Handbook for Sound Engineers

Loudspeakers 603

response to this modulated force, the diaphragms to
move opposite (toward or away from) each other. The
upper limit on the amplitude of the allowed signal
voltage is then equal to half the polarization voltage.
This arrangement is the basis of all modern electro-
static loudspeakers. The result is an acceptably low
level of harmonic distortion, as long as variations in the
distance between plates or the diaphragms are mini-
mized. The movement of the foil diaphragms generates
sound waves. The diaphragms produce equal acoustical
power radiated in opposite directions. This set of char-
acteristics defines a dipole radiator.

It is asserted by the designers of electrostatic loud-
speakers that they overcome certain basic disadvantages
of cone-type loudspeakers, particularly with respect to
the propagation of acoustic energy at the high frequen-
cies. Cone-type loudspeakers are driven by a voice coil
that is attached to a relatively small portion of the total
diaphragm area, and they do not behave as pistons at
higher frequencies. Because the electrostatic loud-
speaker has a diaphragm that is driven uniformly across
its surface, breakup is said to be eliminated. Addition-
ally, the diaphragm can have low mass compared to the
air load on the diaphragm. This enhances
high-frequency and transient response.

Electrostatic loudspeakers may be constructed in
several different ways. Two of the most common
construction types are:

1. Stretching the diaphragm between supports around
   its periphery and leaving an air gap between the
   diaphragm and two stationary electrodes, Fig.
   17-14.


2. Using an inert diaphragm that is supported by a
   large number of tiny elements disposed across the
   entire surfaces of the two electrodes. These
   elements act as spacers to hold the diaphragm in the
   center between the electrodes, Fig. 17-15.

In the latter type of loudspeaker, the diaphragm is a
thin sheet of plastic on which has been deposited a very
thin layer of conductive material. It is supported by
multiple small elastic elements that hold the diaphragm
in place but permit it to follow audio-signal waveforms.
The electrodes on each side of the diaphragm are acous-
tically transparent to avoid pressure effects from
trapped air as well as to permit acoustic energy to prop-
agate away from the diaphragm. This type of construc-
tion permits the diaphragm to be of arbitrary size. The
performance per unit area is the same for any area of the
diaphragm. The actual loudspeaker is a thin surface
curved in the horizontal, forming a section of a cylinder.

A surface that is large with respect to wavelength

becomes increasingly directional at high frequencies.

Since an electrostatic loudspeaker is designed to

couple directly with the acoustic resistance of air, the

mass of the diaphragm is quite small and can be

neglected with little effect on the accuracy of predictive

models. The velocity of the diaphragm is directly propor-

tional to the electrostatic force applied, except as altered

Figure 17-14. Electrostatic- or capacitor-type loudspeaker.

Figure 17-15. Cutaway view showing the internal construc-

tion of an electrostatic loudspeaker.

Back

plate

Foil

Insulating

layer

Polarizing

voltage

Foil

Insulating

layer

Back

plate

60°

C

+

R

Supporting

frame

Outer

electrode

Inert thin

conductive

diaphragm Inner

electrode