Handbook for Sound Engineers

Loudspeaker Cluster Design 653

of attenuation), and width equal to the bandwidth of the
aberrations (typically half an octave), the frequency
response variations throughout the arrays coverage can
minimized.
The third frequency zone of wavelength related
behavior for arrays based on full range modules, is then
at the frequencies above which the horn is effective. Let
us assume that the horns depicted in Fig. 18-12 place the
acoustic center towards the rear of the cabinets, and that
their opening angle also matches that of the trapezoidal
sides of the cabinet. Based on these assumptions, the
array performance will exhibit minimum interference for
frequencies above 1–2 kHz which happens to be the
effective directivity cutoff of the horn. Each additional
module simply adds additional coverage to the array.

18.6.2 Vertical Woofer Arrays

18.6.2.1 Directivity at Frequencies Where Size Makes
Horns Impractical

Beneficial destructive interference sounds like an
oxymoron, but there are several commercially available
woofer arrays that take advantage of this very tech-
nique. By applying the fundamental physics described
by Harry Olson, directional woofer arrays are now
available that out perform large woofer horns.
When two point sources are superimposed on one
another, their outputs simply add up in all directions. As
the two point sources are spread apart, the output dimin-
ishes along the plane of separation due to phase cancel-
lation. At exactly ½ wavelength, a pure null occurs, and
we achieve the classic figure eight, dipole polar pattern.
The current commercially available systems take advan-
tage of this phenomena, directivity through off axis
attenuation, by placing woofers in a vertical array and
spacing them to create this dipolar pattern at frequencies
below which horns become too large.
Fig. 18-13 is an example of one such array. Termed
Tri-Polar by it’s designer Vance Breshears, it uses the
vertical spacing between the three woofers with appro-
priate signal processing to maintain consistent low
frequency pattern control from 400 Hz down to below
100 Hz. One of the first systems available was devel-
oped by Craig Janssen, termed Tuned Dipolar, it uses
two separate arrays. With drivers, spacing and signal
processing appropriate for their respective passbands
Tuned Dipolar offer exceptional low frequency pattern
control over an extended bandwidth. Even subwoofers
are now benefitting from this type of technology. Meyer
Sound is achieving cardioid patterns at lowest frequen-

cies from its PSW-6, providing significant attenuation

of those frequencies directly behind the enclosures.

18.7 Line Arrays and Digitally-Steerable Loud-

speaker Column Arrays

For the communication between a source and a listener

to be effective, it is important that the listener receive

and comprehend the message. In large spaces where

people gather, including auditoria, houses of worship,

sports venues, transit terminals and classrooms, often the

acoustic requirements that enable effective speech are in

conflict with the architectural needs of the spaces. When

the acoustics of a venue cannot be altered to enable

effective speech communication, designing a sound rein-

forcement system to do so, can be a challenge. Recent

advances in efficient amplification and digital signal

processing have enabled a new class of loudspeaker; the

digitally steerable column or line array as its often

called. The acoustical and architectural benefits of these

Figure 18-13. Reference Point Array using four 40q×40q

mid-high enclosures and six low frequency modules in

Tri-Polar configuration for vertical pattern control, along

with appropriate small full range systems for downfill.