Sound generation and air entrainment by breaking waves in the surf zone

This paper presents the results of acoustic and optical measurements of individual breaking waves in the surf zone. Two hydrophones, horizontally separated and deployed in 2 m of water off La Jolla Shores beach, California, were used to measure the source spectrum of breaking surf, and characterize propagation through the surf zone over length scales of order 10 m. The acoustic data show an acoustically active region within a wave which propagates shoreward. The production of sound within the active region is associated with the formation of bubble plumes. Above 500 Hz, the sound is consistent with the radiation from individual bubble oscillations. Pictures were taken of the bubble plumes formed beneath the breaking surf, providing estimates of the plume size, and bubble size distribution and void fraction within a plume. The density of bubbles scales as $a^{\text{−2.5}}$ for $\text{a<1\hspace{0.17em}}\mathrm{mm}$ and $a^{\text{−4.5}}$ for $\text{a>1\hspace{0.17em}}\mathrm{mm},$ where $a$ is bubble radius, and total void fractions of 0.3–0.4 were measured. Theoretical calculations show that radiation from bubbles within a plume is strongly damped, and only a thin shell of bubbles on the exterior of the plume contributes to the wave noise. A “moving bubble sheet” source model for the wave noise is presented which shows fair agreement with the acoustic measurements.