Handbook for Sound Engineers

Assistive Listening Systems 1553

important, as in corporate meeting rooms. For this
reason and because of its outstanding sound quality,
infrared is the system of choice for professional theaters
and concert halls.

42.2.5.1 Components of an Infrared System

The basic components of an infrared system are:

• The control transmitter (which is connected to the
  audio source)


• Slave emitters, daisy-chained together (if needed).

•The receivers.

42.2.5.2 Categories

Any installation generally falls into four categories:

1. Personal listening systems (PLS) for living rooms,
   bedrooms, offices, etc.


2. Medium area systems (MAS) for lounges, meeting
   rooms, courtrooms, classrooms, small theater,
   churches, etc.


3. Large area systems (LAS) for auditoriums, large
   theaters, churches, arenas, etc.


4. Large area multichannel Systems (up to 32 chan-
   nels) for simultaneous interpretation and other
   applications.

42.2.5.3 Coverage

Transmitter and emitter panels with present
state-of-the-art design and components allow over 70 ft^2
of coverage per IR light emitting diode in single
channel IR systems. The shape of the polar pattern of a
panel is nearly identical to the pattern of a single diode.
The half-power angle of luminosity for the presently
used LEDs is approximately ±25 degrees. More LEDs
increase the IR intensity, and the area of coverage multi-
plies by the number of diodes in the panel. The radiation
pattern can be considered the same for horizontal and
vertical orientations, and is scarcely influenced by the
arrangement of diodes or the housing of the array.
Since there is a physical limit to the light output
power of any LED, the total output has to be shared
between channels in a multichannel system and the
available coverage area per radiator has to be divided by
the number of channels in the system. Conversely, for
the same required coverage, the number of radiators
should be multiplied by the number of channels in the
system, or twice the amount of emitters in a stereo

system as would be required in a single channel installa-

tion in the same venue.

Reflection and scatter off walls, ceilings, floors and

furnishings broaden the coverage and make it largely

nondirectional. Infrared light behaves a lot like visible

light as it reflects best off bright and smooth surfaces

like white walls, and is absorbed by dark and rough

materials like black velvet curtains.

Emitters should be placed in a manner to provide

even illumination throughout the room. The are usually

mounted 10–40 ft above the floor and pointed down

toward the audience. When emitters are placed on both

sides of the stage, they should be cross-fired into the

audience. Any number of receivers can be used in the

system as they will not effect the signal source.

LEDs have degradation of light output over time,

however, by using good electronic circuit design, a

projected continuous operating life under standard condi-

tions of more than 100,000 h, before the light output

diminishes to 70% of its original value, can be obtained.

42.2.5.4 Ambient Light

Infrared systems work in virtually any environment

except for direct sunlight. Systems can even be installed

in shaded outdoor areas. Rooms with very high ambient

light levels or poorly filtered fluorescent ballasts may

require additional emitters for a sufficient SNR.

42.2.5.5 The infrared Link

Infrared systems can be either narrow band or wide

band, depending on your requirements. Table 42-2

gives the technical specifications for infrared systems.

The infrared link uses a specially doped gallium

arsenide light emitting diode (LED) to transmit the

signal. Each diode emits about 10 mW total radiant

power, requiring up to 143 LEDs in each array to

produce adequate power. The wavelength of the emitted

light is 930 nm and is neither monochromatic nor

coherent, so any number of diodes can be used together

without interference between them. The useful coverage

pattern of the emitter varies with distance and the

number of channels being transmitted, Fig. 42-12. The

number of emitters required depends upon the size and

shape of the area to be covered and the number of chan-

nels in use. Emitters are usually employed in pairs,

located at each front side of the audience and cross-fired

across the seating area so that each person receives an

infrared beam from each side. This cross-firing helps to

eliminate shadowing from other people in the audience.