LOUDSPEAKERS

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Book Excerpt: Loudspeaker Types and How They Work
10,000 volts (10kV). In addition, the audio signal
is stepped up from several tens of volts to several
thousand volts by a step-up transformer inside
the electrostatic loudspeaker. These high voltages
are necessary to produce the electrostatic fields
around the diaphragm and stators.
To prevent arcing—the electrical charge
jumping between elements—the stators are
often coated with an insulating material. Still, if
an electrostatic loudspeaker is overdriven, the
electrostatic field strips free electrons from the
oxygen in the air, making it ionized; this provides
a conductive path for the electrical charge. Large
diaphragm excursions— i.e., a loud playing
level—put the diaphragm closer to the stators
and also encourage arcing. Arcing can destroy
electrostatic panels by punching small holes
in the membrane. Arcing is a greater problem
in humid climates than in dry climates because
moisture makes the air between the stators more
electrically conductive.
Electrostatic panels are often divided into
several smaller panels to reduce the effects of
diaphragm resonances. Some panels are curved
to reduce the lobing effect (uneven radiation
pattern) at high frequencies. Lobing occurs when
the wavelength of sound is small compared
to the diaphragm. Lobing is responsible for
electrostatics’ uneven high-frequency dispersion
pattern, which Stereophile magazine founder J.
Gordon Holt has dubbed the vertical venetianblind
effect, in which the tonal balance changes
rapidly and repeatedly as you move your head
from side to side.
Electrostatic panels are of even lighter weight
than planar magnetic transducers. Unlike the
ribbon driver, in which the diaphragm carries the
audio signal current, the electrostatic diaphragm
need not carry the audio signal. The diaphragm
can therefore be very thin, often less than 0.001".
Such a low mass allows the diaphragm to start
and stop very quickly, improving transient
response. And because the electrostatic panel is
driven uniformly over its entire area, the panel is
less prone to breakup. Both the electromagnetic
planar loudspeaker (a ribbon) and the electrostatic
planar loudspeaker enjoy the benefits of limited
dispersion, which means less reflected sound
arriving at the listening position. Like ribbon
loudspeakers, electrostatic loudspeakers also
have no enclosure to degrade the sound.
Electrostatic loudspeakers also inherently have a
dipolar radiation pattern. Because the diaphragm
is mounted in an open panel, the electrostatic
driver produces as much sound to the rear as to
the front. Finally, the electrostatic loudspeaker’s
huge surface area confers an advantage in
reproducing the correct size of instrumental
images.
In the debit column, electrostatic loudspeakers
must be plugged into an AC outlet to generate
the polarizing voltage. Because the electrostatic
is naturally a dipolar radiator, room placement
is more crucial to achieving good sound. The
electrostatic loudspeaker needs to be placed
well out into the room and away from the rear
wall to realize a fully developed soundstage.
Electrostatics also tend to be insensitive, requiring
large power amplifiers. The load impedance they
present to the amplifier is also more reactive than
that of dynamic loudspeakers, further taxing the
power amp. Nor will they play as loudly as dynamic
loudspeakers; electrostatics aren’t noted for their
dynamic impact, power, or deep bass. Instead,
electrostatics excel in transparency, delicacy,
transient response, resolution of detail, stunning
imaging, and overall musical coherence.
Electrostatic loudspeakers can be augmented
with separate dynamic woofers or a subwoofer to
extend the low-frequency response and provide
some dynamic impact. Other electrostatics
achieve the same result in a more convenient
package: dynamic woofers in enclosures mated
to the electrostatic panels. Some of these designs
achieve the best qualities of both the dynamic
driver and electrostatic panel. Audition such hybrid
speakers carefully; they sometimes exhibit an
audible discontinuity at the transition frequency at
which the woofer leaves off and the electrostatic
panel takes over. Listen, for example, for a change
in a piano’s timbre, bloom, projection, and image
size as it is played in different registers. Acoustic
bass in jazz is also a good test of woofer/panel
discontinuity in dynamic/electrostatic hybrid
loudspeakers.
One great benefit of full-range ribbons and
full-range electrostatics is the absence of a
crossover; the diaphragm is driven by the entire
audio signal. This prevents any discontinuities in
the sound as different frequencies are reproduced
by different drivers. In addition, removing the
resistors, capacitors, and inductors found in
crossovers greatly increases the full-range
planar’s transparency and harmonic accuracy.
Even hybrid planars put the crossover frequency
between the dynamic woofer and the planar
panel very low (below 800Hz, a frequency
nearly an octave above middle A), so there’s no
discontinuity between drivers through most of the
audible spectrum.
Finally, the large diaphragms of electrostatic
and ribbon drivers are gently driven over their
entire surface areas, rather than forcefully over the
relatively small voice-coil area of a dynamic driver.
This high force over a small area contributes to
the dynamic driver’s breakup described earlier, a
phenomenon less likely to occur with large planar
diaphragms.
Excerpted and adapted from The Complete
Guide to High-End Audio (Fourth Edition).
Copyright © 1994–2013 by Robert Harley.
hifibooks.com To order call (800) 841-4741.
22 Guide to High-Performance Loudspeakers www.theabsolutesound.com
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