Experimental Model Studies of Nonlinear Effects in Normal Mode Propagation

Nonlinear acoustic phenomena associated with parametric sonar are examined in the light of modal propagation in shallow seas. An analysis of the end-fired generation of difference frequency sound in this type of environment has identified several potential advantages and limitations of the parametric technique. Among the advantages is the selective narrow beam excitation of discrete modes with a resulting smoothing of the propagation curve. Among the limitations are the destructive interference effects brought about by phase reversals at boundary reflections within the zone of interaction. These problems have been studied in some experiments at ultrasonic frequencies in a large model tank. The results are interpreted with regard to theory developed for the application of parametric arrays to shallow water propagation.