Theatre Design & Technology - Summer 1983 - 7

PIVOT

arated by an insulating layer. The plates
are charged with a very high voltagepositive on one and negative on the
other-thus creating an electrostatic potential between the plates. An audio
voltage is superimposed on one of the
polarizing voltages causing the potential
between the plates to vary in step with
the audio. As the electrostatic force
changes, the thin foil plate will vibrate in
step with the changing audio signal.
This movement is passed on to the air
around the plate, thus creating sound
waves. Electrostatic loudspeakers can
do an outstanding job of reproducing
the higher sound frequencies, say from
about 1000 Hz up. They are quite unsatisfactory in bass reproduction-the
foil plate cannot make the large movements required to create long wavelengths with useful intensities. While
these loudspeakers are used as highfrequency units in some very good and
costly home systems, they are not useful
in the theatre because they do nor
"speak" loud enough.
Piezoelectric
Certain crystaline materials are piezoelectric; that is, they have the ability
to generate electrical current under pressure or, conversely, to bend, twist, or
contract when an electrical current is applied. Many hearing aids and some
headphones use materials that bend or
twist to operate a small diaphragm (Fig.
2). These units are capable of fairly
good response at low levels.
A more practical loudspeaker is
made by the Motorola Company (Fig.
3). In this device a 2.5-inch-long rod of
piezoelectric material is anchored to the
mounting frame on one end. The other
end is connected to an aluminum diaphragm that forms one wall of a pressure cham ber. When an electrical voltage is applied to the rod it contracts in
step with the variations in the electrical
signal. The attached diaphragm moves
in and out alternating the pressure in the
air chamber causing pressure waves to
move through the small throat and on
out the attached horn.
Because of the short range of contraction of the drive rod, this device is
not capable of generating long wavelengths at useful volume levels. Its useful range is restricted to the upper two
octaves of the audio band. Despite its
relatively low cost, this loudspeaker is
only occasionally used as a super tweeter
in professional equipment.

LNKAGE

CRYSTAL OR

CERAMIC

P<>t....AfUZNG VOlTAGE

Figure 2: Pie::.oeleclric Loudspeakerhearing aid or headphone

Figure 1: Eleclros!alic Loudspeaker

Dynamic
The vast majority of loudspeakers
manufactured today are of the dynamic
type. Except for an occasional piezoelectric tweeter, dynamics are the only
type of interest for professional loudspeakers in the theatre.
A dynamic loudspeaker consists of
three basic parts: 1) a large permanent
magnet, 2) a coil of fine wire around the
magnet, and 3) an air-moving diaphragm connected to the coil. When an
audio current is applied to the coil, a
varying magnetic field is created which
interacts with the fixed field of the permanent magnet causing the coil and its
attached diaphragm to move. The
amount (distance) of movement corresponds to the intensity (or loudness) of
the electrical energy, and the rate of
movement corresponds to the frequency.
There are two branches of the dynamic loudspeaker family: I) the cone
drivers-sometimes called direct radiators and the most familiar of all loudspeakers; and 2) the compression drivers
-very important although less wellknown. The cone driver is often referred to as a direct radiator because the
cone can be used to create sound pressure waves directly while the compression driver must be coupled to the air
through a horn. Cone drivers work better at low frequencies than at high.
Compression drivers, on the other hand,
do not work at all at low frequencies yet
are able to generate very high pressure
levels at high frequencies.

Cone Drivers
Figure 4 shows the principal parts of
a cone driver in cross-section. A large
magnet-sometimes weighing several
pounds-has formed in it a circular slot
called the magnetic gap in which the

voice coil is suspended by the cone. The
cone is attached to the frame or basket
by a flexible area called the surround.
The surround is flexible enough to allow
the cone to move freely as the voice coil
rides in and out in the magnetic gap, yet
it is stiff enough to return the cone and
coil to the mid-position when there is no
electrical current in the coil. In large
units, an additional support, called a
pi del', is added near the voice coil.
There are several factors that limit
the performance of a cone driver. Its
size (or diameter) tells much about how
loud it can reproduce low frequencies.
The ability to create a low bass tone is
not, in itself, a function of loudspeaker
size; but the ability to do so loudly depends upon how much air the cone can
set in motion. Low-frequency loudness
depends largely upon the size of the cone
and the travel distance of the voice coil
in the magnetic gap.
There are limitations on how far the
voice coil can move in the gap. If the
coil leaves the gap-or even reaches the
edge-the interaction of the magnetic
forces can no longer take place and the
sound from the loudspeaker becomes
very distorted or stops completely if the
coil leaves the gap. The mechanical limitations of the cone, surround, and spider determine the limit of low-frequency
loudness.
ote that we ha e not given numbers
to either "low frequency" or to "loudness." These terms are relative to the
size of the loudspeaker. The limitation
on a loud low frequency from an 18"
cone is certainly not the same as from a
4" unit. One very basic rule of loudspeakers is the rule of the Three L's: For
low frequencies to be reproduced loudly, the source must be large.

SITT /SUll1ll1cr 1983/Theatre Design & Technology

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