Audio Engineering

738 Chapter 25

25.3.4 Electrostatic

The electrostatic headphone, like the electrodynamic, uses a thin plastics diaphragm, but
instead of a copper track it requires only to be treated to make it very slightly conductive
so that the surface can hold an electrostatic charge. It can consequently be very light.

The diaphragm ( Figure 25.5 ) is stretched under low mechanical tension between two
perforated conductive plates to which the audio signals are fed via a step-up transformer.

The central diaphragm is kept charged to a very high voltage with respect to the outer
plates using a special type of power supply, capable of delivering only a nonlethal, low
current, high voltage from the house mains, or, alternatively, by an energizer, which uses
some of the audio signal to charge the diaphragm to a similarly high but safe voltage.

The diaphragm experiences electrostatic attraction toward both outer plates. The spacing
between the plates and diaphragm, the voltage between them, and the tension on the

Step-up transformer

Input from

power amplifier

High voltage DC power supply

Light plastics

membrane

diaphragm with

conductive

coaling

Diaphragm

vibration

Acoustically

transparent

perforated

metal plates

Figure 25.5 : Electrostatic headphone. The transformer steps up the audio signal for feeding

to the outer metal plates. The central diaphragm is given a high DC charge with a power

supply. An audio signal causes the diaphragm to be attracted alternately to the outer plates.