Page 25 - January 2007

UNDERWATER AMBIENT NOISE
Peter H. Dahl
Applied Physics Laboratory, University of Washington Seattle, Washington 98105
James H. Miller
Department of Ocean Engineering, University of Rhode Island Narragansett, Rhode Island 02882
Douglas H. Cato
Defence Science and Technology Organisation and University of Sydney Institute of Marine Science Sydney, NSW 2009, Australia
Rex K. Andrew
Applied Physics Laboratory, University of Washington Seattle, Washington 98105
Introduction
Underwater ambient noise, long an important area of study in underwater acoustics and acoustical oceanography, has recently entered the forefront of public aware- ness. A renewed emphasis on its study is driven in part by basic questions concerning the relation between anthropogenic noise and the ecology of marine mammals. For example, there is concern about the degree to which marine mammals are possibly habituating to, or otherwise being affected by increasing anthropogenic noise contributions. Natural ambient noise has always provided the back- ground noise limitation on the use of sound by marine mammals, but now ambient noise contains a significant anthropogenic component.
Here we provide a brief overview and perspective on the subject of underwater ambient noise, of interest to a diverse set of behavioral, biological and physical science professionals involved in its analysis. In addition, this article may also serve as an introduc- tion of the subject to those involved in the analysis of human community noise, and motivate a useful informa- tion exchange. That said, we state up front that this article is neither compre- hensive nor exhaustive on the topics presented.
We quote directly from the National Research Council’s 2003 report, Ocean Noise and Marine Mammals1 to define ambient noise as “The noise associated with the back- ground din emanating from a myriad
“Is there a convenient and physically correct way to compare measurements of sound in air and water? This question has caused
a number of interesting discussions involving air and underwater acousticians, journalists, and the general public.”
relating to the use of decibel notation in both the air and underwater noise communities. Next, underwater ambi- ent noise is described in terms of its spectrum, or frequency content. This is a useful and informative summary of underwater noise, but here we can only briefly allude to the noise field’s varia- tion in time, space and angle of arrival (angular intensity distribution). The major anthropogenic and natural con- stituents of the spectrum are itemized, and two spectra, corresponding to nominal high and low ambient noise levels, are introduced to illustrate the dynamic range of underwater ambient noise. These spectra are then com- pared with several examples of field measurements, and some historical trends in field measurements are men- tioned.
Finally, an interesting perspective is gained by a brief examination of ambient noise as we might experience it in air, including a familiar impact concept, speech intelligibility. For this, measurements of noise originating from highway traffic that also exempli- fies time variation and contributions from multiple sources, which is com- mon in underwater ambient noise, are used. Of course, discussion of sound in air and in an underwater environment has frequently led to confusion, so we tread carefully, using genuinely compa- rable physical units. The intent here is to provide some background to under- stand the effects of noise on marine mammals by comparing sound in air and water, and the ambient noise in the two environments.
of unidentified sources. Its distinguish- ing features are that it is due to multiple sources, individual sources are not identified (although the type of noise source—e.g., shipping, wind—may be known), and no one source dominates the received field.” This definition excludes the anthropogenic noise due to individual sources more localized in both time and space. Such sources are, for example, close shipping, sonars, seismic air guns, and pile driving and dredging devices. Suffice to say that an understanding of such noise sources including their potential impacts on marine mammals requires an under- standing of the background, ambient noise conditions.
This article begins with a restate- ment of some important definitions
Underwater Ambient Noise 23