Bidirectional Electrostatic Microphone With Segmented Back-Plate

This study investigates the directivity of electrostatic (condenser) microphones featuring a circular membrane and a back-plate segmented into four parts. We analyze the relationship between the direction of arrival of incoming sound pressure waves and the non-axisymmetrical displacement field of the membrane using both analytical and numerical models. Since the output voltage of an electrostatic microphone is proportional to the mean value of the membrane displacement over the fixed electrode surface, the membrane displacement field influenced by the acoustic pressure field inside the transducer must be accurately calculated. By subtracting the output voltage signals from two opposite electrodes, we obtain two directional outputs with bidirectional directivity patterns perpendicular to each other. To validate our theoretical findings, we conduct measurements on an experimental transducer specimen. The experimental results, obtained using a scanning laser vibrometer to analyze the membrane displacement field and directivity measurements performed in an anechoic chamber, confirm our theoretical predictions. Bidirectional patterns at both outputs, which are perpendicular to each other, are observed in the frequency range corresponding to the passband of the transducer, except at very low frequencies where phase problems occur.