Mechanical-Thermal Noise in Micromachined Acoustic and Vibration Sensors

Since the introduction of the micromachining process, wherein mechanical structures are etched from blocks of silicon, a number of very small acoustic and vibration sensors have been built. This size reduction is attractive for many applications but the small moving parts are especially susceptible to mechanical noise resulting from molecular agitation. For sensors designed for small-signal applications (microphones and hydrophones, for example), this mechanical-thermal noise is often one of the limiting noise components. While this component is often neglected in design and analysis, it is relatively easy to estimate, since, like electrical-thermal noise, the magnitude of mechanical-thermal noise depends only on temperature and the magnitude of mechanical damping. This paper reviews several techniques for calculating the mechanical-thermal noise in acoustic and vibration sensors in general and in micro-machined sensors in particular.