Handbook for Sound Engineers

Microphones 561

nication, are less critical with regard to audio band-
width; 300 Hz–3 kHz is telephone quality, and
50 Hz–8 kHz is excellent for an intercom.
Dynamic range is probably the most critical aspect
of performance for natural sound. A good compandor
system will provide 80–85 dB of dynamic range,
assuming the microphone is adjusted to 100% modula-
tion on the loudest sounds. Leaving a margin of safety
by turning down the microphone modulation level sacri-
fices SNR. Even with extra headroom and a working
SNR of 75 dB, the microphone will still have about
twice the dynamic range of a typical optical film sound
track or television show.
The system should provide at least 40–50 dB SNR
with a 10μV signal and 70–80 dB SNR with an 80μV
signal. This shows up when no audio signal is being
transmitted.
When an electret condenser microphone is used, a
major limitation in dynamic range can be the capsule
itself, not the wireless system. Typically, an electret
powered by a 1.5 V battery is limited to about 105 dB
SPL. Powered by a 9 V battery, the same microphone
may be usable to 120 dB SPL. The wireless microphone
system should be able to provide a high enough bias
voltage to ensure adequate dynamic range from the
microphone capsule. Although the condenser may be
hotter in output level than a dynamic microphone, its
background noise level is disproportionately higher, so
the overall SNR specification may be lower.
Wireless intercom systems do not need the same
dynamic range as a microphone. They do not have to
convey a natural musical performance. However,
natural dynamics are less fatiguing than highly
compressed audio, especially given a long work shift.
So aside from greater range, there are other benefits to
seeking good SNR in the intercom: 40 dB or 50 dB
would be usable, and 60 dB or 70 dB is excellent. An
exception is in a very high-noise industrial environ-
ment, where a compressed loud intercom is necessary to
overcome background noise. A good intercom headset
should double as a hearing protector and exclude much
of the ambient noise.
Distortion is higher in a wireless system than in a
hard-wired system—a radio link will never be as clean
as a straight piece of wire. Still, total harmonic distor-
tion (THD) specifications of less than 1% overall distor-
tion are available in today’s better wireless
microphones. In these microphones, one of the largest
contributors to harmonic distortion is the compandor, so
distortion is traded for SNR. The wireless intercom can
tolerate more THD, but lower distortion will prevent
fatigue and improve communication.

16.10.2 Receiving Antenna Systems

RF signal dropout or multipath cancellation is caused by

the RF signal reflecting off a surface and reaching a

single receiver antenna 180° out-of-phase with the

direct signal, Fig. 16-145. The signal can be reflected

off surfaces such as armored concrete walls, metal

grids, vehicles, buildings, trees, and even people.

Although you can often eliminate the problem by

experimenting with receiver antenna location, a more

foolproof approach is to use a space diversity system

where two or more antennas pick up the transmitted

signal, as shown in Fig. 16-146. It is highly unlikely that

the obstruction or multipath interference will affect two

or more receiver antennas simultaneously.

There are three diversity schemes: switching diver-

sity, true diversity, and antenna combination.

• Switching Diversity. In the switching diversity
  system, the RF signals from two antennas are
  compared, and only the stronger one is selected and
  fed to one receiver.


• Tr u e D i v e r s i t y. This receiving technique uses two
  receivers and two antennas set up at different posi-
  tions, Fig. 16-147. Both receivers operate on the
  same frequency. The AF signal is taken from the
  output of the receiver that at any given moment has
  the stronger signal at its antenna. The probability of
  no signal at both antennas at the same time is

Figure 16-145. Phase cancellation of radiofrequency

signals due to reflections.

Reflecting object

Transmitter Receiver

Direct

signal

Reflected

signal

Combined (phase-

canceled) signal at

receiver antenna