Transmission loss (TL) as a function of frequency, for fixed source and receiver positions, provides a means to investigate the influence of environmental mechanisms on broadband acoustic propagation. These curves exhibit an analogous behavior to cw loss curves as a function of range, with null-peak interference patterns observed in both experimental data and numerical simulation. An analytic approximation for the resulting patterns in a Pekeris type range-independent medium employing normal-mode theory is developed. Broadband signals are partitioned into bands for which accurate modeling can be achieved. Narrow-band approximations are then used to derive formulas for the null-spacings of the curves. These expressions connect the interference structure, through the modal group velocities and appropriate boundary conditions, to volume and seabed structure and composition. This work extends a previous development for a zeroth-order ocean [Badiey et al., J. Acoust. Soc. Am. 98, 2897(A) (1995)]. A qualitative assessment of the formulation is achieved by comparisons with both parabolic equation simulations and high-resolution geoacoustic/acoustic experimental data, for several different environments. The evolution of TL versus frequency patterns in range will be discussed in light of range-dependent environmental parameters.