Extending bubble measuremenets below 20 μm: in memory of Ralph Goodman’s contributions.

No matter which aspect of underwater acoustics we worked on, conversations with Ralph Goodman invariably added insight and often motivated new lines of investigation. This was certainly true of our interest in measuring bubble size distributions. We discuss the extension of acoustical measurement of bubbles to radii below 20 μm, a topic of particular relevance to the study of optical scatter in the ocean. Extending the frequency range of a resonator to 1 Mhz would in principle allow measurement down to ∼3 μm, but significant challenges arise due to the confounding effects of geometric scatter from larger bubbles. This problem becomes greater for measurements of smaller bubbles, requiring a careful analysis of the accuracy limits imposed on the inversion procedure. Preliminary steps along this path are discussed with examples drawn from recent field measurements.