An active bubble curtain surrounds an offshore pile driving installation.
An active bubble curtain surrounds an offshore pile driving installation. Image courtesy of Georg Nehls.
Close-up view of a bubble curtain in operation.  One can see the streams of bubbles exiting the hose of the bubble curtain.
Close-up view of a bubble curtain in operation. One can see the streams of bubbles exiting the hose of the bubble curtain. Image courtesy of Georg Nehls.

Click either choice below to hear pile driving activity before and after a bubble curtain system is activated. Please note, this sound recording demonstrates the effectiveness of this mitigation option and is not meant to demonstrate how it is actually done (e.g., the bubble curtain would already be up and operational before any piling started in a real-world scenario):
 
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Sound recording of 36 inch diameter steel pipe (1” wall thickness) being driven by a diesel impact hammer at a water depth of 7.5m. Approximately midway through the recording, a dual-ring bubble curtain (400 cubic ft per minute flow rate, 75 psi air pressure) is activated. One should be able to hear a difference in the pile-driving sounds before and after the bubble curtain is activated. The sound were recorded by a hydrophone in 20m of water, approximately 1m above the seabed.
Sound credit JASCO Applied Sciences, www.jasco.com; released under Creative Commons Attribution, Non-Commercial.
Description
Bubble curtains can be deployed to reduce or mitigate impacts from high noise levels produced by pile-driving. Air streaming from closely spaced release points creates a “wall” of bubbles around the pile. Because air and water have a substantial impedance mismatch, the bubble curtain acts as a reflector. The bubbles also resonate in response to sound and absorb sound energy. Up to a 30 dB reduction in sound has been measured when bubble curtains are deployed with pile driving.
 
References

  • Lucke et al. 2011, "The use of an air bubble curtain to reduce the received sound levels for harbor porpoises (Phocoena phocoena)." J. Acoust. Soc. Am. 130 (5): 3406–3412. 
  • Reyff, J. 2009, "Reducing Underwater Sounds with Air Bubble Curtains." TR News. Transportation Board of the National Academies. 262: 31-33. 
  • Wursig, B., C. R. Greene, Jr., T. A. Jefferson. 1999, "Development of an Air Bubble Curtain to Reduce Underwater Noise of Percussive Piling." Marine Mammal Research, 49:79–93. 
Additional Resources

  • Reyff, J. A. 2003, "Underwater Sound Levels Associated with Construction of the Benicia-Martinez Bridge: Acoustical Evaluation of an Unconfined Air-Bubble Curtain System at Pier 13." Illingworth & Rodkin, Inc., Petaluma, California.