Handbook for Sound Engineers

614 Chapter 17

the loudspeaker, it is most common for the required hori-
zontal coverage to be relatively narrow at the greatest
distance from the source and to become successively
wider at closer distances. In addition, it is desirable for
the vertical angle of greatest intensity to be as large as
possible — i.e., for greater energy to be directed to the
seats at the greatest distance from the loudspeaker in
order to produce similar SPL values throughout the audi-
ence. One early attempt to address this requirement was
the JBL 4660, shown in Fig. 17-34.

Another design, developed for a specific applica-
tion, is the IMAX® PPS (Proportional Point Source)
loudspeaker, developed by the author. Fig. 17-35 is the
high-frequency horn used in this loudspeaker, and
Figure 17-36 is its 4 kHz isobar.

Dave Gunness, chief engineer at Electro-Voice at the
time, developed a family of asymmetric directivity
horns in the late 1980s and early 1990s. These were
known as Va r i - I n t e n s e devices.

Optimized (asymmetric) directivity is an attractive
engineering goal, but there are a number of obstacles to
its widespread acceptance:

1. Computer-based sound system prediction software
   is required in order to visualize its effectiveness and
   optimize aiming and device placement,
   2. Exactly what constitutes ideal directivity is a strong
   function of the space in which the device is to be
   used. The ideal directivity will vary, for example,
   for different loudspeaker elevations within the same
   space.
   3. With the exception of the proprietary IMAX loud-
   speaker, there are currently only high-frequency
   devices available with this type of directivity.
   Achieving uniform sound pressure levels
   throughout the seating, but at high frequencies only,
   is of limited value.

17.7.6.6 Acoustic Lenses

Although an acoustic lens is not generally regarded as a

directivity control device, it can function as a directivity

alteration device. While acoustic lenses are used to

widen a pattern, they can also be used to narrow a

horn’s directivity. An acoustic lens is usually formed

with parallel plates of strategically chosen shapes

placed at an angle to the direction of sound propagation.

Differing path lengths through different portions of the

lens create arrival time relationships for the associated

components of the wave that generate specific direc-

tivity characteristics.

The slant-plate lens assembly, shown mounted on a

JBL studio monitor in Fig. 17-37, is one notable imple-

mentation of an acoustic lens. Note that the device has

concave openings in the plate array. As the wave leaves

the horn and progresses through the lens plate array, the

center of the wave reaches the air on the outside first,

due to the shorter path through the lens. The outer

portions of the wave travel through longer paths within

the lens and are therefore delayed in time relative to the

portions that came from the center. The net effect is to

produce arrivals that are better synchronized—and

therefore stronger—at positions that are off axis in the

horizontal, thus widening the polar pattern in that direc-

tion. The vertical pattern would ideally be unaltered.

Lenses have the undesirable property of causing rela-

tively strong reflections back into the horn.

17.7.6.7 Folded Horns

One of the practical drawbacks of horns, particularly

those intended for use at low frequencies, is their phys-

ical size. Folded horns were developed in response to

this problem and have been in use in various forms for

more than half a century. A folded horn is produced by

truncating the shape at point, providing a reflecting

surface to change the direction of the outgoing wave,

and continuing the horn’s expansion in another direction,

Figure 17-33. Whelen Engineering horizontal diffraction
horn with multiple drivers. Courtesy Whelen Engineering.