Sound is energy in the form of a wave. In the dark depths of the sea, whales and other marine mammals use gentle sound pulses to communicate about feeding, mating, and to keep their groups together.

But as humans increasingly plumb the ocean for their own forms of energy, the loud sounds of exploration, development, and construction send powerful waves that can confuse and harm even the mightiest denizens of the deep.

Offshore oil and wind power companies are studying an unusual but promising means of lessening the impact of sound on marine mammals: bubble curtains. Adapting a technique that proved successful in underwater bridge building, energy firms are testing the benefits of surrounding their operations with walls of bubbles that actually alter the shape of the noise waves.

It is too early to say whether the method will be effective. But research into this technology and other means of dampening human sound will be crucial for marine mammals living in the Arctic and coastal habitats now being eyed for their vast fossil and renewable energy potential.

A Bridge to Insight

The retrofit of the San Francisco-Oakland Bay Bridge provided evidence that bubbles can be used to lessen the impact of undersea sound.

In 2003 and 2004, the California Department of Transportation (Caltrans) needed to drive new pilings—8 feet (2.44 meters) in diameter and 300 feet (91.44 meters) long—into the seabed for the project. Caltrans was concerned that the short, powerful pressure waves created by the enormous hammers striking the pilings would affect the behavior of nearby seals. Bud Abbott of Oakland, California, a marine biologist consulting on the project, warned the waves might kill fish as well.

Sure enough, during an early test, a number of dead fish appeared on the surface. Abbott collected the fish and conducted necropsies. The swim bladders of the fish had burst and their kidneys had suffered terrific damage. Abbott's verdict: The pressure wave compressed the air in the swim bladder, which then quickly expanded again, bursting the bladder and damaging the kidneys.

The discovery, alarming as it was, pointed the way to potential methods for lessening the impact of sound waves.

"When a pressure wave hits an air bubble, it will compress the bubble, then it will expand again, so energy is lost," Abbott explained. Although scientists disagree on the amount of energy lost in this process, Abbott said, there is no doubt that the air bubble actually changes the shape of the wave.

"Sound travels faster through water than air," said Abbott. "It slows down as it hits the air bubble." This creates a much smoother wave, altering it from a brief percussive bang to a longer, weaker wave.

For the Bay Bridge project, Caltrans placed a wall of air bubbles directly against each pile as it was being driven. This attenuated the waves caused by the pile driving. Overall fish deaths were reduced and delayed mortalities did not occur beyond 69 meters (226 feet) from the piling.

Germany's Offshore Wind Discovery

In Germany, where offshore wind farms are an important component of the nation's ambitious plans for expansion of renewable energy, the impact on the rich marine life in the North and Baltic seas has been a growing concern. Building a wind farm into the sea floor is a massive undertaking; turbines in Germany's first offshore wind project each stood about 150 meters (490 feet) high and weighed 1,000 tons.

A government study found that porpoises avoided swimming within 20 kilometers (13 miles) of a wind farm, Alpha Ventus, while the complex of 12 wind turbines was being built in 2009 and 2010 some 45 kilometers (28 miles) north of the German island of Borkum.

In the wake of this research, the German government has set a mandatory 160-decibel limit on the sound levels allowed in wind farm construction, measured at a distance of 750 meters (half a mile) from the pile or source of noise. The German Federal Agency for Nature Conservation (BfN) listed bubble curtains as an option to meet the new standard and mask the sound of underwater wind turbine pile driving.

A new wind farm now under construction in the North Sea, Borkum West II, being developed by a consortium of German power companies, will be the first offshore application of bubble curtains. Scientists familiar with the effort say the developers expect to be able to meet the German standard of 160 decibels.

The Whales of the Arctic

The oil company Shell* also is researching the use of bubble curtains to protect marine life from the noise of its undersea operations. This issue is of special concern in its proposed Arctic exploration in U.S. waters, because the marine mammals of the Beaufort and Chukchi seas off the coast of Alaska, where Shell currently holds leases, are among the most diverse in the world, according to the U.S. Geological Survey. The 15 species common to the area include ice seals, walruses, baleen and toothed whales, and polar bears. For the endangered bowhead whale, almost an exclusively Arctic species, noise, oil pollution, and warming are all important concerns, USGS says.

The ocean is actually filled with sound from waves, rain, thunder, and marine life (bowhead whale songs and tones have been measured at ranges from 128 to 189 decibels). And ocean ambient sound levels have been increasing, due to human-generated sounds, including an increase in commercial shipping.

For comparison, normal conversation is about 60 decibels, 130 decibels is painful to the human ear, and the sound of a jet engine is about 140 decibels. But sound in water is not the same as sound in air. Hal Dreyer, president of Gunderboom, an Anchorage, Alaska-based engineering company that designs and builds aquatic filters and bubble curtains, notes that a sound that is 100 decibels on the surface, just "on the threshold of being uncomfortable" to human ears, is roughly equal to 162 decibels underwater.

The U.S. Marine Mammal Protection Act requires that permits be issued for any underwater human activity, like oil exploration and drilling, that could affect protected species. A continuous sound (like that of a drilling ship) measured underwater at greater than 120 decibels or an "impulsive" sound (like that of a seismic survey air gun) greater than 160 decibels could disturb the behavior of marine mammals and requires a permit, say scientists at the U.S. National Oceanic and Atmospheric Administration (NOAA). In Shell's current Arctic exploration plan, NOAA's National Marine Fisheries Service estimated that an area of 481 square miles (1,245 square kilometers) would be exposed to 120-decibel sound or greater for one of two drill ships Shell is considering using, and 30 square miles (78 square kilometers) for the other. NMFS estimated an area of 74 square miles (191 square kilometers) would be exposed to seismic survey sounds greater than 160 decibels.

Shell's work on bubble curtains is still at an early stage, so the technology is not included in the current mitigation plans for the Chukchi and Beaufort seas that the company submitted to the U.S. government for proposed exploratory drilling in the 2012 season.

But Mitch Winkler, manager of the Arctic Technology Program at Shell International Exploration and Production, said the company recognizes that "marine sound is important."

"We are focusing on the use of air bubbles and their impact on sound waves as a means of reducing the sound transmitted from stationary sources," he said. By surrounding a rig or ship with bubbles of air, "we are targeting a reduction in the amount of noise by as much as ten decibels," he said.

He said the sound created by oil exploration and extraction is an even, constant noise, different from the staccato bursts emitted by driving piles. Winkler said the drill and other equipment on the sea floor create very little noise. Rather, he said, the noise that is the biggest issue for undersea life is from the engines and machinery onboard the ship. That noise reverberates through the hull into the surrounding water.

Michael Stocker, executive director of Ocean Conservation Research, a group that advocates reducing marine noise pollution, agrees that ship sounds are the issue of greatest significance. "Drilling is not as concerning as the thrusters," on the floating platforms, he says. "Thrusters stabilize the platform as they drill and explore. It's a continuous noise." But he says that seabed noises also may be a concern. "There is seafloor processing and separation of oil, gas, sand, and brine," he said. "They use the pressure to drive the system." He said the noise fields created by the oil industry need to be mapped so scientists can better understand how noise pollution affects sea life.

Shell is developing bubble curtains that use "passive means for attenuation, namely reflection and energy absorption," according to Winkler. This means the air is already contained in the curtain and the curtain does not require any energy once it is in place.

Researchers are still trying to figure out the best design for bubble curtains. The University of Texas Applied Research Laboratory has shown that a softball-size air bubble is optimum for reducing sound. But once released from the source, these big bubbles quickly break into smaller bubbles. Shell is experimenting with ways of encapsulating the air bubbles to control their size and placement. The concepts include using oversized ping-pong balls, toroids, and hoses suspended from floats that would surround the ship or platform.

Such details are important. During the Bay Bridge retrofit, Abbott recalls, Caltrans first used stacks of rings to guide the bubbles to the surface, but the tide and currents would sometimes push the bubbles away from the piling. Other methods had to be tried to contain the bubbles where they would be effective.

Brendan Cummings is senior counsel for the Center for Biological Diversity, one of the groups that filed legal challenges opposing Shell's Arctic drilling plans. He said he lauds any efforts to develop technology to protect marine mammals from noise. But, he said, noise-mitigation technology alone would not alleviate his group's concerns about oil exploration in the ecosystem.

"We're certainly in favor of using and testing any new methods of reducing sound from human activities underwater," he said. The idea of "bubble curtains is raised pretty frequently, although there are few real-world applications of it. But there are far bigger problems than the noise impacts, including the simple fact that there is no technology, methodology, and—most important—no infrastructure in place to deal with an oil spill in the Arctic."

The Search for Sound Solutions

Meanwhile, there may be alternatives in the offing that could soon compete with bubble curtains for dampening underwater sound. Karl-Heinz Ehlmer, a researcher at Leibniz University in Hannover, Germany, who was involved in the first study of air bubble curtains in Germany, said he found that air bubbles could be sensitive to tide currents and that large bubbles showed a "chaotic behavior." He said that although there have been several tests of air bubble curtains in recent years, the technology is expensive and "insufficient in the lower frequency range when using large monopiles."

Ehlmer said there are several alternatives in development. One that he is working on, called Hydro Sound Dampers (HSD), uses nothing more sophisticated than fishing nets and foams to lessen the impact of sound waves. Ehlmer believes it is an improvement on air bubble curtains because it appears to be more effective (lowering noise by at least 20-25 decibels) and less costly. "Hydro Sound Dampers do not need compressed air supply, they can be exactly tuned to the necessary frequency range, and they are not influenced by tide currents, as air bubbles are," he said.

Other scientists are looking for less noisy ways to affix wind turbines to sea beds, said Mark Simmonds, international director of science for the Whale and Dolphin Conservation Society, based in Great Britain. "They can anchor them, they can also drill the hole and drop the turbine column in to it," he said. "But these are expensive."

Simmonds said the scientific committee of the International Whaling Commission is planning a workshop to consider ways to mitigate the noise caused by pile driving.

"There is technology which could be deployed further," he said. "But at the moment, I don't think there is enough of a public perception that this is even an issue in order to push companies and regulators to address this."

—With additional reporting by Jimmy Langman and Marianne Lavelle