Instrumentation for <i>i</i> <i>n</i> <i>s</i> <i>i</i> <i>t</i> <i>u</i> acoustical measurements of bubble spectra under breaking waves

Abstract

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 μm; 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 known 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. These ocean data compare very well with recently published laser measurements in a water–wind tunnel over radius range 30&amp;lt;a&amp;lt;270 μm, but agree with older photographically obtained values at sea only for radii 50&amp;lt;a&amp;lt;100 μm.