User`s manual

3-3
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
located on the Cat. No. 513 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 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.