In a testimony to the speed at which science is progressing, two separate groups, one in China and the other in Canada, have reached the same conclusion – that it is possible to observe negative refraction with sound waves.
For their part, Xiangdong Zhang of Beijing Normal University and Zhengyou Liu of Wuhan University have used computer simulations to design 2D ‘phononic’ crystals that behave like the ‘left-handed’ optical materials that are able to negatively refract light.
A ‘phononic’ or ‘sonic’ crystal is the acoustic equivalent of a photonic crystal – a material that contains a periodic arrangement of air-filled voids that have a lower refractive index than the host material. It is the periodic variation of the refractive index that creates an optical band gap in the photonic crystal, which means that only certain wavelengths of light are able to pass through it.
Similarly, phononic crystals contain acoustic gaps, which means that only certain wavelengths of sound can pass through the material.
So much for the theory. In Canada, John Page of the University of Manitoba and his colleagues have actually created a simple phononic crystal from tungsten carbide beads that actually demonstrates the effect.
When the researchers looked at the crystal’s effect on ultrasound frequencies at 1.60MHz, generated by a small source placed just above an 8mm-thick slab of the crystal, the waves spread out as they passed through the phononic material. But sound waves just slightly lower in frequency, at 1.57MHz, behaved in quite the opposite way – the crystal bent them back toward a focal point below the slab.
Like the researchers in China, the Canadians also created a theoretical model of the propagation of signals through the crystal and got ‘tremendous agreement’ with the experimental results.
<b>More information on the Chinese work can be found on the Institute of Physics site <link>here=http://www.iop.org/news/782</link>. Details of the Canadian research are on the Physical Review Focus site <link>here=http://focus.aps.org/story/v14/st3</link>.</b>