In this work waves formation in laser-produced plasma is investigated by an analysis of time-of-flight signal of the electron pulse. Electrons are extracted from a non-equilibrium plasma, generated by pulsed laser ablation on a solid Ge target. The process is represented by ion-acoustic waves, which are generated from an external perturbation, given by the positive bias voltage of a Faraday cup. The characteristics of the waves depend substantially on the geometry of the plasma expansion chamber and on laser fluence, while are independent on bias potential. A KrF excimer UV laser was employed for plasma generation. Measurements were performed at two different laser fluence, 4 J/cm2 and 7 J/cm2. The plasma created propagates with a mean velocity of about 1.1 ∙ 104 m/s. A movable Faraday cup was employed in order to collect electrons at different bias voltage values.