564 Chapter 16
other fluids. The hydrophone uses a piezoelectric
sensing element, which is frequency compensated to
match the special acoustic conditions under water. A
10 m high-quality audio cable is vulcanized to the body
of the hydrophone and fitted with a standard three-pin
XLR connector. The output is electronically balanced
and offers more than 100 dB dynamic range. The 8011
hydrophone is a good choice for professional sound
recordings in water or under other extreme conditions
where conventional microphones would be adversely
affected.
16.11 Multichannel Wireless Microphone and
Monitoring Systems
By Joe Ciaudelli and Volker Schmitt
16.11.1 Introduction
The use of wireless microphones and monitoring
systems has proliferated in the past few years. This is
due to advancements in technology, a trend towards
greater mobility on stage, and the desire to control
volume and equalization of individual performers.
Consequently, installations in which a number of wire-
less microphones, referred to as channels, are being
used simultaneously, have increased dramatically. Now
theatres and studios with large multichannel systems,
greater than thirty channels, are common. Systems of
this magnitude are a difficult engineering challenge.
Careful planning, installation, operation, and mainte-
nance are required.
Wireless systems require a transmitter, transmit
antenna, and receiver to process sound via radio
frequency (RF) transmission. First, the transmitter
processes the signal and superimposes it on a carrier
through a process called modulation. The transmit
antenna then acts as a launch pad for the modulated
carrier and broadcasts the signal over the transfer
medium: air. The signal must then travel a certain space
or distance to reach the pickup element, which is the
receiving antenna. Finishing up the process, the receiver
—which selects the desired carrier—strips off the signal
through demodulation, processes it, and finally reconsti-
tutes the original signal. Each wireless channel needs to
operate on a unique frequency.16.11.2 FrequenciesManufacturers generally produce wireless microphones
on ultrahigh frequencies (UHF) within the TV band
with specifications outlined by government agencies
such as the Federal Communications Commission
(FCC). The wavelength is inversely proportional to the
frequency. Higher frequencies have shorter wave-
lengths. UHF frequencies (450–960 MHz) have a wave-
length of less than one meter. They have excellent
reflective characteristics. They can travel through a long
corridor, bouncing off the walls, losing very little
energy. They also require less power to transmit the
same distance compared to much higher frequencies,
such as microwaves. These excellent wave propagation
characteristics and low power requirements make UHF
ideal for performance applications.16.11.3 SpacingIn order to have a defined channel, without crosstalk, a
minimum spacing of 300 KHz between carrier frequen-
cies should be employed. A wider spacing is even more
preferable since many receivers often exhibit desensi-
tized input stages in the presence of closely spaced
signals. However, caution should be used when linking
receivers with widely spaced frequencies to a common
set of antennae. The frequencies need to be within the
bandwidth of the antennas.16.11.4 Frequency DeviationThe modulation of the carrier frequency in an FM
system greatly influences its audio quality. The greater
the deviation, the better the high-frequency response
and the dynamic range. The trade-off is that fewer chan-
nels can be used within a frequency range. However,Figure 16-150. DPA 8011 hydrophone. Courtesy DPA
Microphones A/S.