Relationships are developed between internal‐wave vertical velocity and acoustic phase. For source–receiver separations of many internal‐wave wavelengths, it is shown that the mapping of internal‐wave energy onto the phase of a single acoustic path is highly directional. Internal waves propagating perpendicular to the acoustic path produce the greatest phase modulation, and modulation of acoustic phase is reduced in all other geometries. For the omnidirectional internal‐wave spectrum proposed by Garrett and Munk, the acoustic phase‐modulation power spectrum is proportional to ω−3andj−3, where ω is the internal‐wave radian frequency andjis the internal‐wave vertical‐mode number. The first‐mode internal wave contributes more than half of the acoustic phase‐modulation power, and in this sense, the first‐mode internal wave dominates the acoustic phase spectrum.