What's the Problem?

Sound in the marine environment behaves differently from the way it behaves in air.

Generally speaking, an equivalent sound in air will travel 5 times faster and 60 times farther when generated underwater. Sound is not measured per se (you can't have a cupful or a pound of sound) but is referenced using units called decibels (dB). Sound in either environment is not just heard, it's a physical wave even if we can't see it with our eyes. Very intense, acute sound can actually knock you down, such as that produced by a nearby explosion -- the "shockwave" -- but its intensity decreases (attenuates) very quickly.

Underwater, however, a similar explosion could cause cerebral hemorrhage or kill you outright. Equally intense but more prolonged sounds, such as those produced by low-frequency sonars, can travel literally for hundreds of kilometers without much attenuation if the conditions are right.

Problems between loud manmade sounds and marine mammals arise when the animals are too close or exposed too long to these sounds. Their own normal calls and songs can be "masked" by manmade sound, making them unable to communicate with each other or to navigate; conversely, this "masking" effect can also prevent them from being able to hear important sounds, such as the approach of a ship or a predator.

If cetaceans are too close to the source of a loud sound, they can be temporarily deafened (temporary threshold shift, or TTS), or permanently deafened (permanent threshold shift, or PTS) if they are repeatedly exposed to loud sounds in areas they frequent or if the source is mobile.

They can be killed if the sound is intense enough to cause bleeding in the brain, eyes and ears, or has caused bubbles to form and expand as the animal ascends quickly trying to get away from the sound source, similar to "the bends" human divers can get.

Equally serious but far less studied is the effect of intense sound on fish. The few studies that have been conducted indicate that fish, and especially fish larvae, are highly susceptible to intense sounds. With 90% of the world's commercial fisheries in serious decline or collapse as of 2003, this is of concern not only for the marine food web but also for humans as an important food resource.

For further reading about underwater acoustics, we highly recommend reading Marine Mammals and Noise by Richardson, Greene, Malme and Thomson, 1995, published by Academic Press. Also: NOAA's Acoustic Monitoring Program: Underwater Acoustics Tutorial