1264 Chapter 34
34.3.2.8.7 Low-Frequency Loudspeakers and Feedback
The published Q for a two-way packaged loudspeaker
system is a compromise between the relatively high Q of
the high-frequency horn and the lower Q of the woofer.
The same is true of three-way and four-way packaged
loudspeaker systems. Using this published Q in the PAG
and NAG equations may provide an acceptable result.
However, the lower Q value of the woofer may, in some
systems, result in feedback in the woofer’s frequency
range.
34.3.2.8.8 Direct Sound and Feedback
The PAG and NAG equations assume the microphone is
entirely in the reverberant field of the loudspeaker clus-
ter. If the microphone is receiving any significant
amount of direct sound from either low- or high-fre-
quency loudspeakers, the PAG and NAG equations will
not accurately predict feedback problems. This potential
direct sound feedback problem must be considered
qualitatively in the design of the cluster by aiming the
loudspeakers away from the microphones. Since the
low-frequency loudspeakers are normally low Q and
cannot be effectively aimed away from the microphone,
they are often placed nearest the ceiling to be as far
from the microphone as physically possible.
34.3.2.8.9 Other Considerations
At this point, the basic cluster design is finished. Con-
sideration should be given, of course, to overall system
head room, frequency response, distortion, and other
sound-quality factors that help to answer Question 5,
“Does it sound good?”
34.3.2.9 Answering Question 2: “Can Everybody Hear”
The cluster design process outlined in the last section
includes little help in actually choosing and aiming the
horns. Trigonometric methods may suffice for a simple
room. For more complex rooms, most designers use
EASE or Modeler or another computer software design
tool.
34.3.2.9.1 History of Manual/Graphical Cluster Design
Tools
Before the advent of EASE, Modeler, or other software
design tools, system designers used one of several man-
ual/graphical tools that were extensions of architectural
mapping techniques. A user of one of these tools would
overlay a graphical representation of the loudspeaker
coverage pattern, known as an isobar. onto a specially
prepared angular map of the room.
Because both the room map and loudspeaker isobar
were angular, moving the isobar around the room map,
was equivalent to re-aiming the horn. Thus, the user
could aim the loudspeaker to estimate optimum
coverage patterns. More than one loudspeaker isobar
overlay could be used to estimate the coverage patterns
of multiple, overlapping horns.
The concepts behind these methods were developed
by several engineers including Thomas McCarthy of
North Star Sound, and first commercialized as Altec’s
Array Perspective. Fig. 34-20. The disadvantage of the
Altec method was its lack of accuracy, especially when
horn aiming angles were far from the zero line defined
for the room map.Another manual/graphical method, developed by
John Prohs and David Harris at Ambassador College,
used a transparent plastic sphere onto which the room
was mapped. Semispherical plastic overlays represented
horn patterns and could be moved around on the room
map to simulate aiming the horns. This method resolved
most of the accuracy problems of the flat transparency
method and was commercialized by Community Light
and Sound as the “Cluster Computer.”
A number of noted engineers and consultants
contributed to the development of these manual/graph-
ical design tools. Among them were Ed Seeley, Thomas
McCarthy, Ted Uzzle, John Prohs and David Harris,
Farrel Becker, Peter Tappan, Gene Patronis, and Bob
Thurmond.Figure 34-20. Array Perspective, a manual/graphical design
method. Courtesy Bosch/Electro-Voice.