Modelling of silicon condenser microphones

Several models concerning the sensitivity of capacitive pressure sensors have been presented in the past. Modelling of condenser microphones, which can be considered to be a special type of capacitive pressure sensor, usually requires a more complicated analysis of the sensitivity, because they have a strong electric field in the air gap. It is found that the mechanical sensitivity of condenser microphones with a circular diaphragm, either with a large initial tension or without any initial tension, increases with increasing bias voltage (and the corresponding static deflection), whereas the mechanical sensitivity of other capacitive pressure sensors does not depend on the static deflection. It is also found that the mechanical sensitivity increases with increasing input capacitance of a preamplifier. In addition, the open-circuit electrical sensitivity and, consequently, the total sensitivity too, also increases with increasing bias voltage (or static deflection). However, the maximum allowable sound pressure at which the diaphragm collapses, an effect that has to be taken into account, decreases with increasing static deflection in most cases, ulthnately resulting in an optimum value for the bias voltage. The model for microphones with a circular highly tensioned diaphragm has been verified successfully for two microphone types.