Measured capacitance of a condenser microphone as a function of diaphragm displacement

This paper shows that the capacitance of a polarized condenser microphone measured with a sinusoidal electrical signal is quantitatively related to the displacement of the microphone diaphragm. In fact, the capacitance measured with an ac signal is the "input capacitance" of the microphone when it acts as a transmitter. The expressions for the diaphragm displacement and the input capacitance are derived with damping taken into account. The input capacitance differs from the static capacitance with bias by an additional term which varies with the polarization voltage and frequency. Therefore, in general, it is impossible to measure the static capacitance with bias using a low frequency ac signal without the diaphragm being blocked. In order to measure the true static capacitance with bias, a static method based on the dc voltage distribution is presented. The experimental results for the input capacitance and the static capacitance with bias, obtained with the abovementioned ac and dc methods, respectively, give a high level of confidence in the theory. The experimental results reported by some others also support the theory.