The static pressure and temperature coefficients of laboratory standard microphones

The sensitivity of condenser measurement microphones depends on the environmental conditions, which affect the acoustic properties of the air enclosed between the diaphragm and the back electrode and in the cavity behind the back electrode. A theoretical investigation has been performed based on an extended lumped parameter representation of the mechanical and acoustic elements of the microphone. The extension involves the frequency dependency of the dynamic diaphragm mass and stiffness as well as a first-order approximation of resonances in the back cavity. It was found that each coefficient, for a given type of microphone, can be described by a single function when the coefficients are normalized using their low-frequency value and the frequency is normalized with respect to the individual resonance frequency of the microphone. The theoretical results are supported by experimentally determined coefficients for about twenty samples of microphone types B&K 4160 and B&K 4180.