By Jean Kumagai
Hearing, like all the senses, is highly subjective. Standing five metres away from loudspeakers at a death-metal concert is heaven for some people and hell for others. In the ocean, the impact of noise depends on, among other things, the temperature and depth of the water, the topography of the ocean floor, and the type of creature doing the listening. In general terms, though, "water is more viscous and denser than air," explains Robert Gisiner, of the Office of Naval Research [U.S.].
Confusingly, sound measurements taken underwater are not directly comparable to those in the air. Like the Richter scale for earthquakes, decibels are logarithmic, so if one sound is said to be 100 decibels stronger than another, that means it's 10 billion times as strong. Decibels are also relative units, not absolute ones. More precisely, a sound's intensity in decibels is the ratio between the sound wave's intensity and a reference pressure.
But scientists use different reference pressures for water and air—1 for water, 20 micropascals for air. (Decibel measurements cited elsewhere in this article are referenced to 1 micropascal.) To compensate, you need to add 26 dB to an air sound measurement to get the equivalent underwater pressure measurement. Also, because of the difference in impedance between air and water, you need roughly 35,000 times as much power—equivalent to 35.5 dB—in air to produce an equivalent pressure in water. That yields a 61.5 dB correction factor between air and water. So, for example, the U.S. Occupational Safety and Health Administration says that workers should not be exposed to on-the-job noise above 85 dB over an 8-hour period. The underwater equivalent would be 146.5 dB.
"But it's a dangerous comparison," says John Hildebrand, a professor at the Scripps Institution of Oceanography, in La Jolla, California. Dangerous because it doesn't factor in how prevalent the noise source is and how likely you are to encounter it.
At well over 300 dB, the mother of all underwater noise sources is a nuclear explosion. But there hasn't been one of those in a decade, and there may not be another for many years.
The effects of naval sonar, though they can be deadly, are similarly limited to certain areas and certain times. Commercial ships, on the other hand, produce less intense noise, but they're ubiquitous, contributing to a doubling of the oceans' ambient noise intensity every decade.
Also making comparisons difficult is that whales' ears "are air-adapted ears that have had to re-adapt to an aquatic environment," notes Gisiner. "We don't know what the eardrum is for anymore, if anything. We don't know if the middle ear is functional. Under these circumstances, it is hard to make comparisons with the subjective world of sound we know.
Hildebrand says sonar-related strandings grab people's attention because they are acute, dramatic events. "When there's an animal on the beach, dying or dead, people get excited," he says. "But there's this other hidden issue. If the ambient noise is steadily rising, how is that affecting entire populations of animals over broad areas?"
To probe further
The National Research Council has issued four scientific reports that look at anthropogenic sound and marine mammals, available on the National Academies Press Web site, http://fermat.nap.edu. The most recent, "Marine Mammal Populations and Ocean Noise," was released in 2005.
The Natural Resources Defense Council's "Sounding the Depths ll" (http://www.nrdc.org) and the Whale and Dolphin Conservation Society's "Oceans of Noise" (http://www.wdcs.org) provide overviews on the sources of underwater anthropogenic noise and the risks they may pose to marine mammals.
The U.S. Navy's Whales and Sonar Web site, http://www.whalesandsonar.navy.mil/, lays out the Navy's case for the necessity of sonar.
Discovery of Sound in the Sea, http://www.dosits.org , has sound samples and explanations of ocean acoustics for a lay audience.
- IEEE Spectrum
(publication of the International Electrical
and Electronic Engineers, Inc.)