications

CHAPTER EIGHT
TYPICAL APPLICATIONS
In order to select a microphone for a specific
application, and to apply it properly, it is first necessary
to know the important characteristics of the sound
source(s) and of the sound system. Once these are
defined, a look at the five areas of microphone specifications
will lead to an appropriate match. Finally, proper use
of the microphone, by correct placement and operation,
will insure best performance. This section presents
recommendations for some of the most common meeting
facility sound applications.
Lectern
The desired sound source, for a lectern microphone,
is a speaking voice. Undesired sound sources that may
be present are nearby loudspeakers (possibly overhead)
and ambient sound (possibly ventilation, traffic noise, and
reverberation). The sound system in this and the following
examples is assumed to be high quality with balanced
low-impedance microphone inputs.
The basic performance requirements for a lectern
microphone can be met by either dynamic or condenser
types, so the choice of operating principle is often
determined by other factors, such as appearance.
In particular, the desire for an unobtrusive microphone is
best satisfied by a condenser microphone, which can
maintain high performance even in very small sizes.
If phantom power is available, a condenser is an excellent
choice. If not, dynamic types, though somewhat larger,
are available with similar characteristics.
For the microphone to match the desired sound source
(the talker’s voice) it must first have a frequency response
which covers the speech range, (approximately 100Hz to
10kHz). Within that range the response can be flat, if the
sound system and the room acoustics are very good, but
often a shaped response will improve intelligibility. Above
10kHz and below 100Hz, the response should roll off
smoothly, to avoid pickup of noise and other sounds outside
of the speech range, and to minimize proximity effect.
The choice of microphone directionality that
will maximize pickup of the voice and minimize
undesired sounds, is unidirectional. This type will
also reduce the potential of feedback since it can be
aimed toward the talker and away from loudspeakers.
Depending on how much the person speaking moves
about, or on how close the microphone can be
placed, a particular type may be chosen: a cardioid for
moderately broad,
close-up coverage;
a supercardioid or
a hypercardioid
for progressively
narrower or more
distant coverage.
The electrical
characteristics of
the microphone are
primarily deter-
mined by the sound
system: in this
case, a balanced
low-impedance
type would match
the inputs on the
mixer. Of course,
this would be the
desired choice in
almost all systems
due to the inherent
benefits of lower
noise and longer
cable capability.
The sensitivity of the
microphone should be in the medium-to-high range since
the sound source (speaking voice) is not excessively loud
and is picked up from a slight distance. Again, this is most
easily accomplished by a condenser type.
The choice of physical design for a lectern micro-
phone must blend performance with actual use. The most
effective approach is a gooseneck-mounted type, which
places the microphone close to the sound source and
away from both the reflective surface of the lectern and
noise from the handling of materials on it. Another
approach is the use of a boundary microphone on the
lectern surface, but this method is limited by lectern design
and by the potential for noise pickup. As mentioned
above, the desired physical design may also suggest the
operating principle. The most effective small gooseneck
or boundary styles are condensers.
The ideal placement of a lectern microphone is 6 to
12 inches away from the mouth, and aimed toward the
mouth. This will give good pickup of the voice and
minimum pickup of other sources. Also, locating the
microphone a few inches off-center will reduce breath
noise that might occur directly in front of the mouth. It is not
recommended that two microphones be used on a lectern
as comb filtering interference is likely to occur.
29
MEETING FACILITIES
Audio Systems Guide for
Lectern mic