Bubbles Below the Surface

By Jenny Blair in 40 Acres, Sept | Oct 2012, Special on September 5, 2012 at 2:15 pm | No Comments

Thanks to humans, the ocean is a noisy place. Pile drivers, seismic tools, sonar, and ships’ engines fill the deep with sounds that can travel vast distances and outblast a jet engine. As shipping and oil exploration increase and more offshore wind farms are built, it’s only getting louder under the sea. But associate professor of mechanical engineering Preston Wilson, BS ’90, MS ’94, has discovered a clever way to hush some of the din.

For ocean animals, underwater noise is more than a nuisance—it’s a trauma. Military sonar has been linked to mass whale strandings and brain hemorrhages from barotraumas, or pressure damage. The delicate ears of fish can be damaged by human racket. Noise stresses and drowns out countless species that use sound to signal, hunt, and navigate.

Wilson’s surprise tool for reducing the pain: bubbles. “We’ve known since the early 20th century that bubbles attenuate sound,” he says. With a syringe, he shoots air into a glass of water and taps the side with a fork. The air creates bubbles, and the clink is muffled.

Companies responsible for large amounts of underwater noise use this idea in “bubble curtains,” created by pumping air through a perforated pipe. As they rise, the short-lived bubbles absorb some high frequencies, but they’re too small to absorb manmade low frequency noise—that would call for bubbles as large as 10 centimeters across.

With a grant from Shell Global Solutions, Wilson, research scientist Mark Wochner, and postdoctoral fellow Kevin Lee, BS ’01, PhD ’09, set out to build bigger, better bubbles. The team hit upon a simple innovation: enclose air in a thin shell. A fabricated bubble of that sort could be enlarged and permanently tethered to an underwater line.

At Lake Travis and in a test tank, the engineers experimented with different shell materials by sinking a network of bubbles around a submerged loudspeaker. Party balloons muffled noise nicely but proved too fragile. Boat fenders: too stiff. Dodgeballs and fire hoses: ditto. Then, at last, the team found a material that’s cheap, rugged, and flexible.

“That’s the secret sauce,” Wilson says, laughing. He can’t reveal what it is until a pending patent application is accepted but says it’s not an unusual substance. “It’s just something that people didn’t think they would ever use for what we’re using it for.”

The team founded a company called AdBm to develop and market their bubbly noise abatement system. One size won’t fit all, Lee explains: the bubbles’ diameter and spacing can be customized. They can be affixed to a noisy object, hung around it like a curtain, or anchored to the sea floor like strands of kelp.

Wilson even imagines a day when shipping lanes are lined by guardrail-like bubble strands, ships have bubbles built into their hulls, and harbor aquariums are shielded from noise. If this idea floats, then the ocean depths may one day afford sea animals a little more peace and quiet.