How does the sound vary with distance?

The Sound of the Ocean - Comprehensive long-term study of the background noise in the Southern Ocean published

12.01.2017Alfred Wegener Institute, Helmholtz Center for Polar and Marine Research

For almost three years, AWI scientists listened to the Southern Ocean with underwater microphones and heard a “chorus of whales and seals. The recorded noises provide new insights into the natural soundscape of the ocean as well as the behavior and occurrence of the animals.

It is never completely quiet in the world's oceans. Above all, wind and waves provide a continuous background noise. Human activities, such as shipping or resource use, add additional noises in some places, which, depending on the distance, can drown out the natural background noise. The Antarctic is an exception, because so far it has been largely unaffected acoustically by mankind due to its remote location. This is why there is no other ocean in the world that is so well suited for an acoustic study of marine mammals and the natural background noise underwater as the Southern Ocean. Scientists from the Oceanic Acoustics working group at the Alfred Wegener Institute, Helmholtz Center for Polar and Marine Research (AWI) recorded this special background noise in the Antarctic for almost three years. Their results have now been published in the journal Royal Society Open Science.

Sebastian Menze, the first author of the study, and colleagues have identified the sounds of leopard seals, Antarctic blue whales, fin whales and southern minke whales in the Southern Ocean, which merge into a kind of monotonous background choir. The composition of the animal sounds varies in terms of time and location, so that the recorded sounds provide new insights into the behavior and occurrence of the animals.

The sound track of the southern minke whale, for example, follows a 24-hour rhythm in the winter months from April to July: the measurements showed that minke whales contribute more to the background noise at night than during the day. This could be related to their main prey, krill, which migrates vertically in an identical day-night rhythm. The scientists also collected data on the animals' seasonal cycle. Antarctic blue whales contributed to the background noise all year round, but fin whales and southern minke whales only for a few months.

The marine biologists and physicists were also able to find out how great the influence of the sea ice is on the background noise in the Southern Ocean. In the winter months it lies like a sound-insulating carpet over the ocean: “In the Antarctic, it is impressively quiet under the ice cover. Then it is no longer mainly physical phenomena such as storms and waves that shape the soundscape, but also the animal world, says Sebastian Menze. The acoustic recordings show that not only the area of ​​the sea ice plays a role, but also its concentration and texture.

The scientists used two acoustic recorders for their recordings in the Atlantic part of the Southern Ocean from March 2008 to December 2010, which they anchored 217 and 260 meters deep in the water. It is the first long-term study of underwater background noise that has been carried out in the higher latitudes of the Antarctic Ocean. “Comparable studies usually only show a few weeks in the southern summer, says Sebastian Menze. “Especially with regard to the seasonal occurrence of marine mammals, there were significant gaps in knowledge that we can partially close with our records.