Acoustic waves at the interface between a polymeric solid and a glass-forming liquid are studied through impulsive stimulated thermal scattering (ISTS), a photoacoustic method. The transverse character of the interface waves provides a unique window into the shear relaxation dynamics of supercooled liquids. Real and imaginary parts of the shear modulus of glycerol are extracted as functions of temperature and acoustic frequency. Extension of ISTS to interface waves permits characterization of shear dynamics even under conditions of very strong acoustic damping which preclude more conventional ultrasonic methods. (c) 2006 American Institute of Physics.