Microphone calibration for the measurement of acoustic transfer functions

Calibrated microphones are necessary in acoustic measurements if quantitative results are required. However, a relative calibration of the microphones against each other is sufficient if transfer functions are to be measured. A measurement set-up in the form of a tee-junction for the relative calibration of two microphones was investigated. The frequency response of the set-up and the influence of the geometric parameters were calculated with an analytical one-dimensional model of the geometry. Numerical simulations of the full geometry, including an uncertainty estimation, were performed for comparison. The numerical simulations showed that spatially non-uniform input pressure distributions across the input port cross-section equalize rapidly along the input pipe. A spatially uniform input pressure is therefore not necessary and the microphone calibration is insensitive to the pressure distribution of the acoustic driver. The results of the analytical and numerical models and the measurement of two electret microphones are compared.