Abstract
Underwater and in-air recordings were made from a boat anchored near Prudhoe Bay, Alaska, while a Griffon 2000TD hovercraft drove by at or near full power on four passes. At the closest point of approach (CPA, 6.5m), underwater broadband (10–10000Hz) levels reached 133 and 131dB re: 1μPa at depths of 1 and 7m, respectively. In-air unweighted and A-weighted broadband (10–10000Hz) levels reached 104 and 97dB re: 20μPa, respectively. The hovercraft produced sound at a wide range of frequencies. Both underwater and in air, the largest spectral peak was near 87Hz, which corresponded to the blade rate of the thrust propeller. In addition, the spectral composition included several harmonics of this frequency. The shaft or blade rate of the lift fan was barely detectable underwater despite its proximity to the water. The hovercraft was considerably quieter underwater than similar-sized conventional vessels and may be an attractive alternative when there is concern over underwater sounds.
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