Handbook for Sound Engineers

Interpretation and Tour Group Systems 1535

41.2 Multichannel FM Induction Loop

The induction loop system is a development of the
audio loop for assisted hearing that has been around for
many years. This system has the advantage of being less
expensive than the infrared system and more portable
than the hardwired system. It is not affected by
line-of-sight limitations and does not require visible
radiator panels as is required by infrared systems. Origi-
nally these systems operated on the AM band, however,
today’s induction loop systems operate on the FM band
for better quality and less noise and interference.

A closed loop antenna is installed around the perim-
eter of the area if the room is small, 30 ft × 60 ft (9 m ×
18 m), Fig. 41-7A, or as a zigzag or circular pattern in
larger areas with a 10 ft × 15 ft (3 m × 4.5 m) pitch
Figs. 41-7B and 7C. The zigzag or circular system
should always be on or in the floor rather than in the
ceiling because the field strength above and below the
loop is reduced as the horizontal strength is increased.

The antenna should be placed in aisles where pickup is

not essential.

The loop consists of a single conductor with a

minimum cross section area of 2.5 mm^2. Loop induc-

tance should be about 1.5μH/m. This translates to

1.5 mH for a 1000 m loop. For best signal, the FM

transmitter should be capable of delivering 100 mArms

per channel to the antenna. It is common to install the

loop on or in the floor rather than in the ceiling because

the seated listener is usually closer to the floor, and ceil-

ings are apt to have a lot of metal that shorts out the

signal. If the antenna is installed permanently in conduit

in the floor, nonmetallic conduit, such as PVC, must be

used.

An FM modulated-carrier transmits the signal. The

band for the carrier frequencies is limited to

15–150 kHz by international Telecom regulations in

most areas except North America. This bandwidth

limits the number of voice channels to about eight. The

radiation is primarily magnetic and pretty much

confined to within the area defined by the loop, there-

fore the system has reasonable security. Audio quality is

Figure 41-6. Common mode audio. Courtesy Auditel
Systems Limited.

Audio

amplifier

Differential

amplifier

By-

pass

Central unit

Differential

driver

Audio

amplifier

0 dB in

(L/S)

Audio amplifier

Mic

preamplifier

Delagate unit

Differential amplifier

Mic switching

Summing amplifier (differential reject)

Audio highway

0 dB out

(mic)

Transformer rejects common-

mode loudspeaker signals

Audio highway carries loudspeaker drive

signals in common-mode form. Microphone

signals are carried in balanced form.

Figure 41-7. Configuration for inductive loops. Courtesy

Auditel Systems Limited.

Max 10 m

Max 20 m

Direction

of field

Type A Type B

Type C Type D

Type E

D

D