Specifications

Surface-mount or “boundary effect” microphones are
designed for this purpose. They effectively reduce interfer-
ence from the surface to which they are attached. If they
are located at the junction of two or more surfaces, such as
the corner of a room, they reduce interference from each
adjacent surface. In addition, a boundary microphone
exhibits increased sensitivity due to addition of the direct
and reflected sound.
To minimize reflective interference, avoid using
microphones near acoustically reflective surfaces. If this is
not possible, consider using a surface-mount microphone
on the primary reflecting surface.
In addition to interference problems, the use of
multiple microphones creates other potential difficulties.
One of these is due to the fact that as the number of active
microphones in a sound system increases, the overall
potential for feedback also increases . And of course,
each active microphone is adding more ambient noise
pickup to the system.
This leads to a final general rule for microphone
use: Always use the minimum number of microphones.
If additional microphones are not needed, they may
actually degrade the sound system. If the application can
be satisfied with one microphone, use one microphone!
The second key area of microphone use is the
interface of the microphone with the sound system.
As mentioned at the beginning of this section, this involves
primarily electrical considerations. We will develop a few
simple rules for proper interface based on the electrical
characteristics of the microphone output and the sound
system input, and on the requirements for electrical and
mechanical reliability.
In the discussion of operating principle it was
mentioned that all condenser microphones require power
for their operation. This is provided by an internal battery
in some models, or by phantom power in others. If a
condenser is selected, care must be taken to assure that
the appropriate power source (battery or phantom) is
available. A battery-powered condenser is fine for
applications such as portable recording but phantom
power should be always employed for any permanent
installation of condenser microphones.
Phantom power is provided through the microphone
cable itself. It is a DC (direct current) voltage that may
range from 11 to 52 volts, depending on the microphone
requirement and the phantom power source rating. This
voltage is applied equally to the two conductors of a
balanced microphone cable, that is, pin 2 and pin 3 of an
XLR-type connector. The voltage source may be either in
the mixer itself or in a separate phantom power supply
connected in line with the microphone cable. Most recent
mixers have phantom power built-in, and the actual voltage
will be stated on the mixer or in the operating manual.
The voltage requirement for a phantom-powered
condenser microphone will also generally be stated on the
microphone or in the manufacturer’s literature. Some types,
particularly those that are externally charged, may require a
full 48 volt supply. Electret types, which have a permanent
charge, will typically operate over the entire range from 12 to
48 volts. Unless specifically stated otherwise by the
manufacturer, these microphones will deliver their full
performance at any voltage in this range, and further, they
will not be damaged by a full 48 volt supply. Supplying less
than the recommended voltage to either type may result in
lower dynamic range, higher distortion, or increased noise
but this also will not damage the microphone.
Dynamic microphones, of course, do not require
phantom power. However, many mixers have only a single
switch which supplies phantom power to all microphone
inputs, which may include some used by dynamic
microphones. The presence of phantom power has
no effect on a balanced, low impedance dynamic
microphone. It is not possible to damage or impair the
performance of a balanced microphone which is correctly
connected to any standard phantom supply.
If a balanced microphone is mis-wired or if an
unbalanced, high-impedance microphone is used, there
may be a loud “pop” or other noise produced when the
microphone is plugged in or switched on. In addition, the
sound of the microphone may be distorted or reduced
in level. Even in these cases, the microphone will still
not be damaged and will work normally when the
wiring is corrected or the phantom power is turned off.
If an unbalanced microphone must be used with a
phantom-powered input, a transformer should be used to
isolate it from the input. By the same token, it is also not
possible to damage any standard phantom power source
by improper microphone connection.
Good phantom power practices are: 1) check that
phantom voltage is sufficient for the selected condenser
microphone(s); 2) turn system levels down when connecting
or disconnecting phantom-powered microphones, when
turning phantom power on or off, or when turning certain
phantom-powered microphones on or off; 3) check that
microphones and cables are properly wired. Following these
practices will make the use of condenser microphone almost
as simple as dynamics.
Previously, it was suggested that, for the expected sound
level, microphone sensitivity should be high enough to give
a sufficient signal to the mixer input, but not so high as to
19
MEETING FACILITIES
Audio Systems Guide for