  Using a picosecond pump-probe ultrasonic technique, we study the propagation
of high-amplitude, laser-generated longitudinal coherent acoustic pulses in the
viscoelastic fragile glass former DC704. We observe an increase of almost ten
percent in acoustic pulse propagation speed of its leading shock front at the
highest optical pump fluence which is a result of the supersonic nature of
nonlinear propagation in the viscous medium. From our measurement we deduce the
nonlinear acoustic parameter of the glass former in the GHz frequency range
across the glass transition temperature.
