Testing an underwater acoustic source requires measuring its radiated sound field. The standard approach places hydrophones in the water — sensors at known positions that record the acoustic signal directly. This works but requires waterproof, calibrated equipment at every measurement point.
The paper proposes an alternative: record the sound field in the air above the tank. The water surface is an imperfect mirror for sound — some acoustic energy transmits through the surface into air, and the transmitted field carries information about the source below. If the relationship between the underwater field and the airborne field is stable, measuring in air replaces measuring in water.
In a reverberant tank — where reflections from walls create a complex standing-wave pattern — the transmitted airborne field is a smoothed version of the underwater field, with the surface acting as a spatial filter. The reverberation, which would seem to complicate the measurement, actually helps: it fills the tank with sound from all directions, making the surface transmission more uniform and the air-side measurement more representative of the source's total radiation.
The result validates the method: the airborne measurement produces stable, reproducible characterization of the underwater source. The surface that separates two media is not a barrier to measurement but a transformer — it maps one field to another with a well-defined transfer function. You don't need to be in the same medium as the source to characterize it. You need to understand the interface.