Handbook for Sound Engineers

38 Chapter 2

expanding in two dimensions, the level change with
increasing distance is half that of the point source radi-
ator. The sound level from an ideal line source will
decrease at 3 dB per distance doubling rather than 6 dB,
Fig. 2-24. It should be pointed out that these relation-
ships are both frequency and line length dependent, and
what is being described here is the ideal case. Few
commercially available line arrays exhibit this cylin-
drical behavior over their full bandwidth. Even so, it is
useful to allow a mental image of the characteristics of
such a device to be formed.

If the line source is finite in length (as all real-world
sources will be), then there will be a phase differential
between the sound radiated from different points on the
source to a specific point in space. All of the points will
be the most in phase on a plane perpendicular from the
array and equidistant from the end points of the array.
As the point of observation moves away from the
midpoint, phase interaction will produce lobes in the
radiated energy pattern. The lobes can be suppressed by
clever design, allowing the wave front to be confined to
a very narrow vertical angle, yet with wide horizontal
coverage. Such a radiation pattern is ideal for some
applications, such as a broad, flat audience plane that
must be covered from ear height. Digital signal

processing has produced well-behaved line arrays that

can project sound to great distances. Some incorporate

an adjustable delay for each element to allow steering of

the radiation lobe. Useful designs for auditoriums are at

least 2 meters in vertical length.

While it is possible to construct a continuous line

source using ribbon drivers, etc., most commercially

available designs are made up of closely spaced discrete

loudspeakers or loudspeaker systems and are more

properly referred to as line arrays, Fig. 2-25.

2.12 Conclusion

The material in this chapter was carefully selected to

expose the reader to a broad spectrum of principles

regarding sound reinforcement systems. As a colleague

once put it, “Sound theory is like an onion. Every time

you peel off a layer another lies beneath it!” Each of

these topics can be taken to higher levels, and many

have been by other authors within this textbook. The

reader is encouraged to use this information as a spring-

board into a life-long study of audio and acoustics. We

are called upon to spend much of our time learning

about new technologies. It must be remembered that

new methods come from the mature body of principles

and practices that have been handed down by those who

came before us. Looking backward can have some huge

rewards.

If I can see farther than those who came before me, it

is because I am standing on their shoulders.

Sir Isaac Newton

Figure 2-24. Line sources radiate a cylindrical wave (ideal
case). The level drop versus distance is less than for a point
source. Courtesy Syn-Aud-Con.

A 2A

2Dx

Dx

10log A

2A

= 3 dB

Figure 2-25. The finite line array has gained wide

acceptance among system designers, allowing wide audi-

ence coverage with minimal energy radiation to room sur-

faces. Courtesy Syn-Aud-Con.

Pressure maximum

due to phase summation

Pressure minimum

due to phase summation

End of array

End of array

A 2A

2Dx

Dx

10log A

2A



3 dB