Figure 4.9 shows a typical, if simplified, circuit for an electrostatic microphone.
The capacitor part, known as the capsule, consists of the conductive diaphragm and the backplate. A d.c. supply, which is usually in the region of 50–100 V, provides a polarizing voltage on the capacitor. The charge on a capacitor is given by
Q = CV
where Q is the charge (in coulombs), C is the capacitance in farads and V is the voltage across the capacitor. The resistance R in the diagram is of extremely high value, a few hundred megohms, so that when the microphone is ‘switched on’ it takes an appreciable time (compared with the duration of sound wave cycles) for the capacitor to become fully charged. In short, ...