Underwater ambient noise, long an important area of study in underwater acoustics and acoustical oceanography, has recently entered the forefront of public awareness. 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 background 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 introduction of the subject to those involved in the analysis of human community noise, and motivate a useful information exchange. That said, we state up front that this article is neither comprehensive 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 background din emanating from a myriad of unidentified sources. Its distinguishing 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 understanding of the background, ambient noise conditions.
This article begins with a restatement of some important definitions relating to the use of decibel notation in both the air and underwater noise communities. Next, underwater ambient 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 variation in time, space and angle of arrival (angular intensity distribution). The major anthropogenic and natural constituents 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 compared with several examples of field measurements, and some historical trends in field measurements are mentioned.
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 exemplifies time variation and contributions from multiple sources, which is common 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 comparable physical units. The intent here is to provide some background to understand the effects of noise on marine mammals by comparing sound in air and water, and the ambient noise in the two environments.