Calculations were performed using the normal‐mode solutions for the parabolic profile to determine the behavior of the acoustic field when either the axial or maximum profile sound speed was altered slightly from an unperturbed value. It was found that incrementing either the axial or maximum profile speed by small amounts produced a change in the focal length of the parabolic profile, which in turn caused a range‐dependent compression or expansion of the unperturbed acoustic field with no noticeable distortion for frequencies less than 400 Hz and range less than 550 km. Approximating the effect of the first‐mode semidiurnal internal tide by appropriate variations in axial velocity, this fact was used to show that for cw propagation between fixed source and fixed receiver, the fluctuation in acoustic amplitude produced by the internal tide can be obtained by sampling the amplitude function for the unperturbed profile a distance Δreither side of the range in question, where Δris the distance the unperturbed acoustic field is shifted due to the change in axial speed produced by the internal tide. For a moving source, a range‐dependent critical source speed was determined such that for source speeds equal to or less than the critical speed, the effect of the internal tide predominates as a source of acoustic fluctuations, while for source speeds greater than this critical speed the effect of source motion is paramount.