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B2-GRQ-OpGuide-v9-031020.pmd
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Increasing the loudness more than this, or increasing the delay somewhat more than 35 milliseconds,
makes the delayed signal sound like an echo.
The phenomenon describing how the acoustic image follows the signal we hear first is called the
Precedence Effect. The phenomenon that makes two distinct sounds heard less than 35 msec. apart
seem like only one sound is call the Haas Effect. However, the terms are often used interchangeably
in the sound industry.
9.4.4. Three Applications for Digital Delays
9.4.4.1. Application I: Under-The-Balcony Speakers
Delay Figure 5 shows a typical situation where the performer is amplified by a center cluster hanging
above the stage. Almost everybody in the audience will enjoy good sound, except those seated in the
shadow of the balcony. So we add an under-balcony speaker to fill in the shadow.
Now we have sufficient volume under the balcony, but the sound from the two speakers arrives at the
listener’s ears some 55 to 69 milliseconds apart. The two signals, along with their echoes, result in an
unintelligible cacophony. We must delay the sound from the under-balcony speaker to synchronize the
signals. Do we set the Graphi-Q delay to 55 or 69 milliseconds? Obviously, the geometry will not allow
us to exactly synchronize every location under the balcony; we have to compromise.
First, consider the program type. For spoken word programs, you will produce the best intelligibility
if the signals from the under-balcony speakers arrive within 10 msec. of the signals from the center
cluster. Therefore we should set the delay to 65-69 msec. You can allow a little more reverberation
for programs that are mostly music.
Next, we must eliminate comb filter distortion. Find the axis where the levels of the center cluster and under-
balcony speaker are equal (see
Section 9.4.2. Comb Filter Distortion
on p.43) You can use the Graphi-
Q to precisely synchronize the speakers along this axis to eliminate the most severe comb filters. Comb
filters off the equal-level axis are much less of a problem since a louder signal is not affected very much
by a weaker signal.
Finally, you can experiment with adding 5 to 10 milliseconds delay to both sets of speakers to enhance
the Precedence Effect for the audience seated near the performer.
In the final analysis, every setting is a compromise, and your ear has to be the final judge. Check the
sound in several different locations throughout the auditorium and correct the most severe irregularities.
9.4.4.2. Application II: Center Cluster with Front Fills
Delay Figure 6 below describes a typical application that has a stage with a microphone, a center cluster
above the stage, and front fills in front of the stage. There must be thousands of installations throughout
the world like this that "get by" without digital delays. But with the Graphi-Q, you can improve the
intelligibility and add a new quality without ringing up any significant costs. Use the Graphi-Q in this
situation to align the visual image with the acoustic image. The program is much more enjoyable when
the amplified sound seems to be originating with the performer, not the loudspeakers.
Delay Figure 5: Overhead view of under-balcony application.
