We have identified a new hydrodynamic phenomenon in the laboratory that generates sound at frequencies so high that even bats and dolphins would be left struggling! We have observed an extremely high-frequency acoustic oscillations in an intense, femtosecond laser generated solid-plasma. The measured frequencies are close to the terahertz upper limit that can be sustained in such a medium, and around six million times the highest frequency audible to any mammal! The hot dense plasma rapidly expands into the surrounding vacuum. The oscillations arise due to the presence of a density gradient that becomes increasingly steep as one moves deeper into the fluid. This causes the dense plasma to accelerate into the stagnant lower density material, causing a pile-up. This pile-up grows and saturates, causing a repeat of the behaviour further upstream, and the generation of a periodic oscillation or sound wave.
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