Brian H. Houston

This paper discusses progress made on the design of an acoustic velocity sensor. An analytic model was developed for the frequency response of a slender cantilever rod forced by the pressure gradient and particle velocity associated with an acoustic wave propagating in a fluid. The model, validated with acoustic response measurements in air, was used to design cantilever sensors, which respond predominately to acoustic particle velocity. One such design utilizes a short cantilever formed from a 125 μm silica glass fiber immersed in a viscous fill fluid whose lateral tip displacement is detected using a multi-fiber optical probe. This velocity sensor is predicted to be able to detect fairly low acoustic sound levels in water. Progress has been made in instrumenting a large pool at NRL to allow accurate propagating acoustic wave response measurements in water of these new velocity sensors down to frequencies below 5 Hz. Measurements made in this facility on various cantilever sensors will be presented and discussed. [Work supported by ONR.]

A human surrogate was recently developed to emulate the acoustic transmission path of a human ear. This physical model was used to study soldier exposure to impulsive small weapons fire. The ear response of the surrogate and human data are in very good agreement. The effect of the helmet and body armor equipment on a soldier's hearing was determined in this report through laboratory and firing range measurements. A sound pressure level (SPL) reduction of 2 to 3 dB on both left and right inner ears of the surrogate was found when a helmet was worn. This reduction was probably due to the helmet blocking the direct acoustic path from the weapon muzzle to the ear. Body armor appears to have very little effect on the sound reaching the inner ears. This implies that reflections from the torso are a second order effect when compared to the direct acoustic path. The physics of the direct acoustic path from the weapon muzzle to the ears appears to be the most important acoustic mechanism affecting the inner ear SPL levels. The surrogate developed here may serve as a standard for human hearing evaluation of infantry combat equipment and weapons systems deemed relevant to the warfighter.

We have constructed and successfully laboratory tested a device that can periodically change from a submerged to a buoyant state using gas generated by microbes alone. The duration of the buoyant state and the switching frequency from the submerged to buoyant state can be controlled. If the type of microbes used is native to the deployment location, gas generation will not create an identifiable acoustic signature disclose the presence and operation of the device. In this report, we discuss the operation and testing of the device, the microbes that we use to test the operation of the device, and the test results.

We present a design approach that enables monolithic integration of high-quality-factor (Q) radio-frequency (RF) microelectromechanical systems (MEMS) resonators with CMOS electronics. Commercially available CMOS processes that feature two polysilicon layers and field oxide isolation can be used to implement this approach. By using a nonplanar resonator geometry in conjunction with mechanical stress in polycrystalline silicon (poly) gate layers, we create rigid and robust mechanical structures with efficient electromechanical transduction. We demonstrate polysilicon domes with capacitive pickup and arch-bridge resonators with piezoresistive readout. The small footprint of our MEMS structures enables on-chip integration of large arrays of resonators for RF signal processing or sensing applications. Their large surface-to-volume ratio in combination with high rigidity (that alleviates stiction associated with wet chemistry processing) can make these resonators particularly useful for sensors that require surface functionalization.