Phonon-polariton dispersion is characterized in ferroelectric lithium tantalate and lithium niobate through femtosecond time-resolved impulsive stimulated Raman scattering (ISRS). An improvement in the ISRS setup permits optical heterodyne detection of the signals. In addition to substantially increasing the sensitivity and accuracy of the measurements, the phase sensitivity of heterodyne detection makes it possible to fully characterize the polariton wave after it has propagated outside of the excitation region. The detection of propagating responses with heterodyned ISRS is explored theoretically and experimentally. Discrepancies in earlier results reported for these materials are resolved.