Our spreading experiment results and ship receive levels are suggesting that different ships within a single ship class put out different amounts of high-frequency noise (at the same range from our hydrophone and the same speed over ground). Could this be a manifestation of cavitation within many or all classes of ships?
A clue could come from experimental (e.g. laboratory) measurement of cavitation noise. How loud is cavitation from propellers at the standard range of 1 meter?
Thus my literature search began. (Blake, Wolpert, Geib, 1977) show some interesting figures of source spectra for cavitation that have minima near 6 kHz and increasing levels not only at higher frequencies (up to 80kHz) but also at lower frequencies (down to 1 kHz where levels are actually above those at 80kHz). The figure notations in (Kipple, 2002) suggest that cavitation noise primarily occurs at frequencies above 10-20kHz.
They also show a plot of spectral density of cavitation noise at 31.5 kHz that could be compared to single ship spectrum presented by (Hildebrand, 2006), as well ours… Hildebrand computed the 31.5 kHz source spectrum level of the Hanjin to be ~110 dB. The corresponding Blake levels range from 75-100 dB, suggesting that the Hanjin is *really* cavitating!