604 Chapter 17
by the stiffness of the diaphragm suspension. Measure-
ments indicate that for a constant voltage applied to the
electrodes, the acoustic response is uniform (flat) to well
beyond the range of human hearing.
Output at low frequencies is limited by the
maximum linear amplitude of the diaphragm motion,
which is determined by spacing between the
diaphragms and damping in the suspension. The
maximum power output from an electrostatic loud-
speaker of a given diaphragm area is determined by the
strength of the electrostatic field that can be produced
between the diaphragm and the electrodes.
An electrostatic loudspeaker is seen by an amplifier
as a capacitor with a value on the order of 0.0025PF
from electrode to electrode. Thus, the magnitude of the
impedance presented by the loudspeaker to the output of
the amplifier falls off at 6 dB per octave as the
frequency is increased. This presents some problems for
driving electrostatics, as many amplifiers are not
designed to drive purely capacitive loads.
Because electrostatic loudspeakers are relatively
large in area compared to cones, their directivity is high
in comparison to cone systems. Various schemes have
been used by designers of electrostatics to address this
issue. The Quad ESL63 is one example. Here, the
diaphragm is broken into different regions for different
frequency ranges, the smaller ones being used for
higher frequencies, thereby making them wider in
dispersion than a large single panel.
17.5 Piezoelectric Loudspeakers
Piezoelectricity, or pressure electricity, was discovered
in the 1880s by the Curies. It is today a feasible motor
drive mechanism for loudspeakers. In a piezoelectric
material, a voltage applied to the material will result in a
mechanical strain or deflection. The reverse is also true,
and piezoelectric elements can be used in microphones.
This characteristic is attractive for direct-drive units
such as ultrasonic devices. For loudspeakers, however,
some means must be applied to mechanically amplify
the inherently low excursion so that a loudspeaker
diaphragm may be driven properly.
One of the earliest discovered piezoelectric
substances is Rochelle salt. Although Rochelle salt is
still widely used, it suffers from poor mechanical
strength, low temperature breakdown (55°C), and
extreme sensitivity to humidity. Barium titanate is the
first piezoceramic to be developed. Although it is not as
electrically sensitive, it is still widely used, exhibiting
many superior characteristics over Rochelle salt. The
most widely used piezo material today is lead zirconate
titanate, developed first in Japan in the 1950s. This mate-
rial (PZT) is now highly refined and exhibits the best
properties of any piezo material for loudspeaker use.
PZT material is formed by baking a ceramic slurry or
clay into bars about 1 inch in diameter and then slicing
the bars into thin wafers. Two wafers are bonded
together in opposing polarity, with electrodes on their
flat surfaces, forming a bimorph bender. As voltage is
applied to the bender, deformation of the disc results in
greater displacement at its center.
Early commercial attempts at the application of
bimorph benders to loudspeaker cones involved a rect-
angular drive element anchored at three corners,
allowing the fourth corner to drive the loudspeaker cone
fore and aft. Other attempts used a cantilever structure
anchored at one end with the loudspeaker cone mounted
at the other. In 1965 when Motorola, Inc. first manufac-
tured a piezoelectric loudspeaker, they used a length
expander tube driving a horn-loaded cone directly. This
device, like most piezoelectric loudspeakers made until
that time, still lacked sufficient voltage sensitivity to be
coupled directly to conventional systems without using
an auxiliary step-up transformer.
The development of the circular bimorph using a
corrugated center vane represented the next step
forward in piezo loudspeaker technology. The action of
the two disks working against each other, one
expanding while the other contracts, functions as a
mechanical transformer, giving impedance reductions of
about 20:1. The basic operation is as follows: The driver
dishes in and out; it pumps the cone fore and aft or, in
the case of the horn, into a compression chamber that is
then coupled to the throat of a horn via a slot and flared
rib construction. The driver is allowed to hang free in
space, working against its own inertia to pump the cone.
Further advancements in the state of the piezoelec-
tric art came from Tamura and coworkers in their work
on piezoelectric high-polymer films. This concept of a
diaphragm possessing piezoelectric properties and thus
coupling directly to the air without the use of any sepa-
rate motor structure represents a substantial advance-
ment toward the ideal acoustic transducer.
Another problem area in the development of the PZT
loudspeaker was in the power-handling capability of the
driver. The theoretical failure mode of a piezoelectric
tweeter is the depolarizing of the driver through exces-
sive drive level and/or high temperature. The Curie
point (depolarizing temperature) of the PZT used here is
above 150°C, and the depolarizing voltage is
10 V/25Pm thickness, or about 35 Vrms for the basic
driver. These numbers describe a fairly impressive
power-handling capability, but unfortunately one that is