Physicists have generated a lot of excitement in recent years by dreaming up specially structured materials with novel applications like invisibility cloaks. What’s more, some of these “metamaterials” have been built in the laboratory and shown to work over a narrow range of electromagnetic wavelengths. Now, a group of researchers from Korea and China has created an acoustic metamaterial that causes the bizarre effect of a reverse Doppler effect. This is an important stepping stone to an acoustic cloak, according to the researchers.

As every physicist is taught in school, the Doppler effect is what causes a pedestrian to hear a high pitch siren as a police car speeds towards them, and a lowering pitch as it races away. Surprisingly, a new material has defied physics textbooks by reversing this effect.

Chul Koo Kim of Yonsei University and his colleagues have achieved this feat by creating an elastic tube that transmits sound with a negative phase velocity. “We have successfully fabricated an acoustic metamaterial whose acoustic refractive index can be controlled; the theoretical models can now be implemented to realize acoustic cloaking as well as other applications,” Kim told physicsworld.com.

Witchcraft and wizardry

In 2006 a group at Duke University, North Carolina, captured public interest when they demonstrated a trick previously confined to the pages of Harry Potter books. Led by David Smith, they created a cylinder from artificial “metamaterials” capable of hiding an object from microwave radiation — waves were literally “steered” around the object as if it wasn’t there.

Another bizarre optical effect to be demonstrated in the past few years is “negative refraction”: light passing between two media, including a metamaterial, is bent in the opposite direction to classic refraction. This effect is most pronounced when the metamaterial is “double negative”, possessing both negative electric permittivity and magnetic permeability.

This research is an important breakthrough Jose Sanchez Dehesa, University of Valencia

This latest research takes the principles of negative electromagnetic refraction and applies them to acoustic vibrations. Here the parameters to be made negative are material density and modulus, the latter relating to a material’s elasticity. Until now engineers have only created metamaterials with either of these properties, but Kim and colleagues have successfully combined them to create the world’s first “double-negative” acoustic metamaterial (arXiv:0901.2772v2 . Their acoustic tube is constructed from thin membranes under tension fed by a carefully controlled air flow, and this manages to create a negative phase velocity for sound travelling through.

Sound is passed into the tube from a moving source via holes pierced periodically along the device. Inside the tube a fixed detector receives the sound before sending an electrical signal to a loudspeaker. According to Kim, the major engineering hurdle was to develop effective absorbers at each end of the tube. “This enabled us to so as to prevent reflections and ensure the quality of data,” he said.

New sound

Kim and colleagues tested the apparatus using sound of 350 Hz with a source moving 5 m/s towards then away from the direction of wave propagation. Contrary to classic Doppler experiments they found that frequency was down-shifted as the source moved towards the receiver and up-shifted as it moved away from it. What the experimenters heard was a decreasing pitch as the source moved towards the detector and increasing one as it moved away from it.

Kim told physicsworld.com that the next stage of this research is to translate their “1D design” into various types of 2D and 3D acoustic metamaterials. “These developments may find uses in medicine and industry. Also easy control of acoustic refractive index will spur new research directions in fiber acoustics,” he said.

Jose Sanchez Dehesa, a metamaterials researcher of the University of Valencia said, “This research is an important breakthrough; if we can now shift this structure to 2D and 3D it could be used to achieve things like sub-wavelength resolution in ultrasonic imaging and many other interesting devices.”