Theatre Design & Technology - Summer 1983 - 26

bright daylight. The infrared light that
is a natural part of daylight wipes out
the lower power modulated light from
the system. The system can also receive
interference from incandescent lamps at
very high levels such as on a TV set.
Partially dimmed incandescent lamps
can be a problem in some situations.
The reduced voltage to the lamps causes
a shift toward red that greatly increases
the infrared output of the lamp. This increase in in frared interference can, on
rare occasions, be a problem where audience downlights are left at a dimmed
setting if the infrared beam from the
system is also weak. A likely trouble
spot is deep under a balcony. When this
problem occurs, it is necessary to dim
the lights lower or add more emitters to
the infrared system.
The infrared system hardware consists of a transmitter, power supply, and
two or more emitter panels. The useful
coverage pattern of the emitter varies
with distance and the number of channels being transmitted as shown in Fig.
1. The number of emitters required depends upon the size and shape of the
area to be covered and upon the number
of channels in use. Figure 2 includes
both a practical rule-of-thumb and system costs for emitters required in typical
Number of
Seats
Up to 750
to 1,500
to 2,500
to 4,000

Emitters
2
4
6
8

Approximate
Cost
S 4,500
8,000
11,000
14,000

Figure 2
theatre installations. 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 crossfiring helps to eliminate
shadowing from other people in the
audience.
Infrared transmitters are the most
costly systems for the hearing-impaired.
The receivers are also more costly than
other systems. A single-channel receiver
costs about S150, while a multichannel
unit costs about 5275.
The manufacturer provide detailed
in tructions for planning and installing
the ystem. A useful tool in aiming the
emitters is a low-co t black/white TV
camera and monitor. Most monochrome TV cameras have useful sensi-

tivity in the infrared region. Just turn
off the room lights and observe the illumination pattern of the infrared emitter on the TV monitor. As a corollary to
this procedure, the infrared/TV viewing
system can be used to view a darkened
stage-say for coordinating rigging and
prop moves in a fast, complicated
change in the dark.
Receivers are required with all systems, though fewer are needed with an
induction loop because many patrons
will have aids equipped with T-coils.
Most manufacturers of systems for the
hearing-impaired offer several types of
earphones with their receivers. Typically
these include a single earpiece,
stethoscope-type dual earpiece, and an
induction loop for use with a person's Tcoil. Most users report a strong preference for the dual earpiece instruments,
and a strong dislike for an over-the-head
type headphone. Common complaints
about the head-band units are that they
are uncomfortable for long periods of
use, too conspicuous, and destructive to
hair styles.
Many theatres with hearing impaired
systems have found that normal hearing
persons use the system. This is especially
true in larger houses with seats in areas
where natural acoustics are poor.
ormal-hearing patrons prefer the dual
earpiece both because it sounds more
natural and because a single earpiece
leaves one ear available to receive live
sound from the stage with a time-delay
that depends upon the distance from the
stage. This delay (echo) can be very distracting to those with less than profound
hearing impairments.
Battery replacement and earpiece
sanitizing are the principal maintenance
problems with the hearing-impaired systems. Some theatres have reported a
battery life of one year, although that
seems like an uncommonly long life if
the receivers are being used often. The
infrared receivers have rechargeable batteries that the manufacturer suggests
hould be recharged after each use. Earpieces are most commonly sanitized by
replacing the plastic ear-tips (about 5C
per pair) or by using replaceable foam
balls (about 251! per pair).
Most theatre provide receivers for
their patrons at no charge or for a small
fee to cover the cost of handling, batteries, and sanitizing. A driver's license
or credit card i usually required as a
return security. Some theatres have been

successful in selling receivers to regular
patrons, especially in communities
where several theatres and churches use
the same technology.
System Design and installation of the
broadcast and infrared systems consist
of little more than following the manufacturers' instructions. Magnetic induction loop systems require no special or
unique equipment, so a system can be
assembled by the theatre's technical
staff. The parts are a small power
amplifier (20 to 50 watts) and a length of
#20 or #22 gauge wire with an
impedence-matching transformer to
match the wire loop(s) of the amplifier.
In seating areas greater than about
30' x 90 " it is necessary to divide the
area to be covered into smaller blocks,
each served by a separate loop with all
the loops connected in series. There is a
mathematically-based design procedure
for determining the optimum loop size
and for determining the correct matching transformer. A monograph detailing these procedures is available to
anyone interested from the author at
21120 Vanowen Street, Canoga Park,
CA 91303.
0
This article was adapted from a paper
given at the 1983 USITT convention.
Directory of Manufacturers
By-Word Corp.
64 Fort Point Street
East orwalk, CT 06855
magnetic induction receivers
Communications Company
3490 Noell Street
San Diego, CA 92110
magnetic induction receivers
IPAS International Corp.
1440 Broadway-Suite 2250
ew York, Y 10018
complete infrared systems
LPB Inc.
28 Bacton Hill Road
Frazier, PA 19335
10 sy co-ax and carrier current AM
transmitters
Phonic Ear
250 Camino Alto
Mill Valley, CA 94941
complete FM systems
Williams Sound Corp.
6844 \\ ashington Ave. South
Eden Prairie, I 55344
complete AM and FM systems

SITT Summer 1983/Theatre Design & Technology

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