Instrumentation of in situ acoustical measurements of bubble spectra under breaking waves

A floating acoustical resonator has been developed to determine numbers and sizes of bubbles in the region of spilling breakers in the open sea. The change of Q of several modes of the one-dimensional resonator has been used to infer, simultaneously, bubble populations of nine radii between 30 and 270 mu m; smaller radii bubbles can also be studied. To demonstrate the accuracy of the technique, theoretical predictions of resonance broadening due to bubbles were compared with measured broadening for a know bubble population in the laboratory. Statistics of bubble densities under and near spilling breakers were then obtained at a depth of 25 cm below the ocean surface.