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In high‐pressure discharges, arc instabilities may occur due to acoustic resonances inside the discharge vessel. We have observed experimentally the presence of longitudinal acoustic resonances in a high‐pressure sodium discharge well below the threshold above which the arc is unstable. A dye laser, tuned to about 17 140 cm-1, proved to be able to excite the acoustic standing waves, and their galvanic effects have been measured. A local thermodynamic equilibrium discharge model is presented, which qualitatively describes the measurements. It is shown that the instability threshold is correlated to an acoustic pressure amplitude in the ventral segments of about 6 kPa.