Ultrasonic Bandgap Materials

Ultrasonic bandgap materials are to sound waves what semiconductors are to electrons and photonic bandgap materials to light waves: they allow some energies (or frequencies) and not others. The hope is to fabricate the acoustic equivalent of various electronic or optical elements, such as mirrors, lenses, even switches and "transistors" in some future acoustic integrated circuit.

The trouble is that, as with the optical counterpart, it has been difficult to achieve full exclusion of certain acoustic frequency bands in "phononic" materials. Pressing ahead anyway, a group of physicists in Spain have produced an ultrasonic wedge which, even without perfect acoustic bandgap performance, can split a beam of sound waves or steer the sound through an angle of 90 degrees.

At the Instituto de Fisica Aplicada in Madrid (contact Jose Aragon, joseluis@iec.csic.es, 34-915-618-806 x 251) researchers create a material consisting of mercury cylinders inserted into a slab of aluminum (see figure at Physics News Graphics). The researchers noticed that in refracting through their device the sound waves did not conform to Snell's law, the classical equation governing the propagation of waves from one medium into another, a phenomenon (probably related to the interaction between the waves and the compound crystalline environment of the wedge) which might be applicable to the case of light waves. (Torres et al., Physical Review Letters, 7 May 2001; text at Physics News Select).