Acoustic metamaterials are mesostructures (of a scale between microscopic and macroscopic) that are able to control, manipulate and direct sound waves by changing their underlying characteristics. In recent years, advances in these structures have given rise to so-called phononic materials that can actively sculpt the flow of sound waves in highly specified ways.
For example, phononic materials that offer zero or even negative refraction can control sound at the subwavelength scale, allowing for precise control of the sound field and acoustic energy.
This opens up a range of practical possibilities, including technologies such as acoustic camouflage. It also provides a means to explore quantum mechanics, by understanding more about the underlying physical properties of the sound waves themselves.
“We wanted to discover entirely new properties in sound wave physics,” explains Johan Christensen, research fellow at the University Carlos III of Madrid. “The other motivating factor was to improve certain technological aspects for acoustics in our daily life,” he notes.
These two strands formed the backbone of the EU-funded PHONOMETA project, supported by the European Research Council. “This means taking a deep look into quantum mechanics and, using some of the knowledge from this field, attempting to transfer it to acoustics,” adds Christensen, who acted as PHONOMETA project coordinator.
Diving into theoretical physics
The PHONOMETA project created a class of artificial materials known as PT symmetric systems. PT symmetry is a fundamental concept in quantum mechanics, in which a system would evolve in exactly the same way whether time ran forward or in reverse, while being transformed into its mirror image.
In acoustics, PT symmetric systems are artificial materials which contain gain units (that amplify sound waves) and loss units (that reduce the wave amplitude).
“It is the combination of both, a concept borrowed from quantum mechanics, that leads to highly unusual wave propagation characteristics for sound waves,” explains Christensen.
Loss units are easy to produce in acoustics: any sponge or foam-like material causes sound to lose energy. But the gain component is difficult. In optics, this could be done using a laser. But for acoustics, this sort of technology doesn’t exist.
The PHONOMETA project originally planned to use piezoelectric semiconductors, materials which can cause some kind of acoustic gain when an electric field is applied to them.
During the later stages, the team discovered that carbon nanotube films were better candidates. These films, made up of a stack of graphene sheets, have exceptional electrical, mechanical and optical properties providing higher levels of control. Christensen and his team used these materials as the gain component in the PT symmetric systems that they created.
An inaudibility cloak
One of the key contributions of the project was to harness the reflectionless properties in these new materials. At a certain threshold when loss and gain contrast is increased, the material appears acoustically transparent, where it generates no echoes, and is effectively rendered invisible. The team took this concept further, to create an invisibility cloak.
“For centuries people have dreamt of an invisibility cloak that can make someone indiscernible for the naked eye when hidden underneath it,” remarks Christensen. In optics, this would mean suppressing not just the reflected light back from an object, but also the shadow behind it to make it completely disappear.
The team managed to do exactly that acoustically, hiding an object the size of a refrigerator. “Acoustic gain and loss are combined to produce zero reflection and a diminishing effect of the acoustic shadow,” says Christensen. If developed further, the technology could be useful for providing drastically improved stealth technology for larger objects such as submarines.