ASA 127th Meeting M.I.T. 1994 June 6-10

3pUW6. The propagation of very-high-frequency sound in a strongly ducted waveguide.

M. F. Werby

Code 7181, Naval Res. Lab., Stennis Space Center, MS 39529

The method used in the normal mode code SWAMP based on a new mathematical scheme enables one to obtain accurate answers for very high frequencies. To date 1000 modes have been calculated for a strongly ducted waveguide obtained from a Medd. profile with a typical fluid bottom. The water column here is 80 m and calculations have been conducted to 25 kHz. With increasing frequency, strong refraction dominated and the relevant modes are confined by and large in the ducted regions. The higher modes, which produce a phase averaging effect, result in a relatively weak signal where they are dominant and strong bending of waves is rather dramatic with increasing frequency. On the other hand, when a source is placed in the nonducted region, a paradoxical result occurs in which the signal appears trapped in the nonducted region because the modes relevant in the ducted region can not be stimulated from a source outside the ducted region at very high frequency. The fact that the modes contributing in the nonducted region are very high angle--as expected---is evident in the two-dimensional plots presented. Many interesting results will be illustrated graphically over a variation in frequency. [Work sponsored by the Naval Research Laboratory 6.1 Program Element 0601153N.]