When a projectile enters water, it creates a cavity — a column of air that trails behind the descending body. The cavity elongates, narrows at the surface, and pinches off. The pinch-off disconnects the cavity from the atmosphere, and the trapped air pocket begins to oscillate. The oscillation radiates sound.
Bodily, Yousefian, and collaborators (arXiv 2602.22761, February 2026) combine experiments, simulations, and theory to characterize the acoustic signature of water entry. A cone-nosed cylinder drops into water. Hydrophones record the underwater sound. High-speed cameras capture the cavity dynamics. The question is whether the sound can be predicted from the impact parameters — whether the splash has a fingerprint.
It does. The dominant cavity oscillation frequency falls nearly linearly with the Froude number, which encodes the ratio of inertial to gravitational forces at impact. The frequency is governed by the projectile's nose geometry and its mass — not by the cavity volume alone. A free-floating bubble of the same volume would oscillate at a different, lower frequency. The rigid projectile core stiffens the oscillation, raising the frequency above what bubble theory predicts.
The sound unfolds in three stages. A weak acoustic pulse at the moment of initial contact. Then silence as the cavity grows. Then the main event: sustained pressure oscillations persisting for more than twenty cycles after pinch-off, as the trapped air pocket rings like a tuned cavity. The frequency of the ringing encodes the impact conditions.
The cavity is not a bubble. It is a bubble attached to a rigid body, and the rigid body constrains the mode shape. The boundary condition changes the resonance, and the resonance encodes the boundary. The splash sounds different from what its volume would predict because the projectile is still inside it.