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In most cinemas constructed today, echo problems can generally be dealt with by ensuring that the back
wall is very absorptive and that substantial damping is installed behind the screen on the baffle-wall.
C. The Role of the Acoustical Consultant:
An acoustical consultant should be chosen on the basis of previous jobs well done. There is much that is
learned simply by having encountered — and solved — many problems. Stating it another way, an experienced
consultant has probably seen most of the common mistakes and knows how to spot them before they become
problems. While much of what a consultant does may seem obvious, and even simple, it is the breadth of
experience that qualifies a good consultant to take on a difficult task and succeed at it.
In addition to the points discussed so far in this section, the consultant will look for potential difficulties in
the following areas:
1. Flanking leakage paths. When acoustical isolation has been addressed in wall construction, flanking paths
through, or around, the wall may become significant. For example, sound often leaks through electrical or air
conditioning conduit, even though the wall itself may act as a good barrier to sound transmission. Such paths can
crop up in many places and need to be identified early in the construction phase of the project.
2. Integrity in construction. Many building contractors routinely take shortcuts, and somebody needs to watch
them carefully. The acoustical isolation of double wall construction can be nullified by the presence of material left
between them bridging the air barrier between the two sections.
3. Impact and structure-borne noise. These are some of the most difficult problems to fix, since they are literally
“built in.” Plumbing noises, elevator motors, and air handling machinery located on the roof are just a few of
the offenders here. Once the installation has been made, the problem is very expensive to correct, and a good
consultant will have an eye out for such things at the design stage of the project. Related problems, such as
projector noise and other noises associated with concession activities need to be identified early in the project
and corrected before construction begins.
As standards for film exhibition continue to improve, such points as we have raised here will become more
important. Loan Allen of Dolby Laboratories has stressed the need for noise ratings in the cinema lower than NC-
25, with NC-30 representing the worst acceptable case (7).
IV. SPECIFYING THE CORRECT LOUDSPEAKERS AND AMPLIFIERS
A. Hardware Class vs. Room Size:
JBL has cinema loudspeaker product groups that fall into three basic categories:
The 3000 Series is a lower cost line for use in small to medium cinemas. The series now includes the
3622N and 3632 ScreenArray systems.
The 4000 Series includes JBL’s traditional line of cinema products, which have been the standard of the
industry for fifteen years. The series now includes the 4622 and 4632 ScreenArray systems.
The 5000 Series consists of 3-way high performance, low distortion systems intended for use in critical
motion picture creative environments and in showcase theaters around the world.
Specifying loudspeakers and amplifiers for cinemas is a straightforward design problem. Some years ago,
a survey of modern cinemas found relatively few major variations in the design parameters. The average room
volume per seat is about 5.6 cubic meters (200 cubic feet), and this leads to a method of determining average
room dimensions for theaters of various sizes (Data courtesy of THX
®
Division of Lucasfilm, Ltd.) Furthermore, the
reverberation time in modern theaters is so low that considerations of so-called “room gain,” or reverberant field
contribution at middle and higher frequencies, in determining reproduced levels at various distances from screen