User`s manual

3-2
It is not practical for the entire cinema industry to standardize on a
single make and model of loudspeaker. In any event, the different
acoustical characteristics of individual theaters would, to some extent,
negate any such standardized speakers. Electronic equalization of each
loudspeaker system achieves consistent results in a broad spectrum of
environments, 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 theaters)
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 (or a multi-microphone setup with multiplexer)
is placed in the auditorium to receive the pink noise reproduced by the
loudspeaker. The output of the selected microphone is fed to a real time
analyzer (RTA). The RTA displays the 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 CP45 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 display a perfectly flat response on the RTA from
pink noise. In an enclosed space such as a theater, 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 theater.