A foil electret transducer for use under varying static pressure

Abstract

We have built a foil electret microphone for use under‐the‐cuff in an automatic blood pressure monitoring system. The microphone uses a 50‐μ‐thick FEP diaphragm charged to − 250 V and suspended 35 μ above a back electrode. A flat sensitivity over a pressure range of 40 to 250 mm Hg (5.3 × 104 to 3.3 × 105 dyn/cm2) is required for this transducer. The sensitivity is maintained at a constant level by allowing the first contact between the electret and the back electrode to occur at 20 mm Hg. Although the microphone senistivity changes quickly from 0 to 20 mm Hg, it is relatively flat in the pressure range of interest. This design technique canbe used to provide a flat sensitivity for any foil electret microphone subjected to a varying static pressure.