Dolby Laboratories CP65 - Manual (Page 25)

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and with a broad range of speakers. Accurate equalization requires the use of standardized
acoustic measurement procedures.
A pink noise generator provides a continuous random noise signal that covers the total
bandwidth and is used to measure and adjust the response of the loudspeakers. The use of
random noise eliminates the problems inherent with tones (standing wave patterns in the
theatres) and enables the frequency response of the entire system to be observed. Each
channel can be measured and adjusted independently of the other channels.
A calibrated microphone is placed in the auditorium to receive the pink noise reproduced by
the loudspeaker; the output of the microphone is fed to a real time analyzer (RTA). The RTA
displays the complete audio spectrum received by the microphone in the form of a frequency
response curve. Pure pink noise would yield a “flat” horizontal line on the RTA. Thus, the effect
of adjustments to the equalizers is quickly and easily seen.
One of the problems inherent in equalization is the nature of the environment. In an open
space, a perfect loudspeaker, radiating a perfectly flat response in all directions, placed in
front of a perfectly flat microphone, producing perfectly flat response to sounds arriving from
all directions, will produce a perfectly flat response on the RTA from pink noise. In an enclosed
space such as a theatre, the results are different. When the pink noise generator is first turned
on, all of the sound that initially reaches the microphone comes directly from the loudspeaker;
the response is flat–for a few milliseconds. Then reflected sound from the walls, ceiling, floor,
seats, etc. starts to arrive at the microphone together with the direct sound from the
loudspeaker. This indirect or reflected sound reinforces the direct sound. The system soon
settles into an equilibrium condition. As much energy is being absorbed at the walls, ceiling,
etc. as is fed into the room. Since high and mid frequency energy is absorbed when sound
is reflected, the displayed response appears to have a rising bass and a falling treble
characteristic. At first glance, rolling off the bass and boosting the high frequencies may
appear to be the logical approach for a flat steady-state response, but such an arrangement
works only on sustained sounds. Dialogue contains short, impulsive sounds and will yield a
much-too-bright result 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 theatre.
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 theatre
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 theatres in which Dolby stereo films are
mixed and for all Dolby stereo-equipped cinemas.
The size of the theatre 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 theatres. The treble
control on the Cat. No. 64 Equalizer card can be adjusted to reduce the output on the response
curve by approximately 1 dB at 8 kHz for very large theatres; an increase of 1 dB at 8 kHz may
be in order for a very small theatre. Any such adjustment should be based on an evaluation
by ear of actual known films rather than as a rule of thumb.