652 Chapter 18
manufacturer L-Acoustic uses a type of path length
equalizer to force the emerging wavefront to conform to
the opening angle of their horn and also puts the
acoustic center behind the cabinet. In the case of ARC,
the cabinet’s trapezoidal side walls also serve as the
waveguide for the high frequencies. As the waveguides
opening angle matches that of the cabinet, this is
certainly an elegant solution to creating minimum inter-
ference arrays at the frequency where the horn is
effective.
In the KF900 series from EAW, simple phase horns
for the mid and high frequencies put the acoustic center
as close to the rear of the cabinet as possible, while their
opening angles also match the trapezoidal sides of the
enclosure. The relatively large size of the KF900 series
enclosures and horns brings minimum interference
performance to frequencies lower than those based on
smaller waveguides. Remember, that this technique for
minimum interference arrays, including the True Array
Principle, only holds true for those frequencies where
the horn is effective.
18.6 18.6 Low Frequency Arrays: Beneficial
Interference
In the preceding paragraphs, I outlined the parameters
necessary to minimize destructive acoustic interference
between adjacent cabinets or horns in an array. But
these techniques are only beneficial at the frequencies
where the horns are effective. Yet these very systems or
horns are used at frequencies well below their direc-
tivity cutoff and lower, down to frequencies where the
woofers piston size offers no directional control at all.18.6.1 Horizontal Woofer Arrays: Maintaining Wide
DispersionFor our first example, let’s look at the additional prob-
lems and opportunities we create when arraying small
(12 inch woofer, 1 inch compression driver) full range
loudspeaker enclosures as in Fig. 18-12. For a full range
array module, there are three frequency zones that
exhibit different wavelength related behavior. At the
lowest frequencies, or longest wavelengths, these
modules exhibit only beneficial interference or mutual
coupling. Each additional module creates additional on
axis acoustic output. The opportunity here, is that less
equalization is required to make the array’s frequency
response flat down to these lower frequencies as
compared to a single cabinet.
A potential problem is created when the array
becomes too wide however. Four or five element arrays
are wide enough as to become quite directional in the
forward plane at those lower frequencies (20 Hz to
roughly 500 Hz or more, dependant on the module).
Without a signal processing scheme, this array cannot
be equalized to have the same frequency response
through out it’s intended coverage. It will sound boomy
in the middle and thin at it’s coverage extremes. A solu-
tion is to taper the length of the array in the horizontal
plane in order to maximize horizontal dispersion of the
lower frequencies. The entire array can be used for the
lowest frequencies as the wavelengths are longest
(20 Hz up to about 200 Hz), but at higher frequencies,
as the wavelengths get shorter, the array length must
also get shorter to maintain wide dispersion. This is
achieved by low passing the outermost woofers of the
array, such that only two or three max woofers are used
at frequencies higher than this.
The second frequency zone that can be problematic
in arrays based on full range modules, occurs at wave-
lengths where cabinet spacing no longer supports
mutual coupling, and the horn has yet to attain it’s direc-
tivity cutoff. This typically applies to a small half
octave range where adjacent cabinet spacing approaches
a wavelength. Here we observe combinations of
destructive and constructive interference at various
observation points around the array intended coverage,
causing frequency response variations greater than
±6 dB. Fortunately there is a signal processing tech-
nique that can minimize this effect. By simply notching
this frequency range from every other cabinet with a cut
equal to the greatest amount a variance (typically 6 dBFigure 18-12. TRAP arrays can be quite small; however, the
size of the horns will determine the lower frequency limit at
which the True Array Principle ceases to operate.