An optoacoustic experiment was set up to study generation, propagation, and damping of broadband acoustic pulses in the MHz region. The sound pulse front is detected interferometrically. The analysis is based on linear hydrodynamic theory and comprises diffraction of the sound pulse. To introduce relaxational effects, it was extended by splitting the specific heat into a "fast" and a "slow" part. Thus, a good description of the observed pulse shapes and speeds could be obtained. Results for the glass forming 1-methyl-3-ethyl-imidazoliumchloride-AlCl3 mixture at three concentrations and for temperatures between -80 degrees C and +20 degrees C are reported.