JBL CINEMA SOUND SET UP Manual - page 14
Acoustical engineers make use of what are called Noise Criterion (NC) curves in attempting
set a noise performance goal for cinemas. The octave band values of these curves are shown in
Figure 9. In implementing this data, the acoustical designer settles on a given criterion and then
determines the cost and other factors involved in realizing it. Low-noise air handling requires large
ductwork and is expensive. Even more likely to be a problem is through-the-wall isolation from
adjacent cinemas. The general recommendation made by Lucasfilm Limited (5) is that interference
from adjacent cinemas should be audible no more than 1% of the time. Considering that NC-30 may
represent a typical air conditioning noise level for a cinema, the desired degree of isolation between
adjacent spaces does not represent a hardship in terms of wall construction. The need for improving
NC standards in cinemas is a natural consequence of better recording technology and is the only way
that the benefits of Dolby SR and digital formats can be fully appreciated.
As an example of what may be required, let us assume that the normal maximum levels in a
multiplex cinema are 95
with levels exceeding this value only about 1% of the time. It is clear
that the isolation from an adjacent cinema must be on the order of 65
if the NC-30 criterion is to be
met, and this will call for a wall structure that will satisfy a Sound Transmission Class (STC) of 65
There are a number of double wall, or single concrete
wall, constructions that will satisfy this
requirement, and economic considerations usually take over at this point. Acoustical engineers and
consultants are usually on firm scientific ground in these matters. Typical standard STC curves are
shown in Figure 10
The isolation task is certainly easier with new construction, since buffer areas can be designed
between adjacent exhibition spaces. The most difficult problems occur when older spaces are to be
subdivided to make multiplex cinemas, inasmuch as the chances of coupling through walls or through
common air handling are compounded.
It is obvious that the architect must work closely with an acoustical engineer if the job of
isolating adjacent spaces is to done correctly. All of this yields to straightforward analysis, but the
job is often a tedious one.
B. Control of Reverberation and Discrete Reflections
After the problems of sound isolation have been addressed, the acoustical engineer then turns
to those problems that are generated entirely
the cinema itself; i.e., reverberation and echoes.
The acoustical ‘signature’ of a cinema
neutral. Reverberation per se is not generally
apparent in most houses, and any perceived
of reverberation or ambience during film exhibition
normally comes as a result of surround channel program.
This is not to say that the cinema environment should be absolutely reflection-free. Strong
initial reflections from the sides of the house may be beneficial in a concert hall, where they are
needed to produce a sense of natural acoustical space; however, in the cinema, pronounced initial
reflections from any direction should be eliminated.
Traditionally, reverberation time in auditoriums increases at low frequencies and decreases at
high frequencies. This is a natural consequence of the fact that many surfaces that are absorptive at
middle and high frequencies are not very effective sound absorbers at low frequencies. At higher
frequencies, there is additional absorption due to the air itself, and this excess attenuation of high