Abstract:

A flat frequency response, reasonably good signal-to-noise ratio, mechanical robustness, and low cost make omnidirectional electret microphones ideal for telecommunications applications. Differential microphones offer the potential additional advantage of improving the acoustic signal-to-noise ratio by rejecting unwanted background noise. Background noise rejection results from two independent acoustic mechanisms---directional selectivity and proximity. Modeling the acoustic performance of a differential microphone requires accurately representing these mechanisms, which means properly accounting for diffraction and damping effects. For these reasons, differential microphones are more sensitive to mounting conditions and housing geometry, and therefore, designing a differential microphone into a product can be a daunting task. This paper discusses some of the design principles that dictate noise rejection, and suggests methods for modeling differential microphones using numerical techniques such as the boundary element method (BEM). A coupled BEM model enables an acoustical interface between the microphone diaphragm and nearby cavities and ports. This is especially important near resonance. Damping plays a key role in microphone performance, and is an overriding factor in the directional behavior of a differential microphone. Techniques for representing these effects in a BEM model using SYSNOISE will be discussed. [SYSNOISE is a registered trademark of LMS Numerical Technologies, Belgium.]