Figure 7. Sound pressure level produced from winch-generated noise measured over ambient conditions and its attenuation with distance from the point source in Cook Inlet, Alaska

ERDC TN-DOER-E14

August 2001

a. Ambient conditions

b. Attenuation with distance from the point source

Figure 7. Sound pressure level produced from winch-generated noise measured over ambient conditions

and its attenuation with distance from the point source in Cook Inlet, Alaska

frequency of 82.10 Hz, or 35.4 dB re 1 Pa-m above peak ambient conditions (Figure 8). This

dredging event was generally quiet, producing slightly more intense sounds than the bucket-closing

event, which was less than 10 dB lower at 99.25 dB re 1 Pa-m. SPL measurements for other

dredging events at this distance were higher by 5 dB re 1 Pa-m for bucket digging and 15.4 dB re

1 Pa-m for bottom striking. Figure 8 illustrates a rapid attenuation of noise for this event, falling

to near-ambient conditions beyond 1,200 m from the source.

Emptying Barge at Placement Site: Whereas the preceeding events are repetitive on a time

scale of minutes during active dredging, emptying the barge occurs much less frequently, generally

at intervals of several hours. Underwater sound data were collected while drifting near the

placement site, which was several km from the dredging site. At the time when the split-hull barge

opened to release the dredged material, the distance to the source was measured to be 316 m. The

placement event lasted approximately 60 sec. Figure 9 illustrates the sounds generated by the

barge-emptying event compared with ambient conditions. Peak SPL (dB rms) was measured at 108.7 dB

re 1 Pa-m at 45.8 Hz. From 20 to 1,000 Hz, lowest SPL was measured at 96 dB re 1 Pa-m. Noise

levels ranged from 22.8 to 34.8 dB re 1 Pa-m above peak ambient conditions.

Bucket Dredging in Unconsolidated Sediments: The dredge Crystal Gayle was performing

maintenance dredging operations near the port facility in an area characterized by soft sediments.

Peak SPL (dB rms) for a bottom strike event in soft sediment was measured at 107 dB re 1 Pa-m

at 91.5 Hz. At a range of 555 m, peak noise levels were 11.6 dB re 1 Pa-m quieter than comparable