Explicit relations are derived between the acoustic pressure and a sound speed that is periodic. They are derived for the Pekeris waveguide, and extended to the range and depth-dependent adiabatic mode/WKB case. As expected, acoustic pressure is found to fluctuate at the sound speed fundamental and its overtones. A Pekeris model simulation qualitatively agrees with the intensity fluctuation spectrum from a range and depth-dependent parabolic equation (PE) simulation of a shelf edge experiment off the New Jersey coast. In this paper we show that the energy of the higher overtones can be significant and derive a modulation index that determines the factors that contribute to the overtone energy.