Boxoffice-March.1988

SOUND ADVICE (continued)
that the proper reverberation is important
in all theatres not only in the big presentation
houses. No amount or type of electronic
equalization or amplification can
correct reverberation problems in motion
picture theatres.
The goal of acoustic design in motion
picture theatres is to create an environment
which will not affect a film's soundtrack.
The acoustics of a motion picture
theatre should be neutral, unlike the
"live" acoustics of a concert hall. The
ambient sound and sound perspective of a
film is recorded on the sound track and
will be recreated for the audience by the
theatre's sound system.
Now lets look at an average-sized
theatre, say sixty feet wide, eighty feet
long, twenty-two feet high at the projection
wall, and twenty-seven feet high at
the screen wall— a room of 117,600 cubic
feet in volume. Table 3 shows the relationship
between volume and absorption
needed to reach the reverberation specification
shown in Figure 1. We see we
should have a reverberation of just over
.61 seconds at 500Hz, and a total absorption
of about 6100 square foot sabines.
Table 4 shows the square foot area of
each surface in the room, the area of
those surfaces to be treated, and the treatments
NRC. As you can see we just
exceed the absorption needed. Note the
projection wall has an area of 1,000
square feet that can be covered, due to
seating and entrance ways. We want to
make this wall as absorptive as we can as
this is the first wall the sound from the
screen encounters that has a direct path
back to the screen. For this reason, we
used panels made of 2 inch material,
which have an NRC of ,85, giving us 850
Sabines. At this point, as John stated, we
could play "what if?", and change the
materials and area covered to see the
effect.
One thing to keep in mind: this technique
will only predict the reverberation
between 250Hz and 2000Hz, In order to
comply with the specification shown in
Figure 2, other work must be done (to
comply with the 31,5Hz, 63Hz, and 125Hz
octave bands,)
If you still have questions regarding
acoustical design, find an acoustician that
specializes in acoustical design (some are
specialists in noise and vibration control)
and pick their brain. Remember, good
experienced design is much cheaper then
misdirected research,
Table 2
Material
Bare Concrete
Painted concrete
Concrete Block
Gypsum Board
Typical
Light Curtain
Heavy Curtain
Commercial Carpet / Underpad
Tectum Brand (1")
NRC Values
for common building materials
Cloth Covered Ridgid Glass-Fiber Panel
Cloth Covered Ridgid Glass-Fiber Panel
NRC
(1) Some materials are advertised with
absorption coefficients of greater than 100%,
hut these are measurements for individual
thick panels located on flat surfaces, and it
is the edges (basically multiple surfaces) of
the panels that provide the extra absorption
IH
Clyde McKinney is technical director for
Lucasfilm I.td's Theatre Operation Division