Simulation und Entwurf eines Messrohres zur Mikrofonvermessung

Diploma thesis

In collaboration with the company NXP, the ideal dimensioning of a measurement tube is developed according to existing literature on wave propagation in tubes. The research goals are the measurement of the free field frequency response and the achievement of high sound pressure levels. The actual paramters of the tube, such as diameter and length, are considered as well as the selection of the appropriate loudspeaker to be used as sound source. First of all plane wave propagation is required to avoid annoying reflections. Further design issues for the measurement tube are reduced disturbances and compact extend. The limiting factors of the loudspeaker, such as maximum sound pressure level and membrane displacement, are taken into account. Methods are laid out in order to determine the ideal positions for the holes of the microphones. This conceptual design is developed into a scheme for measuring MEMS microphones.

The theoretical results are veryfied at an actual tube using cylindrical solutions of the wave equation. The effect of different tube endings are demonstrated. Several measurement at diverse microphone postitions demonstrate the limits of this measurement technique.