Theatre Design & Technology - Summer 1983 - 10

Figure 13:
Folded LowFrequency Horn
Figure 12: Low-Frequency Horn wirh COile Driver

very little a manufacturer can do to
change the directional behavior of the
driver. The depth of the cone and the
cone material will have some effect on
the coverage angle. Thus, some loudspeakers may narrow to around 120°.
These directivity charts do not indicate what the frequency response is going to be, but experience has shown that
most of the larger drivers start to break
up and get ragged at about the same
point where the coverage angle drops
away from 160°. Generally, the point at
which a cone driver begins to beam can
be regarded as the limit of its useful
range.
Low-Frequency Horns (Fig. 12)
Before the recent revival of interest
in the vented box, horns were the mainstay of the professional market; and
they are not yet totally replaced by the
little boxes with holes. Horns have better efficiency and can have a little less
distortion than vented boxes. In most
practical horns, the added efficiency
amounrs to abour 6 dB in the range
from about 150 Hz up. The response
below about 150 Hz drops off fast. In
its operating range, the horn offers directional control tighter than the vented
box. But the beam width from a horn
constantly changes; it narrows as the
frequency goes up-the same beaming
problem as in the high-frequency horn.
A few horns have been built to operate lower than 150 Hz, but the size
gets very large. Remember mouth size
and length determine the low-frequency
limits. One way of increasing 10\\'frequency response while still maintaining reasonable horn size is to fold the
horn (Fig. 13). This scheme works well
at low frequencies but cannot be used

Figure 14: Vellfed Box wirh From Loaded Horn

above a few hundred hertz because the
shorter wavelengths begin to distort as
they move around corners.
Another attempt at adding low bass
to a horn is a combination of vented box
and horn (Fig. 14). While this form
does add some low bass, the level is
about 6 to IO dB below the level of the
mid-bass frequencies. This difference in
level will have to be smoothed out with
the equalizer if the system is to be used
for music reproduction, so little or nothing is gained by the vented horn.
It is the feeling of many system designers today that low-frequency horns
are usually a waste of money and box
size, except in the case of a speech-only
system where tight pattern control in the
mid-bass region is very important or
where the extra 6 dB of acoustical output cannot be made up by extra electrical power.
Among high-power loudspeaker systems, the bass horn system is still more
common than the vented box. But this
is changing as the vented box is being
used in many applications previously reserved for a horn. All the professional
equipment manufacturer have added
\'ented boxes to their line and most are
increasing their attention to vented
boxes as a replacement to the traditional
u e of bass horns.
It was noted thar bass horns are
sometimes useful when a tighter lo\\'frequency pattern is required. The pattern of cone drivers in vented boxes can
be tightened by stacking rwo drivers
close togerher in rhe same box. With
one driver over the orher, the vertical
pattern can be reduced to about 60° at
500 Hz while the horizontal covcragc remains at about 160°. If four drivers are
mounted in a cluster together, both the

vertical and the horizontal patterns will
be about 60° at 500 Hz. In fact, the
behavior of a bass horn can be rather
well duplicated by the correct arrangement of drivers in a vented box. Very
high power level capability can be put
together in this manner also.
Columns
This concept of stacked cone drivers
and the resultant narrow vertical pattern
brings us to a scheme called a column
loudspeaker. There are several columns
on the market with from four to fifteen
loudspeakers in a tall, narrow box.
These units have very narrow vertical
patterns and wide horizontal. They also
have lobes in their pattern reminiscent of
the multicell horns. This lobing and
ragged pattern shape is caused by phase
cancellations among the several drivers.
This lobing is of little concern at low frequencies but its effect in the mid-range
can be very noticeable.
As a result, column loudspeakers seldom sound very good. A column may
be well used for a speech-only sysrem in
a very reverberant space-such as a
church-as a visually less-offensive solurion rhan a horn. In a space with a
reasonable reverberation time (under 2
seconds)-as a theatre surely is-a column is not a good choice. ( 'Iy advice is:
do nor buy a column, and if you own
one, replace it. Chances are that a colUllln is nor the an wer ro any problem
you have.)
Powering the Loudspeakers
Loudspeakers cannot be considered
withour some reference to the system
rhat powers them. Before moving into
a discussion of powcring rhe loudspeakers, it is important to remember a few
points: 1) a large cone driver is needed

USITT /SUI11I11Cr 1983 Thealre Design &

Technolog~

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