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Report No. 6945 BBN Systems and Technology Corporation but continued to apply power for varying periods of time after being stopped. The tlrandom square wave" cannot depict the variety of power settings and periods of propeller cavitation that existed during the 14 minute period, but some generalities may be stated. The ship usually cavitated severely after changing from 'treverseq' to "forward ,If as the ship reversed direction, and it usually cavitated severely after being stopped on a 'forward run by the ice. Also, propeller sounds usually faded away during a propulsion change from one direction to another. In spite of these generalities, it is difficult to see any relationship at low frequencies between the ship activity or condition of cavitation and the sound level. At 20 Hz, it appears that the sound level decreased soon after the ship's forward progress was stopped each time. At the highest frequencies, the 3150 Hz third-octave band, the level was generally high while the ship was going forward and lower while the ship was in reverse. This observation is consistent with the theory that the ship propellers cavitate while the ship is accelerating forward to meet the ice, and that cavitation causes a general increase in level at higher frequencies. The effect does not appear to be present at 500 Hz or at the lower frequencies. In summary, in 14 minutes there were about five cycles of accelerating into the ice followed by backing away to try again. Clear relationships between ship activity and sound level were difficult to find, but we did not have records of the power settings on the ship. The variations in sound level were different in the third octave frequency bands between 20 and 3150 Hz. At the highest frequencies, the levels were higher during the accelerating phase when the ship ran ahead to hit the ice than during the backing phase in preparation for another run.
Report No. 6945 BBN Systems and Technology Corporation REFERENCES FOR APPENDIX B 1. Greene, C. R. , 1987. Acoustic studies of underwater noise and the localization of whale calls. In Responses of bowhead whales to an offshore drilling operation inthe Alaskan Beaufort Sea, Autmn 1986. Report by LGL Limited environmental research associates and Greeneridge Sciences, Inc., for Shell Western E&P Inc., 128 p. 2. Carter, G.C. and A.H. Nuttall. 1980. On the weighted overlapped segment averaging method for power spectral estimation. Proc. IEEE 68(10):1352- 1353. 3. Harris, J. 1978. On the use of windows for harmonic analysis with the discrete Fourier transform. Proc. IEEE 66:51-83. 4. Knudsen, V.O., R.S. Alford and J.W. Emling. 1948. Underwater ambient noise. J. Mar. Res. 3:410-429.
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H BOWERS BAW I ALEUTIAN ARC 3 STGEC
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A BEAUFORT SEA B U WCH SEA CHOPEaAS
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on the ice. The single calves remai
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A 0EAUFaAT SEA BCHJKCH 9 A CHaPE BA
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FIG. 3.2 NOISE SPECTRA IN SHALLOW W
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Adapted From K.H. Jacob (1986) Cali
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FIG. 3.8 ESTIMATED OVERALL SOUND LE
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FIG. 3. Q UNDERWATER NOISE SPECTRA
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FIG- 3.1 1 STATISTICAL ANALYSIS OF
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FIG- 3-12 STATISTICAL ANALYSIS OF C
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-- Alr Gun Array + Vlbrosels -* Ice
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FIG. 3.14 REPRESENTATIVE SHIPS AND
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~ -- -- FIG. 3.1 7 REPRESENTATIVE R
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FIG. 3-18 REPRESENTATIVE CULTURAL S
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FIG. 4.1 CORRECTION TO 300 M REFERE
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FIG. 4.1 1 AIR TO SHALLOW WATER SOU
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