Specifications

cause input overload. In practice, however, most mixers are
capable of handling a very wide range of microphone signal
levels and sensitivity values are not critical. Occasionally, for
extremely high sound levels, an attenuator may be necessary
to lower the output of the microphone. These are built into
some microphones and most mixers. Otherwise, accessory
attenuators are available that may be inserted in line with the
microphone cable.
It has already been mentioned that balanced, low
impedance microphones are recommended for the majority
of meeting facility sound applications. This will allow the use
of long microphone cables, and result in the least pickup of
electrical noise. In any case, the microphone impedance
should be similar to the rated impedance of the microphone
input of the mixer or other equipment. It is not necessary
to match impedances precisely. It is only necessary that
the actual mixer input impedance be greater than the
microphone output impedance. In fact, the actual imped-
ance of a typical mixer input is normally five to ten times
higher than the actual output impedance of the microphone.
When it is necessary to match a balanced, low
impedance microphone to an unbalanced, high
impedance input, or vice versa, transformers with the
appropriate input and output connectors are readily
available. Transformers provide an impedance matching
function, a level matching function, and can also change
the configuration from balanced to unbalanced as
needed. Ideally, transformers should be connected
so that the bulk of the cable run is balanced, low
impedance, for maximum length and minimum noise
pickup. This would normally place the transformer at the
connector of the unbalanced, high impedance device.
Professional (and most semi-professional) equipment
has balanced, low impedance microphone inputs using
3 pin XLR-type connectors. Less sophisticated consumer
electronic products and many tape recorders typically have
unbalanced, high impedance microphone inputs using
1/4” phone jacks or 3.5mm mini-phone jacks. A few mixers
offer both types of connectors for each input channel.
Simple adapters may be used to mate different types of
connectors if no configuration change (high/low impedance
or balanced/unbalanced signal) is necessary. Always use
only high quality connectors and adapters.
Optimum microphone performance depends on the
associated connectors and cables. In addition to quality
connectors of the types described above, it is equally
important to use high quality cables. Beyond the basic
specification of balanced (two conductors plus shield) or
unbalanced (one conductor plus shield), there are several
other factors that affect the performance of cables.
The outer jacket protects the shield and conductors
from physical damage and may be rubber or plastic.
It should be flexible, durable, and abrasion resistant.
Depending on the location it may need to be chemical or
fire resistant. Different color jackets are available and can be
used to identify certain microphone channels or cables.
The shield protects the conductors from electrical noise.
It may be braided or spiral-wrapped wire or metal foil.
It should provide good electrical coverage and be flexible
enough for the intended use: braid or spiral for movable
use; foil only for fixed use such as in conduit.
The conductors carry the actual audio signal and are
stranded or solid wire. They should be of sufficient size
(gauge) to carry the signal, and provide adequate strength
and flexibility. Use stranded conductors for most applications.
Use solid conductors only for stationary connections.
A large percentage of “microphone” problems are
actually due to defective or improper microphone cables.
Microphone cables should be handled and maintained
carefully for long life: 1) position them away from electrical
sources to prevent hum or other noise pickup; 2) allow them
to lie flat when in use to avoid snagging; 3) use additional
cable(s) if necessary to avoid stress; 4) do not tie knots
in cables; 5) coil loosely and store when not in use; and
6) periodically check cables visually and with a cable tester.
Individual, pre-assembled microphone cables are
readily found in a wide variety of styles and quality.
In addition, multiple cable assemblies, called snakes, are
available for carrying many microphone signals from one
location to another, such as from the courtroom to the
equipment rack. The use of only high quality cables and
their proper maintenance are absolute necessities in any
successful meeting facility sound application.
20
MEETING FACILITIES
Audio Systems Guide for
Burbank, California City Council