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

Report No. 6945
BBN Systems and Technology Corporation
third-octave bands. The spectrum in Figure B.1B was begun at time 13:23:26,
when the ship had been pushing ahead but reversed pitch to back away from the
ice. This spectrum contained the highest third octave levels at 20 and 31.5
Hz, and the lowest third octave levels at the highest frequencies.
Figure B.2 contains five spectra composed by sorting the individual
frequency analysis cells over the 53 spectra and determining the minimum,
maximum, and the 5th, 50th (median) and 95th percentile levels. In one point
of view, this figure presents five points on the cumulative distribution of
spectrum levels for each frequency cell from 20 to 4000 Hz. Somewhat
surprisingly, there is relatively little variation between the lowest and
highest spectrum levels at low frequencies (about 20 dB). The sounds at those
frequencies come mostly from the ship's machinery and propellers. Although
the machinery operating speed is constant for all phases of the icebreaking,
the power generated varies substantially. At the high frequencies, we expect
considerable spread in the levels, as is shown, because of the effects of
cavitation. The propellers cavitate most severely during the "bollardft
condition and when changing direction from going astern to goind ahead. When
changing the power setting from reverse to forward, the propellers pass
through a condition where there is no cavitation at all. For comparison, the
idealized spectrum for Knudsen et al. (1948) Sea State 6 is shown at the
bottom of the figure.
Table B.l presents the cumulative distribution information for each of
the third-octave frequency bands. Consistent with the effect seen in the
statistical spectra, the span of levels at low frequencies is only 15 dB at 20
Hz, 10 dB at 31.5 Hz, but at high frequencies it is 38 dB at 3150 Hz.
Figure B.3 presents the variation in sound level vs. time for four third-
octave bands: 20, 50, 500, and 3150 Hz. During the 14 minutes recorded and
analyzed, the ship went through about five cycles of "backing and rammingft a
heavy ice floe in attempting to break it up. To depict these cycles, at the
bottom of each graph we have drawn a tlrandom square wave" representing the
times the icebreaker was going forward and in reverse. Also shown are the
instants of time when the ship's forward progress was seen to be stopped by
the ice. The ship did not go into reverse immediately after being stopped,