Theatre Design & Technology - Summer 1983 - 9

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MULTlCEll

ACOUSTlC L£HS

Figure 7: Polar Diagrams
compression driver is a more delicate
device and will withstand less overload
abuse than the typical cone loudspeaker.
The two great killers of compression
drivers are mechanical failure due to excessive power at the low-frequency end
of its operating range and thermal burnout due to feedback.
Compression drivers are never used
in the bass region. While a few will operate as low as 300 hertz, most are rated
for use above 500 Hz, and some operate
only as super tweeters above 5000 to
7000 Hz. Though most professional
compression drivers are rated at so
many watts above 500 Hz, many of
them can be used with twice as much
power if the minimum operating frequency is moved up to about 1200 Hz.
At 250 Hz, half the rated power will probably kill most of them.
Both system design and system operation must take into account the compression driver's great dislike for low
frequencies and its equally great dislike
for feedback.
Loudspeaker Enclosures
A driver does not operate alone. It
must be enclosed in a box and/or attached to a horn to become a functioning loudspeaker.
High-Frequency Horns
A compression driver can operate
only when it is connected to a horn. The
horn provides the necessary acoustical
coupling between the driver throat and
the outside air. In so doing, the horn
also determines the directional characteristics or coverage pattern of the sound
field. A horn can be designed to give
almost any coverage pattern. Therefore,
in selecting a horn loudspeaker, take
care that its coverage angles are com-

Figure 8: Constant Coverage Horn

FRONT VIEW

Figure 9: Typical Large Cluster
patible with its intended use. Modern
horns range in coverage from the very
wide pattern of 120° by 60° to the very
narrow angle of only 20° by 40°. The
most commonly-available commercial
horns are:
120° x 60°
90° x 40°
60° x40°
20° x 40°

The narrow angle horns are most
commonly used in fixed loudspeaker
clusters intended for speech rein forcement in large halls where the desire is to
confine the coverage pattern to the audience area only and to avoid as much as
possible spilling sound energy on the
walls or ceiling where it would add to the
reverberation or echo problems common in these large spaces. Such clusters
are made up of several loudspeakers,
each covering a section of the audience.
Portable loudspeakers intended for
sound effects or surround music generally have wide coverge angles to ensure
including all the audience in the cover-

age of a single unit. The human hearing
process is such that what is required for
good speech intelligibility is not necessarily what is required for music or even
sound effects. Speech intelligibility requires good control of reverberation levels, but the quality of music is often enhanced by more reverberation. In some
old churches, for instance, the music
sounds great but not a word of the sermon can be understood.
Types of Horns
There are three types of highfrequency horns in general use today.
Figure 6 shows the two workhorses of
the industry: the radial and the multicell.
The radial came first and is still the most
widely used of all horns. Its biggest
problem is that it does not disperse all
frequencies equally. As the frequency
rises the coverage angle grows progressively more narrow. This is called beaming. An untrained listener who remains
in his or her seat may not be aware of
beaming; however, one who is knowledgeable of what should be present in

US ITT /Summer 1983/Thealre Design & Technology

Table of Contents for the Digital Edition of Theatre Design & Technology - Summer 1983