Headphones 739diaphragm are all chosen carefully so that the fi lm does not collapse on to either plate.
Instead it stays in a stable position between the outer plates, attracted to each one equally
during no-signal conditions. When an audio signal is fed to the transformer, it is stepped
up at the secondary from a few volts to around a thousand volts. This unbalances the
forces on the diaphragm in sympathy with the audio signal, causing it to be attracted
alternately to each plate and of course reproducing an analogue of the original sound.
The push–pull action of the transformer and plates effectively produces a linear force
on the diaphragm regardless of its position between the plates—unlike normal single-
ended electrostatic attraction, which follows an inverse square law and would create large
amounts of distortion in a transducer.
The electrostatic headphone is therefore the most linear of all the types available and with
its super-light diaphragm, weighing less than several millimeters of adjacent air, it is not
surprising that good headphones of this type can offer superb quality sound—the best
available. However, this essentially simple technique is the most complex to execute. Not
surprisingly, electrostatic headphones are the most costly to manufacture and buy. Figure 25.6
shows details of the Jecklin Float PS-2 type.
They are generally less sensitive than moving-coil types and are usually operated
directly from the amplifi er’s loudspeaker terminals. Due to the capacitive nature of the
electrostatic element and the complex inductive/capacitive nature of the transformer, they
tend to have more reactive impedance than other types, but this does not usually pose any
problem for good amplifi ers. Air ionization between the diaphragm and the plates limits
the maximum signal and polarizing voltages. Likewise, the signal voltage on the plates
must not exceed the polarizing voltage. These factors impose limitations on the maximum
sound pressure level that can be achieved.
25.3.5 Electrets
Basically, the electret headphone is an electrostatic type but using a material that
permanently retains electrostatic charge—the electrostatic equivalent of a permanent
magnet. The electret has the advantages of the conventional electrostatic, but does not
require an additional external power supply. It is similarly restricted in maximum sound
pressure level, although both types produce perfectly adequate sound pressure levels with
conventional power amplifi ers.