Analysis and Ranking of the Acoustic Disturbance Potential of ...

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.