Stealth in schools: Fish noise reduction and the future of submarines

Schools are very noisy places, especially during recess and when the final bell rings to announce the end of the day’s classes. But schools can also be very quiet, like when fish swim in “schools,” making them surprisingly stealthy underwater with a group able to sound as quiet as just one fish.

The new findings by engineers at Johns Hopkins University in Baltimore, who worked with a hi-tech simulation of schooling mackerel, offer new insight into why fish swim in schools and the potential for the design and operation of much quieter submarines and autonomous undersea vehicles.

“It’s widely known that swimming in groups provides fish with added protection from predators, but we questioned whether it also contributes to reducing their noise,” said senior author, mechanical engineer Prof. Rajat Mittal. “Our results suggest that the substantial decrease in their acoustic signature when swimming in groups, compared to solo swimming, may indeed be another factor driving the formation of fish schools.”

The work has just been published in the journal Bioinspiration & Biomimetics under the title “Effect of Schooling on Flow Generated Sounds from Carangiform Swimmers.” This term refers to a type of swimming by a fish in which undulations are limited to the tail regions.

The team created a 3D model based on the common mackerel to simulate different numbers of fish swimming – varying their formations, how close they swam to one another, and the degrees to which their movements synced. The model, which applies to many fish species, simulates one to nine mackerel being propelled forward by their tail fins alone.

Silent swim: Fish noise reduction

The team found that a school of fish that moved together in just the right way was stunningly effective at noise reduction – a school of seven fish sounding like a single fish.

“A predator like a shark could perceive it as hearing a lone fish instead of a group,” Mittal said. “This could have significant implications for prey fish.” The single biggest key to sound reduction, the team found, was the synchronization of the school’s tail flapping – or actually the lack thereof.

If the fish moved together, flapping their tail fins at the same time, the sound added up, and there was no reduction in total sound. But if they alternated tail flaps, the fish canceled out each other’s sound, the researchers discovered.“Sound is a wave,” Mittal explained. “Two waves can either add up if they are exactly in phase or they can cancel each other if they are exactly out of phase. That’s kind of what’s happening here, though we’re talking about faint sounds that would barely be audible to a human.”

The tail fin movements that reduce sound also generate flow interactions between the fish that allow them to swim faster while using less energy, said lead author Ji Zhou, a Johns Hopkins graduate student who is studying mechanical engineering.

“We find that a reduction in flow-generated noise does not have to come at the expense of performance,” Zhou said. “We found cases where significant reductions in noise are accompanied by noticeable increases in per capita thrust due to the hydrodynamic interactions among the swimmers.”

The team was surprised to find that the sound reduction benefits kick in as soon as one swimming fish joins another. Noise reduction grew as more fish joined a school, but the team expects the benefits to cap off at some point.

“Simply being together and swimming in any manner contributes to reducing the sound signature,” Mittal concluded. “No coordination between the fish is required.”

Next, the team plans to add ocean turbulence into the models and create simulations that allow the fish to swim more “freely.”