  We report ultrafast x-ray scattering experiments of the quasi-1D charge
density wave (CDW) material (TaSe$_4$)$_2$I following photoexcitation with
femtosecond infrared laser pulses. From the time-dependent diffraction signal
at the CDW sidebands we identify an amplitude mode derived primarily from the
transverse acoustic component of the CDW static distortion. The dynamics of
this acoustic amplitude mode are described well by a model of a displacive
excitation, which we interpret as mediated through a coupling to the optical
phonon component associated with the tetramerization of the Ta chains.
