JBL Motion Picture Loudspeaker Systems Instruction Manual

A. Introduction:
Although the company's roots extend back
to the early days of motion picture sound, it
has only been in the last few years that JBL
has made a significant impact on the commercial
theater market. Some of the reasons for this are
the company's preeminence in high-quality sound
reinforcement in general and the development of
the notion of flat power response in particular.
In just the last year, JBL theater systems have
been incorporated into some of the most presti-
geous dubbing theaters and screening rooms in
the film industry. We now feel that the time is
right to prepare a comprehensive applications
manual for our theater dealers to aid them in
system design and specification and to give them
an edge over their competitors.
B. Systems for Smaller Houses:
1. A summary of house types:
By far, most of the theaters being built
today are long rectangular rooms. They are
usually built in groups called multiplex
theaters for reasons of economy. Most of
these theaters are mono, with a single
loudspeaker behind the screen, and the
seating capacity usually ranges from 200
to 500. In general, the larger houses, those
seating 1000 or more, were built some years
ago, and these are usually equipped with
three or five loudspeakers behind the screen
and will have a multi-speaker surround
channel located on the rear and side walls.
A very few houses, usually found only in
larger cities, will have special effects
low-frequency channels, such as Sensurround,
for special screenings of important films.
Instruction Manual
Loudspeaker Systems
2. General Acoustical Characteristics:
Historically, motion picture theaters,
whatever their size, have been acoustically
"dead" rooms; that is, they have consider-
able absorption on the boundaries, and rever-
beration times, even in the larger houses,
rarely exceed 1.25 seconds. More likely, we
will find the reverberation times in smaller
houses to be in the 0.5 second range. The
reason for this of course has to do with arti-
culation and clarity of dialog.
The most common method of treating the
interior of theaters is with velour draping.
This is an interior treatment only; structurally,
the architect relies on solid block walls to
minimize sound leakage from adjacent theaters.
One of the most common problems encoun-
tered in theaters is a slap or reflection off
the back wall, which often goes untreated.
Should the back wall be curved, then the
effect may be even worse, due to focussing of
the reflection back to the front.
3. Sound Level Requirements:
Average peak levels of 85 dB-SPL are common
in the theater. A minimum headroom figure
above this would be 10 dB, but JBL recommends
15 dB for a greater measure of safety. The
following factors are taken into consideration
in determining the kind of system to specify
and the amount of amplifier power which will
be required:
a. Room volume
b. Room boundary area
c. Reverberation time
d. System sensitivity and power
rating
e. System directivity index
Motion Picture