A SMALL-SIZE SILICON MICROPHONE FOR MEASUREMENTS IN TURBULENT GAS-FLOWS

For the first time a silicon microphone specially designed for measure ments in turbulent gas flows has been fabricated and tested. The new d esign, based on surface-micromachining techniques, has a very small pr essure-sensitive polysilicon diaphragm of 100 mum side length and 0.4 mum thickness with polysilicon piezoresistive strain gauges. The small diaphragm makes it possible to resolve and measure the pressure fluct uations of the smallest eddies in a turbulent flow. In order to achiev e a sufficiently high acoustic pressure sensitivity, a relatively deep (3 mum) cavity is formed below the diaphragm by using the sacrificial -layer etching technique. A special vent channel is designed to give a n equalization of the static air pressure between the cavity and the a mbient without degrading the dynamic pressure response of the micropho ne. The device has a very flat frequency-response curve within +/- 2 d B between 10 Hz and 10 kHz and an acoustic sensitivity of 0.9 muV Pa-1 for a supply voltage of 10 V. It has been shown that the new sensor f ulfils the requirements for pressure measurements in turbulence. The m icrophone frequency response has been calculated using an electrical a nalogy. Comparisons with experimental data are presented.