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Listen: 841KB wav file(The recorded signal has been sped up 10 times) 

  NOAA Ship Ron Brown approaching hydrophones sound  

Example of "blade noise" from a large oceanographic vessel. The spectrograms show the acoustic signature of the NOAA Ship Ronald H. Brown as it approaches five different hydrophone sites in the equatorial Pacific Ocean. The spacing of the blade lines is generally consistent in each example. Acoustic signatures are routinely used by the military to identify ship characteristics, class, and often individual vessels.