Reducing bubbles help swimmers make a splash

Humans are noisy swimmers. But just how much noise do we make thrashing around in the water, and is there enough of it to disturb marine wildlife?

To find the answer, a team led by Christine Erbe, director of the Centre for Marine Science & Technology at Curtin University, Perth, Western Australia, outfitted an Olympic-sized pool with acoustical sensors and had people practice swimming, diving, kayaking, and scuba diving in it.

What they found was both reassuring for ocean swimmers not wanting to leave environmental chaos in their wake, and intriguing for coaches looking for new ways to improve in-the-water technique.

Most of the noise, Erbe said last week at a meeting of the Acoustical Society of America in Minneapolis, Minnesota, comes from generating and pushing around clouds of bubbles as we kick, dive, or move our arms. It’s loud enough, she says, to be heard from tens to hundreds of meters away, but almost certainly not loud enough to disturb distant wildlife.

In terms of noise management in the oceans, she says, even the noisiest humans are small fry compared to the big emitters like ocean vessels, offshore construction, deep-sea mining, oil and gas development, or even jet skis and motor boats — “all the things that are obviously loud in the water”.

The experiment was simple and inexpensive, since Erbe’s lab already had all the needed equipment.

“We simply threw the gear we had in the pool,” she laughs. “Everyone swam.”

Sometimes, even scientists want to have fun in the water. Especially in Australia.

But the results were also quite interesting, especially when the scientists compared the recorded sounds to videos of what produced them.

In swimming, Erbe says, “it’s all related to bubbles created when [we] pierce the water with our heads, hands, or fins. In scuba it’s the exhalations and inhalations. Kayak sound was interesting because every time the paddle comes out of the water it dribbles and you get this tinkle sound.”

There was also a difference between swimming strokes (breaststroke was quieter than freestyle) and, most interestingly, between individual swimmers. Once you’d familiarised yourself with a swimmer’s acoustical signature, she says, you could listen blindfolded and know who was swimming and at what speed.

“Some of us use much more force in swimming than others,” she explains. “For example, some swimmers exerted more energy vertically downward — pushing huge and noisy bubble clouds underwater, while other swimmers used their energy more in a forward-propelling sense and created much less bubble noise vertically below them.”

One possible application is to seek a better understanding of shark attacks. “We know sharks don’t go hunting for humans,” Erbe says. “There is a hypothesis that they mistake humans for prey. So one of the things one could look at is how do the signatures of humans compare to the natural sounds of prey for sharks.”

Another application might be security for swimming pools. “You could set up equipment that automatically listens and sends an alarm,” she says.

But an even more interesting application, she says, is as a training device for swimmers and coaches.

“Those who create the most bubbles are not the best swimmers,” she says. “They waste energy. The best swimmers are certainly also the quiet ones. That might be good for performance feedback.”

Furthermore, the needed equipment isn’t all that expensive. “You can do it with a GoPro,” Erbe says. Even a good hydrophone only costs a few hundred dollars.

Andrew Sheaff, assistant swimming and diving coach at the University of Virginia in US, agrees that acoustics might help coaches identify previously overlooked nuances in swimming technique.

“I’m always looking for ways to identify the characteristics for fast swimming,” he says. “Adding an acoustic element would further expand our knowledge of what excellent swimming looks like…we [might] gain insight that was previously missed. Once we identify what these characteristics are, we can begin the process of helping swimmers learn them.”