A Q-switched frequency Nd:YAG laser was focused on copper, aluminum, and lead targets. The acoustic emission accompanying plasma formation was acquired and analyzed in both the time and the frequency domains. Spectral analysis of the shock wave has proved to be a simple and low-cost diagnostic of plasma phenomena. In the time domain, several propagation mechanisms of the shock wave were observed and the velocity profile of the shock wave estimated. Spectral measurements were performed in the acoustic propagation regime of the shock waves. Spectral features related to the plasma formation mechanism were identified and discussed for copper, aluminum, and lead on the basis of the physical properties of these elements, the expansion mechanisms of the plasma, and an empirical parameter representative of the transported energy.