The optimum frequency for acoustic propagation in shallow water is controlled by a number of physical effects and environmental parameters. This article concentrates on the effect of a nonlinear frequency dependence of the sea‐bottom attenuation on the optimum frequency. Experimental data on low‐frequency acoustic propagation in shallow water are presented, for which, over the frequency range where the optimum frequency should occur, no apparent optimum frequency is observed. If there is an optimum frequency for these sea areas, it is much lower than predicted by existing theories. It is demonstrated that the nonoccurrence, or lowering, of the optimum frequency can be easily explained if the sea bottom is assumed to have a nonlinear frequency dependence. Excellent agreement between theory and experiment is achieved for a site in the Yellow Sea for three different seasons using values for sediment attenuation (with nonlinear frequency dependence) and sediment sound speed which were measured independently at the same site [J. X. Zhou, J. Acoust. Soc. Am. 78, 1003–1009 (1985)]. The sea‐bottom attenuation used for the calculations was proportional to frequency to about the 1.8 power.

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