What are Electrostatic Transducers?
Electrostatic Transducers can easily be identified by their flat, thin
(often tall) profile. They basically consist of a very light membrane (often
Polyester film) suspended between two conductive plates (perforated metal
sheeting/wire mesh). In operation, the membrane is charged to a high voltage,
and a differential audio signal is sent to the two conductive plates, varying
the charge on the plates and exerting an electrostatic force on the membrane.
The membrane then moves, moving the air around it.
Why use electrostatics?
Aside from the limitations within the MRI chamber, conventional dynamic
loudspeakers suffer from a number of inherent problems. A dynamic driver
design is a tradeoff between inversely related driver mass and rigidity;
higher mass equals higher driver inertia (limited frequency response, poor
transient response), and lower rigidity equals more driver deformation
(harmonic distortion). The necessity of having multiple drivers to reliably
cover a wide frequency range is related to these properties of mass and
rigidity, and comes with its own problems in the form of time/phase alignment
issues, and dispersion.
Electrostatic transducers circumvent the issues with mass and rigidity.
The driving diaphragm is very thin and light--with mass on the order of
a few millimetres thickness of air. Because of this, diaphragm inertia
is insignificant. Also, because driving force is uniform over its entire
surface, diaphragm deformation is not an issue. The only design-related
nonlinearity is at the edge of the diaphragm, where it is suspended. The
flexibility of the diaphragm makes that insignificant as well.
Problems with electrostatics
First and foremost, the electrostatic drive principle requires very
high voltages to operate. At the headphone level, bias and signal voltages
range from 200 to 800 volts, while large electrostatic loudspeakers can
use up to 8000 or 9000 volts. Conventional amplifiers must
be coupled through a high-quality transformer to raise the output voltage
to suitable levels, and even then, must be exceptionally stable to perform
well because of the reactive load (electrostatic transducers act as large
capacitors). Bias voltage must be supplied separately--in the case of
loudspeakers, an additional wall plug is usually required for each speaker.
In loudspeakers, frequency response on the low end is limited by a very
low diaphragm excursion--necessarily so because the relatively low-magnitude
electrostatic force requires a short gap between diaphragm and stator
(on the order of millimeters). High bias voltages also run the risk of
arcing between diaphragm and stator, damaging both. The transducer's
dipole nature also results in cancellation of longer wavelengths, as
most loudspeakers are built without a baffle.
In a headphone, many electrostatic-related issues are irrelevant.
At the headphone power level, designing a stable, suitable high voltage
amplifier is easier (though not trivial by any means). The concern
of having open (high) voltages around your head is unsettling, but
not particularly serious.
Low frequency response problems are more or less irrelevant in a headphone
design, as far less excursion is required of a headphone driver. With
acoustic dampening, it isn't too difficult to get a headphone with
superb frequency and transient response, and very low distortion. In
effect, applying electrostatic principles to headphone design (with
good workmanship) almost guarantee high performace.