Whales and dolphins have extremely acute hearing, which they use for navigation, communication and pursuing prey. But increasing human activity in the oceans is now raising alarm about the impact that our noisy lifestyle is having on these sensitive creatures. ‘Concern… has led to attempts to establish acoustic safety criteria for underwater noise’, says Paul Wensveen from the University of St Andrews, UK, working with colleagues from the Sea Mammal Research Company in The Netherlands. Unfortunately, measuring an animal's perception of sound volume can be particularly challenging: James Finneran and Carolyn Schlundt conducted thousands of perceived loudness trials with a bottlenose dolphin to measure its hearing in their 2011 Journal of the Acoustic Society of America paper.
However, Wensveen and colleagues explain that an animal's loudness perception can be tested with a simpler method, used on human infants and animals, where the time that it takes a child or animal to respond to a sound is used as an indication of the sound's loudness: humans and animals respond faster to loud sounds than they do to soft sounds (p. ).
Working with Jerry, a young adult male harbour porpoise who was trained to respond to very soft sounds by swimming away from a holding station, Wensveen and colleagues spent several months measuring his reaction times to sounds ranging in frequency from 0.5 to 125 kHz at sound pressure levels (volumes) from 59 to 168 dB re. 1 μPa. The team then calculated a series of auditory weighting functions, which can be used as indicators of how Jerry and other harbour porpoises of a similar age perceive the loudness of sounds. The team says, ‘Behavioural and physiological responses of marine mammals to noise correlate better with the perceived loudness of a sound than with the unweighted sound pressure level’, and they hope that Jerry's auditory functions could be used to set safer limits on human aquatic noise to better protect cetacean hearing.
