In recent years several papers have been written on the existence and nature of the so‐called “optimum frequency” (Fopt) for acoustic propagation in shallow water. These papers characterize the effects of various basic physical and environmental parameters that influence Fopt (such as water depth, sound velocity profile, sediment type, the depth and separation of the source‐receiver pair, sediment sound‐speed gradient, shear wave couple loss, etc.), but do not consider the frequency and the depth dependence of seabottom sound attenuation. In this paper, some experimental data on low‐frequency response (30–2000 Hz) in shallow water are presented. The interesting result is that over the frequency range where Fopt should occur according to the existing theories, no apparent optimum frequency is observed. In order to explain these experimental results, this paper uses a normal mode computer program to examine the frequency dependence of sound propagation for various canonical models of seabottom structure in shallow‐water regions (including thin top surface transition layer with strong positive sound‐speed gradient of 1000 s−1 and negative attenuation gradient) allowing for the possibility that attenuation in the sediments may have a nonlinear frequency dependence. The results show that the experimental low‐frequency responses, including the disappearance of the optimum frequency, are consistent with the earlier results of J. X. Zhou [J. Acoust. Soc. Am. 78, 1003–1009 (1985)], i.e., that bottom attenuation in the low‐frequency range has frequency power law of fn, with values of n ranging from 1.5 to 2.0, although the results could also be explained to some degree by a seabed top transition layer.