Theatre Design & Technology - Summer 1983 - 11

to reproduce low bass loudly, 2) a large
cone driver will add noticeable distortion to any high frequencies it is called
upon to reproduce, 3) a compression
driver is probably the best choice [Q reproduce high frequencies loudly, and 4)
a compression driver must not be subjected [Q bass frequencies. From these
facts it is obvious that the audio spectrum must be divided in order that each
driver will receive only that band of frequencies that it is able [Q reproduce.
The electronic device that performs this
division is called a crossover network.
There are two basic types: the high-level
and the low-level.

Crossovers
The high-level crossover is placed in
the circuit following the power amplifier
where it divides the audio spectrum
sending the low frequencies to the
woofer and the high frequencies to the
tweeter. This high-level crossover also is
used [Q reduce the power level sent to the
compression driver. This high-end
power reduction is necessary because the
compression driver and horn are more
efficient than the cone driver woofer.
The high-level crossover is a passive
device; that is, it contains no amplifier
circuits. The zero failure life should be
twenty-five [Q fifty years.
There is a major limitation to the use
of high-level crossover. It is not practical to build a crossover that will operate at much more than about 200 watts.
This is enough power for most portable
loudspeaker units, but it is far short of
what a high-power central cluster could
need. The large cluster in Fig. 9 can
handle more than 2000 watts. Highlevel crossovers are out of the question
at such power levels.
The low-level crossover system is
often referred [Q as bi-amping because
the frequency division occurs before the
power amplifier and requires separate
amplifiers for highs and lows. The lowlevel crosso\-er may be a passive filter
de\-ice; but, in fact, it almost always is
an acti\-e electronic system. It may also
contain extra response shaping circuits
10 add a little low-end thump or 10 add
eXIra high-end boos I to compensate for
the natural high-frequency falloff of
compression drivers_
By having separate power amplifiers
for the bass and treble drivers, il is possible [Q use high powers and 10 adjusl Ihe

Polar Patterns
The coverage pattern of a loudspeaker is often depicted by a polar pattern. The loudspeaker is at the center of
a group of concentric circles. Each circle represents some change of output
level from the loudspeaker-usually 5
dB per circle. The spokes radiating from
the center represent angles about the
loudspeaker. Zero degree is on-axis,
180 0 is directly behind, while 90 0 and
270 0 represent right and left. The radiation of many loudspeakers is not uniform about the sphere, so the patterns
for both horizontal and vertical must be
considered. The frequency at which the
measurements were made must be specified because loudspeakers will exhibit
different patterns at different frequencies. Polar patterns for several different

H---HH-f-+-t-l'"

...
HORIZONTAL

frequencies are sometimes drawn over
each other. This gets very confusing to
read. To help reduce this confusion,
some manufacturers also use a rectangular graph to indicate the beam width at
different frequencies. The useful coverage pattern (or beam width) is usually
regarded as the angle between the points
where the output has fallen by 6 dB.
When evaluating the beam width of
a full-range system, the most important
part of the audio spectrum is from 1000
to 4000 Hz. This is the major region of
speech intelligibility. The loudspeaker
depicted in the accompanying figure has
a beam width that varies from 125 at
1000 Hz down to only 50 0 vertically at
4000 Hz. For purposes of aiming at the
audience, this loudspeaker would be regarded as having a 60 beam width.
0

- H-+lH:&ft-H-{

2,000 Hz - - 8,000 Hz *******

Wavelength
The purpose of a loudspeaker is to
set air in a wave motion corresponding
to the variations in an electrical current.
The sound waves created in the air
around the loudspeaker have two properties: intensity representing loudness,
and length representing frequency or
tone. We have all heard the analogy of
dropping a stone in a still pond and
watching the waves. If we could stop
one of these waves and cut a cross sec-

...
VERTICAL

tion through it, the distance from the
top of the crest to the bottom of the
trough would be the intensity of the
wave. In sound, this is loudness. The
distance from the top of one crest to the
top of the next crest is the wavelength or
tone of the sound. The wave may be
measured by the time required to pass
by, or it may be measured as a distance.
We may speak of a 1000-Hz tone or a
13 %/I wavelength.
This concept of the relationship be-

WAVELENGTH

FREQUENCY

WAVELENGTH

COl/IiI/lied 01/ page 2/

Thealre Design &

Technolog~

INTENSITY (LOUDNESS)

USJTT Summer 1983

1130

FREQUENCY

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