Daily Archives: December 14, 2009

+3dB noise reduces ‘effective listening area’ 30%

AE.org - website of the Acoustic Ecology Institute

Jim Cummings of the Acoustic Ecology Institute has posted another great synopsis of an important new bioacoustics paper that has big implications for southern resident killer whales.  After defining a new bioacoustic metric “effective listening area” (which is MUCH more intuitive than “active space”), the authors clarify how slight increases in ambient noise can have big impacts for animals that need to listen to sounds that are normally barely audible.

The authors note analyses of transportation noise impacts often assert that a 3dB increase in noise – a barely perceptual change – has “negligible” effects, whereas in fact this increased noise reduces the listening area of animals by 30%. A 10dB increase in background noise (likely within a few hundred meters of a road or wind farm, or as a private plane passes nearby) reduces listening area by 90%.

We know that most commercial ships and recreational boats raise the ambient noise levels near killer whales by 20-30dB for periods of ~30 or 3 minutes, respectively, as the vessels and whales pass by each other.  Clearly it is time to articulate in what common situations the southern residents need to perceive barely audible signals — like distant inter-pod communication signals or echolocation returns from prey — and to model the reduction in listening area during typical noise exposures.  This paper suggests the results may be disconcerting even though southern residents are keystone predators (though one has to wonder if transients appreciate the advantage of the acoustic cloak a noisy freighter offers when trying to pick off a resident calf).

Scientific literature reference:
Barber, Crooks, Fristrup. The costs of chronic noise exposure for terrestrial organisms. Trends in Ecology and Evolution, 2010.
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