Signal Enhancement of Surface Scattered Underwater Sound.
NAVAL POSTGRADUATE SCHOOL MONTEREY CALIF
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The wind-driven surface of a large anechoic tank was used to study scattered underwater sound. Twenty kilohertz CW signals were transmitted by an omnidirectional source. Two directional receivers mounted together, were oriented to individually monitor the direct and surface-reflected signals. Several methods of signal addition by means of digital FFT analysis were used to increase average received power and reduce temporal fluctuations in the output. The most promising technique, when compared to the case of an omnidirectional receiver, increased the average signal power by as much as 12 db, while reducing the coefficient of power variation by as much as 50. The amount of improvement depends on the size and location of the receiver with respect to the surface interference pattern, and the acoustical roughness of the surface, g. Roughnesses in this experiment were approximately g 0.7 and 1.5, where g 4k squaredsigma squaredcossquaredtheta and k is the wave number, sigma is the rms surface wave height, and theta the angle of incidence and reflection.
- Acoustic Detection and Detectors