Handbook for Sound Engineers

656 Chapter 18

bility throughout the audience areas. Digitally steerable
column arrays can offer superior coverage and they can
provide improved D/R. They can provide improved
intelligibility in highly reverberant spaces, plus they
blend better with their surrounding architecture and are
nearly invisible in use.

18.7.4.1 Digitally Steered Column Arrays

When the room size and volume are fixed and adding
absorption to reduce the RT times is not an option, digi-
tally steerable column arrays offer a new solution:

• They have the ability to be much more directional
  than the largest horns.


• The idea is not new; the concepts for these column
  arrays were described by Harry Olson in 1957. Only
  the implementation is new.


• The hardware required to implement these ideas is
  now available.


• Digital Signal Processing required is now a mature
  technology, very powerful and relatively inexpensive.


• Compact, highly efficient Class D amplifiers are
  capable of high-fidelity performance.

Line Arrays are not a new idea. Harry F. Olson did
the math and described the directional characteristics of
a continuous line source in his classic Acoustical Engi-
neering, first published in 1940. Traditional column
loudspeakers have always made use of line source
directivity.
Simple line arrays (column arrays) are basically a
number of drivers stacked closely together in a line, Fig.
18-17. Simple line arrays become increasingly direc-
tional in the vertical plane as the frequency increases.
The spacing between drivers controls the high
frequency limits. The height (length) of the line array
determines the low frequency control limit. Fig. 18-18
shows the line source directivity as described by Harry
Olson in 1957.
The directivity of a line array is a function of the line
length and the wavelength. As the wavelength
approaches the line length, the array becomes omnidi-
rectional, Fig. 18-19. Fig. 18-20 shows the vertical
dispersion pattern of a typical line array

18.7.4.2 Controlling High Frequency Beaming

Simple line arrays become increasingly directional as
the frequency increase., in fact, at higher frequencies
they become too directional. The vertical directivity can
be made more consistent by making the array shorter as

the frequency increases by using fewer drivers. One

amplifier channel and one DSP channel per driver make

this possible.

17.7.4.3 Beam Steering

The beam can be steered up or down by delaying the

signal to adjacent drivers. DSP control also allows us to

develop multiple beams from a single line array and

individually steer these beams.

DSP control also allows us to move each beams

acoustic center up and down the column allowing us to

Figure 18-17. Basic line array theory.

Figure 18-18. Directional characteristics of a line source as

a function of the length and the wavelength. The polar

graph depicts the sound pressure at a large fixed distance,

as a function of angle. The sound pressure for the angle 0q

is arbitrarily chosen as unity. The direction corresponding

to the angle 0q is perpendicular to the line. The directional

characteristics in 3D are surfaces of revolution about the

line as an axis. (From Acoustical Engineering by Harry

Olson.)

Driver spacing controls HF limit

Array height controls LF limit