During the Yellow Shark '95 experiment the broadband frequency response (200--800 Hz) of a shallow-water waveguide was measured for the purposes of performing multimodal inversion of its acoustic and geoacoustic properties. Measurements from vertical receiving arrays deployed every 8 km along a 40-km tomography transect showed that the frequency response was mostly flat with high transmission loss at selected frequencies. This differs from often observed frequency dependence in other shallow-water environments where there is some low frequency with minimum loss, suggesting an optimum frequency of propagation. Range-dependent geoacoustic and seismic data along the transect and water sound-speed data measured during the acoustic transmissions permitted the development of a realistic environmental model. Normal-mode modeling results with C-SNAP indicated that the presence of a soft clay top layer with a speed less than the water column was a decisive factor in the observed frequency dependence, since it acted as an alternative waveguide to lower-order modes. The modeled frequency dependence and absolute level of transmission loss closely agreed with the measurements obtained at the different ranges and depths, resulting in a validation of theoretical arguments involving selected frequencies of high transmission loss as a major feature of this type of environment.