Abstract: An acoustic field in a body of water is monitored using a coherent light field emitter applying a distributed light field across the surface of the water to be reflected, and a sensor is used to sense reflected components of the light field above the surface and to provide a signal representing information in the reflected light and related to movements in the water and caused by the acoustic field. The signal is provided from an interferometry technique and useable to derive information on the underwater acoustic field in a useful form.

Abstract: An acoustic field in a body of water is monitored using a coherent light field emitter applying a distributed light field across the surface of the water to be reflected, and a sensor is used to sense reflected components of the light field above the surface and to provide a signal representing information in the reflected light and related to movements in the water and caused by the acoustic field. The signal is provided from an interferometry technique and useable to derive information on the underwater acoustic field in a useful form.

Abstract: A liquid flow meter for directly measuring the velocity of a liquid is disclosed. The liquid flow meter includes a pair transducers arranged facing each other in a conduit through which the liquid flows. The liquid flow meter also includes a transmitter means for causing the transducers to simultaneously transmit an acoustic wave packet directed for reception at the other transducer. A differential receiver means is also included whereby the differential receiver means has inputs each coupled to a corresponding one of the transducers for detecting an acoustic signal received thereby and determining a difference between the two received signals. The difference being related to the velocity of the liquid within the conduit. The transmitter means and the differential receiver means are each matched to the transducers to ensure substantial reciprocity to thereby substantially avoid phase and/or amplitude variations in the received signal. A method for measuring the velocity of a liquid is also disclosed.