Sonar signal 'leaks' likely audible to some marine mammals

Scientists have found that killer whales and other marine mammals can hear sonar signals better than previously thought.
The commercially available sonar systems, which are designed to create signals beyond the range of hearing of such animals, also emit signals known to be within their hearing range, according to a new study.
The sound is likely very soft and audible only when the animals are within a few hundred metres of the source, researchers said.
The signals would not cause any actual tissue damage, but it's possible that they affect the behaviour of some marine mammals, which rely heavily on sound to communicate, navigate, and find food, they said.

The findings come from a team of researchers at the US Department of Energy's Pacific Northwest National Laboratory, working with marine mammal expert Brandon Southall of Southall Environmental Associates.
A team led by Zhiqun (Daniel) Deng, a chief scientist at PNNL, evaluated the signals from three commercially available sonar systems designed to transmit signals at 200 kilohertz.

The impact of such systems on marine mammals is not typically analysed because signals at 200 kilohertz can't be heard by the animals.
The team found that while most of the energy is transmitted near the intended frequency of 200 kilohertz, some of the sound leaks out to lower frequencies within the hearing range of killer whales and other animals such as harbor porpoises, dolphins and beluga whales.

The three systems studied produced signals as low as 90, 105 and 130 kilohertz.
At the levels measured, the sounds would be quieter than many other sounds in the ocean, including the sounds the animals themselves make, and they wouldn't be heard at all by the animals beyond a few hundred metres.
"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," said Southall.
"These sounds have the potential to affect animal behaviour, 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," added Southall.

The researchers did not directly study the hearing capability of whales and other marine mammals. Instead, the study focused on the sounds produced by sonar systems, discovering that commercial sonar systems are emitting signals within the animals' known hearing range.
The findings were published in the journal PLOS ONE.