We investigated the acoustic signal of Er:YAG laser ablation in a gaseous environment. The high absorption coefficient of water at the laser wavelength of 2.94 micrometer leads to a small penetration depth of the Er:YAG laser pulses into tissue. The deposition of laser energy in a thin layer at the tissue surface causes a rapid evaporization of tissue water. The resulting tissue removal is used, for example, in laser skin resurfacing. The explosive evaporation of the tissue leads to an acoustic signal. We investigated the generation process of the signal caused by free-running laser pulses and its characteristic parameters in the time and frequency domain in correlation to the tissue and laser parameters with the aim to identify different tissues or tissue layers by analyzing the acoustic signal. Porcine skin and gelatin probes were ablated. Acoustic signals up to 1 MHz were measured using a condenser microphone and a piezoelectric airborne transducer. Schlieren photography was performed simultaneously to the acoustic investigations to visualize gaseous and condensed ablation products. We found that the high frequency content of the acoustic spectrum is due to shock waves created by each of the first laser spikes. Later in the laser pulse the acoustic signal is dominated by lower frequency components, because the generation of the high frequency components is inhibited by the interaction of the radiation with the ablation plume. The acoustic signature of free-running Er:YAG laser ablation seems to be particularly tissue specific during the first part of the laser pulse when the radiation interacts directly with the tissue.