1538 Chapter 41
.This does not take into account the wall surfaces,
niches, and obstructions and it assumes that at least 95%
of the radiation is usable. Sennheiser states its large
radiator can cover 11,000 ft^2 /number of channels in the
system.
Layout of the panels is also important. Every seat
must have a view of a panel. The range and coverage of
a radiator are influenced by its orientation to the surface
to be illuminated. A panel that is located so its pattern is
parallel to the floor will have a long footprint with
decreasing signal with distance. A panel that is aimed
straight down will have a circular pattern that has about
the same signal everywhere.
Total infrared power is proportional to the number of
diodes in the panel. Power can be doubled by using two
panels at the same location or using a panel with twice
as many diodes. It is often best to use more than one
radiator to eliminate dead spots in the area. If the over-
lapping areas have reduced signal, the two signals can
add to bring the SNR ratio up to an acceptable level.
The infrared signal is received by the listener
through a belt pack or integral headset type of receiver.
The body pack receiver is about 125 mm × 60 mm ×
28 mm (4.9 inches × 2.3 inches × 1.1 inches) and
weighs 100 g (3.5 oz) and incorporates a channel
selector switch, volume control, and earphone jack. The
integral headset receiver hangs from the ears down
below the wearer’s chin. It includes a channel selector
and volume control, weighs 2.1 oz, and can deliver
110 dB, Fig. 41-10.
Security from eavesdropping is good since an
unwanted receiver cannot see the transmitter or its
reflections. Interference between rooms is also good
since infrared cannot go through solid walls. Infrared
systems are affected by other infrared sources such as
sunlight and incandescent and fluorescent lights.
Infrared is never usable in direct sunlight but can often
be used in shaded areas if high-power transmitters are
used. Rooms with intense incandescent and fluorescent
lighting may require high-power transmitters. Because
it is line-of-sight, objects, including people between the
transmitter and the receiver, can cause dropout unless at
least two transmitters are covering the same area.
Infrared systems are used for portable systems,
where a large room can be subdivided, and for fixed
installations. This system is more expensive than the
induction loop but has much better audio quality, and is
not as susceptible to electrical interference. Infrared is
the system of choice today.
41.4 Tour Group SystemsA tour group FM system consists of a portable FM trans-
mitter and portable FM receivers. A microphone, often
worn over the head, is connected to the transmitter and
broadcasts the presenter’s voice to everyone in the audi-
ence or group. The portable transmitter allows the audio
to be delivered without having to carry a microphone or
be plugged into the wall. Participants wear a portable
FM receiver with an earphone to hear the presentation.
An unlimited number of receivers can be used with one
transmitter as long as the participants are within the
broadcast range, typically up to 150 ft (45.7 m).
The transmitter and the receivers are tuned to the
same channel and depending on the frequency,
three–eight channels can be used simultaneously. This
allows for multiple tours to be conducted at a time
and/or provide language interpretation. Currently tour
group products are available in 72 MHz, 216 MHz, and
863 MHz.
Transmitters and receivers offer a mix of features
and functionality for channel selection, programming,
power, signal strength, and use. Portable transmitters
and receivers are typically battery powered with stan-
dard alkaline or NiMH batteries.D^ total^ emitted^ power (mW)
receiver sensitivity mW me^2 echannel=--------------------------------------------------------------------------------------------------Figure 41-10. Infrared headset with built-in receiver.
Courtesy Sennheiser Electronics.