Biomimetics: Improving sonar by borrowing from nature

A group of engineers believes people can build better sonar systems by borrowing features from one of mother nature's masters: the bat.

Horseshoe bat
EvgeniyQ | Getty Images

Rolf Müller, a mechanical engineer at Virginia Tech, has been studying bats since he was a graduate student and looking for ways to build better systems that use sound to navigate and detect objects.

He and his team have replicated the features of the natural sonar found on a species of bat that has many advantages over mad-made sonar. After studying the bats for years, the team built an early prototype of a sonar system that can give machines the same agility bats have in the close, crowded and often treacherous environments where they live.

The research is an example of the growing field of biomimetics, which holds that the best solutions to engineering problems are sometimes already found in nature.

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Militaries use sonar extensively—most famously in ships and submarines—but the technology has applications in many fields, including unmanned vehicles, or drones, that are being considered by private users, from private farmers to retailers such as Amazon.

There are two forms of sonar—active and passive. Active sonar systems send a sound out into the air (or water) and then use receivers to detect whether that sound bounces off of any objects. Passive sonar does not send out sounds, but instead listens for sounds as they occur in the environment. Because it doesn't intentionally create sounds that give its position away, passive sonar is often preferred by military ships.

Active sonar has limitations: The sound signals it transmits need to be as tightly focused as possible in order to be effective, in the same way that a tight, narrow flashlight beam is brighter than a wide beam. Getting a clear view of a large area using active sonar requires many transmitters and receivers; large ships often have hundreds of transmitters.

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But small craft, such as some drones, lack the space for lots of sonar equipment. Researchers including Müller realized something: So do bats.

Bats use a natural form of sonar called echolocation. They emit squeaks out of their noses or mouths, and use their large, highly sensitive ears to listen as the sounds bounce off objects. They can use their powerful ears alone as passive systems that can hear even the faintest sounds.

They use those systems to navigate and hunt, relying on them so intensely that scientists used to wrongly believe bats had poor eyesight. Hence the expression, "blind as a bat."

Horseshoe bats include several species of bat that live in Europe, Africa and Asia. Many are small—some varieties weigh little more than a penny. But the bat has an on-board sonar system that is exceptionally strong and complex—even among bats—because it has been honed over generations in the dense forests where the bat flies and hunts.

The horseshoe bat's sonar capabilities are far greater in many ways than anything humans have designed.

Horseshoe bats are unusual in that they do not make their sonar noises with their mouths. Instead they use the complex muscles on their noses—one of which looks like horseshoe—to make a complex array of sounds that can tell them different things. Those sounds bounce off cave walls, tree branches, predators or the bats' favorite food: moths. The bats' large, leaf-like ears catch the sounds.

"The ears of the horseshoe bat have about 20 muscles in them," Müller said. "They can completely change shape three times in the same amount of time it takes the human eye to blink once." (Tweet This)

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By shifting the shape of its ears, the horseshoe bat can gather a lot more information from the signals it receives than a simpler system can.

In contrast to the large sonar systems humans use, horseshoe bats have packed a broad range of features into only two organs. The bat's complex nose and ears allow it to do far more with very little.

As a result, horseshoe bats are incredibly agile; the bats Müller has observed in the wild swoop directly into the thick forests below the caves in which they roost, rather than limiting themselves to the much more open sky above the treetops. Their echolocation system is so refined that bats can tell the difference between a leaf and a moth, just by sensing the slight disturbances made by a moth's flapping wings.

After extensively studying their behavior using high-speed photography and other methods, Müller's team built a system that emulates the bats' sound transmission system. They discussed their model this week at a meeting of the Acoustical Society of America and plan to publish a paper on their research soon.

A new robot sonar prototype is inspired by the movable ears and noses that make up the echolocation systems of horseshoe bats.
Source: Philip Caspers | Virginia Tech

They're already working with interested clients, including the U.S. military.

"We are working already with the Navy, so naval sonar is definitely high on our agenda," Müller said. "But we also believe that these micro-air vehicles, such as drones, could benefit from a sense like bats have, because the bats show that if you have that kind of sonar, you can do amazing things."

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Müller is director of a department at Virginia Tech dedicated solely to producing biomimetic technology.

Engineers at several universities, companies and even the military are increasingly turning to nature for inspiration for better designs. In some cases, it's because nature is a tough designer to beat. Nature's "innovations" are adaptations to highly specific needs built over many generations of evolution.

"The engineering value of nature is fantastic," Müller said. "What you see in life is the result of something like 3.8 billion years of trial and error, of testing, of optimization. We will never, ever come close to being able to reproduce that level of optimization in our laboratories."