A detailed study of the physical processes of pulsed high-pressure, heavy-current arcs is appropriate in view of the technological progress in the application of electrothermal launchers. The results of a Fourier analysis of pressure in the electrothermal launcher are presented. The propelling gas was evolved from an organic substance. The spectral analysis of pressure curves showed the presence of radial acoustic waves in the propelling gas in the discharge chamber. These acoustic waves are revealed in a Fourier spectrum by some frequencies that shift in the measured time. The shift of the frequency is defined as given through the input of electrical energy into the discharge. The sound velocity in the propelling gas was determined according to the frequency of the radial acoustic waves.