Study of Wave Breaking Through Spectral Analysis of the Dissipated Sound Energy

This paper presents the first results of the spectral analysis of airborne and underwater sound produced by breaking waves, with the long-term objective to improve breaking wave detectability. The size of an air bubble can be directly linked to the frequency of the sound that is heard using the simple harmonic solution to the Rayleigh-Plesset equation. It indicates the inverse relationship between frequency and bubble size. This relationship has been used successfully to identify wave breaking in general by Manasseh in 2006 [4]. Now this research goes a step farther and examines how the frequency spectrum of the sound changes with time, in an effort to understand the general pattern and from that to deduce an empirical equation that describes the breaking down to turbulence during a wave breaking event. At this point there have been three main indicators identified that are necessary to prove wave breaking when analysing recorded sound: (1) higher frequencies get more pronounced as time passes, (2) amplitude decreases with increasing frequency, and (3) there is a sinusoidal pattern to how the power is distributed throughout the frequencies. This last point is the one that this research focusses on most. It can be concluded from the experimental data that the sinusoidal pattern is most likely due to the probability of how bubbles break down. This probability function depends on the physical properties of the medium the wave is travelling through, or in the case of ocean waves it depends on the properties of the water and air.