Very high frequency underwater noise from breaking waves

In the absence of anthropogenic and biological sources and under wind-driven seas, underwater sound generated by breaking waves is the dominant source of high frequency ambient noise in the ocean. At frequencies above O (1000 Hz), this noise is generated by pulses of sound radiated by air bubbles entrained in actively breaking whitecaps. A history of measurements extending back more than 50 years has shown that the sound radiated by breaking waves is well-correlated with wind speed and extends in frequency to at least 20 kHz. Here, we present measurements of high frequency noise radiated by breaking laboratory waves. Focused wave packets in the Glass Wave Channel at the Hydraulics Laboratory at SIO generated breaking waves ranging in type from spilling to plunging, which were representative of small to medium scale waves on seas driven by 8–12 m/s winds. Measurements made with high frequency hydrophones (ITC 1089D) show that the noise spectrum from these waves extends beyond 400 kHz, challenging the assumption that noise above 100 kHz is dominated by thermal effects. Moreover, the slope of the noise spectrum is roughly −17 dB/decade, consistent with oceanic noise spectra at lower frequencies.