| Samples of ambient ocean noise, 0.05-30 Hz, from the Wake Island Hydrophone Array are compared to measured local winds and estimated local ocean waves. In addition, continuous noise data during the passage of a typhoon directly over the array, and during a 41-day period are spectrally analyzed in fine detail. The noise is divided into six frequency bands, based upon its characteristics. From 0.05 to 0.1 Hz, a region of the ocean noise spectrum with extremely low levels, the Wake data are limited by system noise. However, Rayleigh waves from moderately-sized earthquakes are frequently observed in this band, and the primary pressure signal from local ocean swell is also observed on a hydrophone at 850m depth. Between 0.1 and 0.2 Hz, noise appears to be caused by double-frequency pressure fluctuations from local ocean swell, as predicted by nonlinear wave interaction theory. During periods of large swell, levels of this noise are the maximum in the spectrum. More commonly, however, peak spectral levels are found between 0.2 and 0.3 Hz. Noise in this band correlates less strongly with estimated local ocean waves, and it may have a more distant origin with conversion to Rayleigh wave-type propagation. From 0.3 to 1.5 Hz, noise correlates strongly with both wind and waves, indicating its source is the local wind waves. The frequency correspondence between this noise and the estimated ocean waves, however, is between 5:1 and 10:1, a puzzling result. Between 1.5 and 6 Hz, noise levels increase with wind speed to a clearly defined saturation level probably corresponding to the known saturation of short wavelength ocean wind waves. Between 2 and 5 Hz, noise levels are saturated about 80% of the time. This saturated noise is probably a constant in all the world's oceans, and is called the holu spectrum from the Hawaiian word for deep ocean. From 4 to 30 Hz, noise levels remain constant until wind speeds exceed about 8 m/s, suggesting this noise may be from whitecaps. Levels in this band grow unbounded, and during the typhoon increased by more than 30 dB. |
