Modelling distortion in condenser microphones

Preface In the scope of my studies at the Technical University of Denmark (DTU) in the Engineering Acoustics master program, I have done my final project about modelling distortion in condenser microphones. This subject was originally proposed by Brüel & Kjaer Sound & Vibration Measurement A/S (B&K) under the title Distortion process in condenser microphones. In February 2009 I have started my literature work at DTU for one and a half month. I have been able to gather and analyse about thirty documents related to my subject. In mid-march 2009 I moved to Brüel & Kjaer headquarters in Naerum in order to work on modelling the distortion. Starting with a very simple model, I have increased the complexity gradually in order to reach a model that gives satisfactory results. This project has been very interesting and rewarding. It gave me the opportunity to apply my theoretical knowledge from DTU and make a five months internship in a leading company in acoustic transducers. I would like to take this opportunity to thank my supervisors for their great help. In particular, I would like to thank Erling Sanderman Olesen for guiding me in my project and for all the time he spent answering my questions. Thanks to Erling Frederiksen for all the useful explanations he gave me. I would also like to thank all the others employees at Brüel & Kjaer for their welcome and the very pleasant and stimulating atmosphere. Finally, I would like to thank Sébastien Tardy for rereading my report and Aude-Aline Sastre for her patience and understanding. Summary Distortion in condenser microphones has different origins : electrical, acous-tical and mechanical. According to the preliminar litterature study, the electrical distortion is found dominant compared to the acoustical and mechanical distortions. The electrical distortion is produced by two phenomenon. First, the presence of passive capacitances in parallel to the active capaci-tance (which varies with the acoustic signal) introduces nonlinearity in the transduction principle. Then, the curved deflection profile of the diaphragm is responsible for an inhomogeneous repartition of the active capacitance. Many papers have already been written about the effect of stray (passive) capacitance but the influence of the deflection shape has not been studied into details yet. A better prediction of the membrane deflection profile is made possible by a new model developped during this study. This model takes the variations of the electrostatic force along the membrane into …