Development of an air bubble curtain to reduce underwater noise of percussive piling

Underwater bubbles can inhibit sound transmission through water due to dens ity mismatch and concomitant reflection and absorption of sound waves. For the present study, a perforated rubber hose was used to produce a bubble cu rtain, or screen, around pile-driving activity in 6-8-m depth waters of wes tern Hong Kong. The percussive hammer blow sounds of the pile driver we:re measured on 2 days at distances of 250, 500, and 1000 m; broadband pulse le vels were reduced by 3-5 dB by the bubble curtain. Sound intensities were m easured from 100 Hz to 25.6 kHz, and greatest sound reduction by the bubble curtain was evident from 400 to 6400 Hz. Indo-Pacific hump-backed dolphins (Sousa chinensis) occurred in the immediate area of the industrial activit y before and during pile driving, but with a lower abundance immediately af ter it. While hump-backed dolphins generally showed no overt behavioral cha nges with and without pile driving, their speeds of travel increased during pile driving, indicating that bubble screening did not eliminate all behav ioral responses to the loud noise. Because the bubble curtain effectively l owered sound levels within 1 km of the activity, the experiment and its app lication during construction represented a success, and this measure should be considered for other appropriate areas with high industrial noises and resident or migrating sound-sensitive animals. (C) 2000 Elsevier Science Lt d. All rights reserved.