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

For the first time a silicon microphone specially designed for measurements in turbulent gas flows has been fabricated and tested. The new design, based on surface-micromachining techniques, has a very small pressure-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 fluctuations of the smallest eddies in a turbulent flow. In order to achieve 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 an equalization of the static air pressure between the cavity and the ambient without degrading the dynamic pressure response of the microphone. The device has a very flat frequency-response curve within +/- 2 dB 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 fulfils the requirements for pressure measurements in turbulence. The microphone frequency response has been calculated using an electrical analogy. Comparisons with experimental data are presented.