Capacitor Microphone Reciprocity Calibration
Andrew C. Corney · Metrologia · 1975
Null balance a.c. bridge techniques substantially reduce the uncertainties in the electrical measurements associated with the closed coupler reciprocity calibration of capacitor microphones. Before any significant improvement in the overall accuracy of the calibration can be made, however, the uncertainties in establishing the acoustic transfer impedance of the coupler must also be reduced. To this end, a coupler of stable and easily measured dimensions has been constructed, and a refined technique for measuring the equivalent volume of a microphone has been developed. Systematic uncertainties in the calibration of a microphone at a frequency of 600 Hz have been reduced to ±0.013 dB (± 1.5 × 10-3), and random uncertainties to a standard deviation of 0.0027 dB (3.1 × 10-4). The new apparatus has been used to study the stability of five microphones, and results obtained for the first eight months are presented. With the exception of one microphone, which increased in sensitivity by 0.055 dB after slight damage, the largest change observed amounted to only 0.02 dB.