Specifications

4-6
because there is no time for reverberation to build and add to the original sound.
What is required is a curve that favors such impulsive “first arrival” sound and
implies the same gently falling response that is observed when the output of an ideal
loudspeaker is measured with a perfect microphone in the theater.
The amount of reverberation varies with frequency and the higher the frequency the
more the treble will be absorbed rather than being reflected. A typical reverberation
curve in a theater rolls off at about 3 dB per octave above 2 kHz. This characteristic
is used to define the standard steady-state response curve for all dubbing theaters in
which films with Dolby soundtracks are mixed and for all Dolby processor-equipped
cinemas.
The size of the theater affects the reverberation time and, therefore, the
measurement of frequency response. After alignment to this standard curve, some
slight adjustment of high frequency slope may be found necessary for extremely
large or small theaters. The Treble Control can be used to reduce the output on the
response curve by approximately 1 dB at 8 kHz for very large theaters; an increase
of 1 dB at 8 kHz may be in order for a very small theater. Any such adjustment
should be based on an evaluation by ear of actual known films rather than as a rule
of thumb.
Some loudspeakers used in theaters are far from ideal and require boosting of the
low- and high-frequency extremes in order to produce an approximation of the
standard reference response curve. Bass and treble controls–centered on the
turnover points of typical loudspeakers–lift the ends of the spectrum without the
need for large amounts of narrow-band boost from the third-octave EQ circuitry.
The final factor is masking of the screen. Most films today are shown in a wide-
screen format. The masking curtains of the screen must be drawn back sufficiently
to clear the left and right speakers before any adjustments or measurements are
made. The treble horns should clear the screen frame and be mounted as close as
possible to the screen. Conventional black felt side masking can severely curtail high
frequency response. Consequently, there would be severe losses if the left and right
loudspeakers were equalized with the masking open as for a 2.35:1 film, and then
the masking were brought in for a 1.85:1 film, thus obscuring the outer speakers.
To avoid this problem, some theaters have installed acoustically transparent
masking cloth, and others leave the masking open whenever they are showing a
1.85:1 film with a stereo soundtrack. Moving the speakers towards the center of the
screen so as to clear heavy masking is not a good solution, since the stereo
separation would be degraded.
Repainted screens cannot be used for quality sound playback, since the perforations
which allow the high frequencies through the screen can become clogged with
paint.