In science fiction, cloaking devices are designed to keep people from seeing things such as Klingon starships or Harry Potter. But cloaking devices could also shield objects from acoustical sensing such as sonar, scientists say — and in the real world, not just in science fiction.
In a recent presentation at a meeting of the Acoustical Society of America in Minneapolis, Minnesota, Amanda Hanford, an acoustical engineer from Pennsylvania State University, unveiled a device that looked like a metre-tall cheese grater.
It can bend underwater sound waves around an object, making it appear as though the object doesn’t exist. And sound waves that bounce off the cloaking device are reflected in a complex pattern of phases that cancel each other out, thereby making them inaudible to distant receivers.
In tests, she and her colleagues put the device in a large tank, then beamed sound waves at it, while listening for reflections from several different angles. The results, Hanford reported, confirm that such methods could, in principle, be used to make underwater objects invisible to sonar.
The key is the use of “metamaterials,” which are complex structures composed of arrays of individual components significantly smaller than the wavelength of the sound waves they are intended to hide from. In this case, Hanford says, these components are about 5 to 12 millimetres in size, ideal for hiding from the high-pitched sounds used in her pilot study.
Similar studies have been done with acoustical cloaking in air, she says, as well as for making small objects invisible to light or other forms of electromagnetic radiation. “We are learning a lot from our colleagues in different disciplines,” Hanford says.
Working underwater, however, has been more difficult because it is denser than air and less compressible — something that limits engineering options.
At the moment, she says, the work is still in the proof-of-concept stage.
“We’re not really hiding anything right now,” she explains. “We’re trying to make the acoustic wave interact with this material as if that material wasn’t there.”
Furthermore, she adds, the current device isn’t designed to hide an object from anything other than a narrow band of frequencies. Many complications come into play for a cloak that works across a broad range.
Hiding objects from sonar isn’t the only possible use of this technology, she says. It may also be possible to use it to create acoustic lenses and focused beams. This might allow for anything from improved underwater communications to better emitters for medical ultrasound imaging — any situation in which highly directed sound waves might be of value.
Other scientists are excited. Steven Cummer, an engineering professor from Duke University, Durham, North Carolina, says the new research draws from efforts to do the same things in air.
“The physics is the same,” he says. “You need the right material properties to bend the waves around an object so there is no reflection and no shadow.”
The basic need, he says, is for a property called anisotropy, in which waves travel with dramatically different speed in different directions.
“That’s why the structure in Hanford’s work is based on plates,” he says. “The sound speed is different for propagation parallel to and perpendicular to the plates. If you can tailor those speeds just right, you can make an effective sound cloak.”
Paul Kinsler, a physicist from Lancaster University, UK, agrees. “I haven’t seen this done in water before,” he says.
But he has seen plenty of efforts to do similar things in other settings.
“What a cloaking device does is to steer incoming waves around an object, whilst also conspiring to reintegrate them on the other side, so that to a distant observer nothing seems to have happened,” he explains.
But if Hanford and other researches really want to put their approach the test, he says, it might be useful to look to nature, rather than the lab: “It would be interesting to know if you could make an underwater cloak like this good enough to fool a dolphin.”