New Paper on “200 kHz” sonar with side lobes audible to marine mammals

 

 

On behalf of my co-authors, I would like to let you know about a new paper on the audibility of high frequency sonar systems to some marine mammals. We investigated the acoustic “leakage” of energy into side lobes from the 200 kHz center frequency of three commercially-available sonar systems. These active sonar systems were being used in efforts to detect and track marine mammals around a tidal power turbine site and they were not expected to be audible to the animals based on their very high frequency. However, behavioral observations of killer whales suggested they were in fact detecting them. Our acoustic analysis of the systems indicates that there is sufficient downward spread of energy in the side bands to expect that these odontocete cetaceans could in fact hear them. We conclude that received levels at animals would very likely be well below those that could be harmful, but that they could be audible and potentially affect behavior over ranges of hundreds of meters. A reference to the paper, weblink to the Open Access manuscript, and the article abstract are given below. The lead author was PNNL engineer Zhiqun (Daniel) Deng and both he and I would appreciate any comments or questions on the paper.

Deng ZD, Southall BL, Carlson TJ, Xu J, Martinez JJ, et al. (2014) 200 kHz Commercial Sonar Systems Generate Lower Frequency Side Lobes Audible to Some Marine Mammals. PLoS ONE 9(4): e95315. doi:10.1371/journal.pone.0095315

http://dx.plos.org/10.1371/journal.pone.0095315

The spectral properties of pulses transmitted by three commercially available 200 kHz echo sounders were measured to assess the possibility that marine mammals might hear sound energy below the center (carrier) frequency that may be generated by transmitting short rectangular pulses. All three sounders were found to generate sound at frequencies below the center frequency and within the hearing range of some marine mammals, e.g. killer whales, false killer whales, beluga whales, Atlantic bottlenose dolphins, harbor porpoises, and others. The frequencies of these sub-harmonic sounds ranged from 90 to 130 kHz. These sounds were likely detectable by the animals over distances up to several hundred meters but were well below potentially harmful levels. The sounds generated by the sounders could potentially affect the behavior of marine mammals within fairly close proximity to the sources and therefore the exclusion of echo sounders from environmental impact analysis based solely on the center frequency output in relation to the range of marine mammal hearing should be reconsidered.

Please also see a PNNL press release on the study: http://www.pnnl.gov/news/release.aspx?id=1053

Greenwire put a short piece out on the paper which you can find at: http://www.eenews.net/greenwire/2014/04/16/stories/1059998006

Marine mammals can detect sounds from readily available sonar systems — study

Jessica Estepa, E&E reporter

Published: Wednesday, April 16, 2014

Whales and other marine mammals likely can detect sounds from sonar systems that had been considered out of their hearing range, a new finding that scientists say should play into policymaking.

While marine mammals are known to be sensitive to certain sounds emitted by echo sounders, some systems were considered safe because they operated at frequencies of 200 kilohertz — thought to be outside their hearing range.

But the new report published yesterday in the journal PLOS ONE found that these sounders — commercially available for uses such as exploration and navigation — release a second, lower frequency that can be heard by marine mammals.

A team led by study authors Daniel Deng, a chief scientist at the Pacific Northwest National Laboratory, and Brandon Southall, senior scientist and president of Southall Energy Associates Inc., examined the potential impacts of three different systems as part of a bigger assessment on the environmental impact of a proposed tidal energy project in Puget Sound, Wash.

Deng and his team measured the frequencies from the systems in field tests. The data collected was then used to determine whether the lower frequencies would fall into the hearing ranges of marine mammals.

They found that these lower frequencies fell between 90 to 130 kilohertz, well within the audible range for the animals.

While the mammals could detect the sounds at those rates, the study noted that’s below potentially harmful levels, such as causing tissue damage. They also determined that the frequencies could only be heard by animals who were nearby.

Still, researchers said the signals could affect animal behavior. Southall noted that some species, such as harbor porpoises, may leave local areas if they hear the frequencies.

Sonar systems are “commonly exempted” from environmental permitting requirements, according to the study. Both Deng and Southall suggested that policymakers should at least consider whether high-frequency systems may affect animals.

The study provides scientific evidence that policymakers can use, according to Deng. “We believe this should be a consideration for them,” he said.

Southall said other systems should be investigated in similar ways. He added that there is a need for controlled behavioral investigations about cetacean responses.

“We aren’t saying that these are necessarily a major issue, but managers should revisit just flatly excluding things from the regular kinds of consideration and assessment because the frequency on the side of the box says they should be ultrasonic,” Southall, previously with the National Oceanic and Atmospheric Administration, said in an email.

 

 

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SOCAL-BRS

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