11
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 be 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 within the cinema itself, reverberation and echoes. The acoustical “signature”
of a cinema should be neutral. Reverberation per se is not generally apparent in most houses, and any perceived
sense of reverberation or ambience during film exhibition normally comes as a result of surround channel program
content.
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 frequencies tends to lower the reverberation
time. Figure 10 shows the normal range of reverberation time, as a function of the value at 500 Hz, while Figure
11 shows the acceptable range of reverberation time at 500 Hz as a function of room volume.
The requirements of specifying the right finishing materials for wall surfaces, along with any special needs
for added low-frequency absorption, fall squarely in the hands of the acoustical designer. In smaller houses, there
is often little choice but to make the space acoustically “dead;” however, a slight degree of reflectivity, even though
it may not be easily perceived as such, will be beneficial. The important factor here is that any echoes, which may
exist, should be some 10 dB below the direct sound.
Discrete reflections are likely to be a problem only if they clearly are displaced from the direct sound
in both time and spatial orientation. The listener usually perceives sidewall reflections well within a time interval
which does not allow them to be heard as such. However, a reflection off the back wall can rebound from the
screen itself, creating a “round trip” echo that may be delayed by as much as 100 milliseconds. The effect here is
to render dialog difficult to understand. In older cinemas with balconies, such reflections were often generated
by the balcony front (or fascia) itself. Substantial acoustical damping had to be placed on that surface in order to
diminish the problem.
Figure 10: Variation of reverberation time with frequency rela-
tive to the value at 500 Hz
Figure 11: Suggested range of cinema reverberation
time at 500 Hz as a function of room volume
