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Underwater Noise Revisited

© Kenneth Friedman

In 1998 I wrote a Suite 101 column on the concern over the effect of underwater noise on marine life. Recently I received e-mail from Lee Tepley pointing to his web site where, to use his terms, he debunks the Navy for its own debunking of myths surrounding low-frequency active sonar tests off the Island of Hawaii. The Navy tests sought to measure the effects of low frequency sound waves (active sonar) on singing Humpback whales and Sperm whales by hitting them with high energy sound waves. In essence the Navy wanted to "see what happened," Tepley says.

Tepley, who holds a doctorate in physics, says that he recently ventured into underwater photography but he has had trouble getting near Humpback whales without violating National Marine Fisheries Services (NMFS) regulations. However, while NMFS is busy protecting the whales from photographers, it hasn't been busy protecting whales from the Navy sonar tests, he says.

His debunking effort focuses on an advertisement by the Animal Welfare Institute, which claimed that the sound waves would be greater than those of a jet engine taking off. He says the ad may have been incorrect because it is "confusing to compare decibel levels in air and water." But he also isn't satisfied with the Navy's reply to the accusations, in which the Navy said that a jet engine is 100 times louder. He says that the Navy also claimed that the whales wouldn't be exposed to sound levels near 200 dB and couldn't understand why environmentalists were upset.

Tepley says that the Navy's use of the term "acoustic power" in its comparison of sonar and a jet engine bothered him. "I had read that 'acoustic power' (or 'acoustic intensity') is a fundamental physical property of sound waves but it is not what is measured. What is measured is acoustic pressure or, more precisely, the change in pressure produced by the sound wave," he says. One would think that if acoustic power went up, then acoustic pressure would go up too, and they do, he adds.

But it took awhile, he says, to understand that there are "differences between the properties of sound waves in air and water," and he says he is surprised that the Navy scientists weren't aware of those differences because it turns out that they affect scientists' calculations.

When Tepley crunches the numbers in some complicated-looking mathematics, he finds that although a jet engine puts out 100 times more acoustic power, the low-frequency sonar pressure is six times stronger. He concludes that the Navy must have based its comparison on acoustic power rather than on acoustic pressure. We would have to read a peer review of his analysis to know whether or not his method and conclusion are correct, but absent such analysis, we'll just continue with his argument, which is that the sonar pressure would exceed that of a jet engine "by a factor of about 33."

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