Devices tease out individual sounds from underwater racket

While biologists sort out what levels of noise go unnoticed, are annoying or cause harm to marine mammals, physical oceanographer Jeff Nystuen is giving scientists and managers a way to sift through and identify the sounds present in various marine ecosystems.

Nystuen, from the University of Washington’s Applied Physics Laboratory, presented his latest findings this week at the Ocean Sciences Meeting in Honolulu.

Knowing what sound is already there is needed when trying to establish noise regulations. Someday activities may be modified in zones that have troubling noise levels. “For example, when a lot of whales are present you just might say, ’Let’s wait until tomorrow to do this,’” Nystuen says.

In order to determine the sound “budgets” for different ecosystems, Nystuen and his team use what they call PALs, short for Passive Aquatic Listeners, designed and built at the Applied Physics Laboratory. Moored to the seafloor by long lines, PALs are submerged tens to thousands of meters below the surface and are set to listen for a few seconds every few minutes. They don’t attempt to record every sound – that would take too much memory. Instead Nystuen is developing software that allows the PALs to sift through the racket, identify and sort sound sources by frequencies as they are received.

PALs can identify sounds coming from such things as ships, whales, volcanic eruptions, rainfall and breaking waves. The result is a tally of all the noise and its intensity.

“Those are the two parts of a sound budget, the distribution of different sound sources as a percentage of time and the relative loudness,” he says. In some areas, the sound budget might be dominated 90 percent of the time by breaking waves, 3 percent of the time by rainfall and 1 percent of the time by ships. Other locations may show a sound budget with large components from marine animals or human activities.

With funding from the Office of Naval Research, National Oceanic and Atmospheric Administration and National Science Foundation, Nystuen has done projects at various locations in the Pacific Ocean, South China Sea and Puget Sound, the inland sea on which Seattle is situated.

A pristine area of the Bering Sea is one baseline for comparisons. In the course of a typical week, one ship might pass by. At the other extreme is Haro Strait, an area just north of Puget Sound that’s between British Columbia and the San Juan Islands, where there are so many ships they are difficult to count.

Sixty percent of the time, Haro Strait’s sound budget is dominated by shipping noise, Nystuen has found. It’s also heavily used by killer whales. “If the food is there, do the orcas care?” he asks. “That’s one for the biologists to determine.”