Underwater drones are becoming increasingly important for exploring parts of the ocean that remain poorly mapped. But there is an unexpected problem: the protective shell around a drone's sonar can actually interfere with the very sound waves it relies on to see underwater.
Researchers at Xiamen University in China have developed a relatively simple solution. Instead of relying on more electronics and computing power to compensate for the distortion, they designed a special acoustic lens that physically corrects the sonar beam before it travels through the drone's protective dome.
Why underwater drones have trouble with sonar
Although satellites have produced maps covering the entire seafloor at relatively low resolution, much of the ocean has yet to be surveyed using modern high-resolution sonar.
Ships equipped with multibeam sonar can map large areas, but their ability to reveal small features decreases as the distance to the seafloor increases. Underwater drones can solve part of this problem because they can carry sonar systems much closer to the bottom.
However, putting sonar on a streamlined underwater vehicle creates another engineering challenge.
Sonar transducers are commonly installed behind a smooth, curved protective dome. This design helps shield sensitive equipment while reducing drag as the vehicle moves through the water.
Unfortunately, the curved surface can distort the acoustic wavefront produced by the sonar. Instead of maintaining a narrow and concentrated beam, the sound can spread out, weakening the sonar's ability to distinguish objects clearly.
The effect is somewhat similar to looking through a pair of glasses made for someone else's eyesight: the information is still there, but it no longer arrives in focus.
Engineers can compensate for these distortions electronically, but such systems may require additional hardware, processing power and energy — all valuable resources on an autonomous underwater vehicle.
An acoustic lens that corrects sound physically
The Xiamen University team approached the problem differently.Researchers first determined how the drone's dome alters the outgoing sonar wave. They then designed an acoustic lens that produces the opposite effect, correcting the distortion before the sound leaves the vehicle.
The lens consists of concentric silicone rings containing different concentrations of tungsten. Changing the amount of tungsten alters the speed at which sound travels through each section of the material.
By carefully controlling those variations, the researchers can reshape the acoustic wavefront and produce a much more focused sonar beam.
The concept is comparable to using an optical corrective element to compensate for distortion in a telescope. Instead of correcting light, however, this system manipulates sound.
Another advantage is that the lens can be customized. Different underwater drones use different dome shapes, so researchers can design lenses specifically for the acoustic distortions created by individual vehicles.
Tests showed a dramatically narrower sonar beam
The researchers didn't stop at computer models.During laboratory experiments, the acoustic lens reduced a sonar beam measuring more than 65 degrees wide to approximately 16 to 30 degrees.
A narrower beam allows the sonar system to concentrate its acoustic energy more effectively, potentially making objects easier to distinguish from surrounding noise.
The team then tested the technology under more realistic conditions in China's Jiulong River.
Researchers placed a hollow plastic sphere roughly six feet underwater and attempted to detect it using an underwater drone.
Without the corrective lens, reverberations from the surrounding environment and boundaries made the target difficult to identify. Once the acoustic lens was installed, those reverberations were reduced by 11.98 dB, allowing the sonar system to detect the sphere more clearly.
Why better underwater sonar matters
Improving sonar on compact underwater vehicles could have consequences far beyond finding individual objects in a river.Large portions of the deep ocean remain poorly explored, including enormous numbers of underwater mountains known as seamounts. More than 100,000 are estimated to rise at least 3,280 feet from the seafloor, yet only a tiny fraction have been explored directly.
These isolated underwater environments are particularly interesting to scientists because they may support unusual ecosystems, including organisms adapted to darkness and conditions very different from those found near the surface.
Autonomous underwater vehicles equipped with more precise sonar could help researchers map such environments in greater detail while identifying geological formations, biological habitats and other features worth investigating.
The approach could also be useful because it improves sonar performance without depending entirely on increasingly complicated electronic beam-control systems. For underwater drones, where space, weight and battery capacity are limited, a passive physical component can offer significant advantages.
The next challenge is the real ocean
The technology still has more testing ahead.After experiments involving artificial saltwater and the Jiulong River, the Xiamen University researchers want to evaluate how the acoustic lens performs in real seawater.
The ocean presents a far more complicated environment. Salinity, temperature, pressure, suspended material and marine organisms can all affect how sound behaves underwater.
If the lens continues to perform well under those conditions, the idea could provide underwater drones with a surprisingly straightforward way to improve sonar imaging: rather than using more computing power to repair a distorted signal afterward, correct the sound before the distortion happens.
