Sonar Signals Leak to Low Frequencies Audible to Whales

Sonar signals are more audible to killer whales and other marine mammals than previously realized, according to a new study.

Writing in the journal PLOS One, researchers report that commercial sonar equipment designed to produce signals at frequencies beyond the hearing range of whales also produces noises known to be within the cetacean's hearing range.

"These signals are quiet, but they are audible to the animals, and they would be relatively novel since marine mammals don't encounter many sounds in this range," marine mammal expert Brandon Southall said in a statement. 

These soft sounds are only audible when a marine mammal is within a few hundred meters of a sonar device. The noises do not likely cause physical tissue damage to whales, but the signals may affect whale's behavior and affect their ability to find food, navigate and communicate, the researchers said. 

Southall conducted the research along with Daniel Deng, a chief scientist at the Department of Energy's Pacific Northwest National Laboratory.

Most sonar devices transmit at signals near the 200 kilohertz frequency, but some of that sound leaks out to lower frequencies that are within the audible range of whales, the researchers learned. Three sonar systems included in the study emitted signals as low as 90, 105 and 130 kilohertz.

Southall drew a comparison of whales' hearing range to that of a piano keyboard. In the ocean, passing ships would be hitting the lowest notes, while the sounds the whales themselves make would occupy the middle of the keyboard. The sonar systems included in the study would occupy the highest notes, to the right-most side of the keyboard.

"These sounds have the potential to affect animal behavior, even though the main frequency is above what they primarily hear. It may be that environmental assessments should include the effects of these systems. This may not be a major issue, but it deserves further exploration," Southall said.