Contribution of cross talk to the uncertainty of microphone calibrations by the electrostatic actuator method

Abstract

Cross talk in electrostatic actuator calibrations is defined as the response of a test microphone to the actuator excitation voltage when the microphone membrane is frozen, i.e., when the actuator polarization voltage is zero—an unavoidable nuisance effect that contributes to the calibration uncertainty. Two models for the cross talk are considered. In the first, the actuator excitation voltage is coupled to the microphone output through the stray capacitance between the actuator electrode and the microphone backplate; in the second, it is coupled through the ground resistance, which provides a common element between the input and output loops. In the stray capacitance model, at frequencies below the membrane resonance, the cross talk is independent of frequency, but in the ground resistance model it varies inversely with frequency. The frequency dependence of the cross talk thus serves as a discriminator between the two models. Measurements on condenser microphones of various sizes yield results that favor the stray capacitance model. The ratio of cross-talk signal to actuator pressure signal (at 94 dB re: 20 mPa) is found experimentally to be −74 dB for a 1-in. condenser microphone and nearly the same for 1/2-in. and 1/4-in. microphones.