Acoustic wave fields in an ocean waveguide with a sediment layer having continuously varying density and sound speed overlying an elastic subbottom is considered in this analysis. The objective of this study is to investigate the effects of seabed acoustic properties,including the density and sound speed of sediment layer and subbottom, on the characteristics of the wave fields. This geometry offers a good environmental model which closely resembles a realistic ocean waveguide. This noise model was first proposed by Kuperman and Ingenito in the study of surface-generated ambient noise using normal mode approach.Recent experimental data provided by Hamilton have shown that the sediment layer in the seabed experiences a transitional change in which the density and the sound speed vary continuously from one value at the top to another at the bottom of the layer. Traditionally, in treating wave propagation in a such environment,the medium is represented by a series of layers,each of which has a uniform property within the layer.While this approximation may reasonably describe the variations of the medium as a whole,the details of the acoustic constituent may only be seen when these variations are properly accounted for. Moreover, the subbottom is taken to be a uniform elastic medium that is capable of supporting both compressional and shear waves. For the study of reflection from seabed, various kinds of sound speed and density profiles are employed.The wavenumber spectrum has clearly shown the various kinds wave components in the waveguide,in particular, the Scholte wave mode.The noise intensity in the water column is dominated by the modal and continuous spectrum.For the set of parameters chosen,the horizontal correlation lengths of the noise field tend to increase as the noise sources becomes more correlated, however, the vertical correlation tends to reduce. This indicates that the coherency of the noise field is controlled both by the noise sources and waveguide properties.

The goal of this research was to develop acoustic sensors and actuators for microfluidic applications. To this end, capacitive micromachined ultrasonic transducers (cMUTs) were developed which generate guided acoustic waves in fluid half-spaces and microchannels. An interdigital transducer structure and a phased excitation scheme were used to selectively excite guided acoustic modes which propagate in a single lateral direction. Analytical models were developed to predict the geometric dispersion of the acoustic modes and to determine the sensitivity of the modes to changes in material and geometric parameters. Coupled field finite element models were also developed to predict the effect of membrane spacing and phasing on mode generation and directionality. After designing the transducers, a surface micromachining process was developed which has a low processing temperature of 250C and has the potential for monolithically integrating cMUTs with CMOS electronics. The fabrication process makes extensive use of PECVD silicon nitride depositions for membrane formation and sealing. The fabricated interdigital cMUTs were placed in microfluidic channels and demonstrated to sense changes in fluid sound speed and flow rate using Scholte waves and other guided acoustic modes. The minimum detectable change in sound speed was 0.25m/s, and the minimum detectable change in flow rate was 1mL/min. The unique nature of the Scholte wave allowed for the measurement of fluid properties of a semi-infinite fluid using two transducers on a single substrate. Changes in water temperature, and thus sound speed, were measured and the minimum detectable change in temperature was found to be 0.1C. For fluid pumping, interdigital cMUTs were integrated into microchannels and excited with phase-shifted, continuous wave signals. Highly directional guided waves were generated which in turn generated acoustic streaming forces in the fluid. The acoustic streaming forces caused the fluid to be pumped in a single, electronically-controlled direction. For a power consumption of 43mW, a flow rate of 410nL/min was generated against a pressure of 3.4Pa; the thermodynamic efficiency was approximately 5x10-8%. Although the efficiency and pressure head are low, these transducers can be useful for precisely manipulating small amounts of fluid around microfluidic networks.