Handbook for Sound Engineers

1550 Chapter 42

be adjusted according to the nature of the principal
program material. If the system is used mostly for
music, a compression ratio of about 4:1 will result in
minimal harm to the music. If speech is the principal
program, compression ratios up to 20:1 can be used to
improve both intelligibility and SNR.

42.2.2.4 Installation

If the loop is installed in conduit, it must be nonmetallic
conduit such as PVC and should be placed so that there
is little (or no) steel between the loop and the listener.
Often the conduit is run in the top of a concrete slab or
below a wood-framed floor; but it can also be run in
walls or even the ceiling of a room. When installing a
loop in an existing room, it is often easiest to run the
loop wire under the carpet, using conduit only for the
run to the amplifier.

42.2.3 FM Broadcast

FM broadcast systems have replaced many magnetic
loops in classrooms where hearing-impaired children
are taught because the FM signal is normally free from
noise and provides a more uniform and reliable signal.
Several channels are available so systems can be used in
adjacent rooms. The sound quality is excellent. The
useful receiving range will vary from 30–90 m
(100–300 ft) depending on the amount of steel in the
building. Transmitters are available for operation from
the powerline for permanent installations or by battery
for portable applications.

The Federal Communications Commission (FCC)

has set aside a band of frequencies, 72.025–

75.975 MHz, for FM broadcasting to the hearing

impaired under FCC Rules Part 15. These frequencies

cannot be used for any other purpose, such as language

translation systems or communications applications. No

license is required, although the manufacturer of the

transmitter is required to have FCC approval of the

transmitter design. The FCC restricts radiation to a

maximum field strength of 8000μV/m at 30 m. The

FCC rules require a special antenna connector on the

hearing assistance transmitters to prevent the use of

illegal gain antennas that could result in a higher trans-

mitted field strength than dictated by the FCC. The

system requires no special knowledge to install; suffi-

cient instructions are provided by the manufacturers.

The FCC has opened the 216–217 MHz band for

assistive listening devices. This band falls under the low

power radio services (LPRS) of the FCC, which limits

the power output to 100 mW. While the 72 MHz band

could transmit 500–1500 ft, the 216 MHz band can

transmit 1000–3000 ft.

The systems can be either wide-band or

narrow-band. Wide-band systems have the following

characteristics:

• High fidelity for all applications.
  •Low cost.


• Good rejection of unwanted or external radio signals.


• Limited to six simultaneous channels.

Narrow-band systems have:

Table 42-1. Copper Wire Data

AWG # Ohms/1000 ft Diameter in Inches Current for Heat Rise*

5 oC 10oC 20oC

Melting Current

24 25.67 0.02010 2.3 A 3.2 A 4.5 A 29.2 A

22 16.14 0.02535 3.0 A 4.2 A 5.9 A 41.2 A

20 10.15 0.03196 4.0 A 5.5 A 7.8 A 58.6 A

18 6.375 0.04030 5.5 A 7.8 A 10.0 A 82.4 A

16 4.016 0.05082 7.5 A 10.0 A 15.0 A 117.0 A

*Heat rise based on an insulation thickness of 10 mils. Heavier insulation allows more current for the same heat rise.

Figure 42-8. Induction loop system block diagram.

Digital

delay Equalizer Compressor

Power amplifier

Autotransformer

Inductive loop

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