Handbook for Sound Engineers

208 Chapter 8

the devices are identical, have equal amplitude drive
signals, and are driven in phase. When driven in this
fashion, the array is said to be unprocessed. Modern line
arrays are usually structured from enclosures that are
each full range loudspeaker systems. Each such loud-
speaker system usually divides the audio band into three
or four separate bands. In effect then, one is dealing
with not just a single line array, but rather three or four
arranged parallel to each other. This technique allows
optimization in each frequency band with regard to the
number of devices, device spacing, and individual
device directivity. Fig. 8-7A and B illustrates the perfor-
mance of the low-frequency section of a straight line
array consisting of ten 15 inch diameter woofers with a
separation between woofers of 0.6 meter. Fig. 8-7A
shows the pressure generated by the array relative to
that produced by a single device when operating at
50 Hz, which is the bottom end of the woofer pass band.
At this frequency the woofer itself is omnidirectional
and the vertical directivity is that produced by the array
structure itself. At 300 Hz, which is the upper end of the
woofer pass band, the central lobe has been greatly
narrowed and there are numerous side lobes as illus-
trated in Fig. 8-7B.

The operation in the other frequency bands of a full
range line array that is unprocessed is qualitatively the
same even though the number of devices and spacing
between individual devices is, in general, different. The
large increase of the pressure on-axis is the desirable
attribute whereas the accompanying narrowing of the
central lobe and the generation of side lobes are undesir-
able. The latter behavior can be mitigated to some
extent by arraying on an arc rather than a straight line.
The mounting hardware linking the devices in an array
is structured so as to allow a splay between individual
units with an adjustable angle in the range of 2° to 5°.
This shapes the array into an arc of a circle rather than
in a straight line. In such an arrangement, there is some
reduction in the maximum pressure on axis, but the
central lobe retains a more uniform width particularly at
the upper ends of the various frequency bands. Mathe-
matical details may be found in the first reference at the
end of this chapter.

Another arraying technique worthy of mention is
that of the Bessel array first introduced by the Dutch
industrial giant Philips. The Bessel array in its simplest
configuration employs five identical elements and
although it only doubles the on-axis pressure in the far
field, it does so while having a coverage pattern both
vertically and horizontally that matches the coverage
pattern of the individual elements from which it is
constructed. The individual elements might be woofers,

horns, or full range systems of any type. In the simplest

configuration, five identical devices are arrayed along a

straight line either horizontally or vertically as close

together as possible with parallel axes. The unique

properties of the array are brought about by weighting

the voltage drive to the array in the sequence 0.5, 1,1,

1, and 0.5. For example for a vertical array, one-half of

the available voltage drive is applied to the top and

bottom elements. This is easily accomplished in prac-

tice by connecting these two elements in series with

each other. The interior elements are then connected

together in parallel with the lowest interior element

being operated in reverse polarity. This physical and

electrical arrangement appears in Fig. 8-8.

When observed in the far field this arrangement

produces twice the pressure of that of a single device

Figure 8-7. Woofer line array operating at 50 Hz and

300 Hz.

80

60 

40

20 0 20 40 60 80

Vertical polar angle

Behavior at 50 Hz

10

9 8 7 6 5 4 3 2 1

(^80
60
40
20020406080)
Vertical polar angle
Behavior at 300 Hz
10
9 8 7 6 5 4 3 2 1 0
A. Woofer line array operating at 50 Hz.
B. Woofer line array operating at 300 Hz.
Relative response
Relative response