Sonic crystals make the sound barrier
Sep 7, 2000
Physicists at Hong Kong's University of Science and Technology have fabricated 'sonic crystals' a few centimetres in size that are capable of blocking out everyday noise. Zhengyou Liu and co-workers found that a 2 cm slab of the crystals can absorb sound that would normally require a much thicker acoustic shield. The new material breaks this 'mass density' law by at least one order of magnitude at 400 Hz, which corresponds to a wavelength of about 80 cm (Zhengyou Liu et al 2000 Science 289 1734). Human hearing covers the range 20 Hz to 20 kHz.
The crystals themselves are constructed from lead balls 1 cm in diameter, coated with a 2.5-mm layer of silicone rubber, and placed inside an epoxy matrix. The strong periodic variation in density creates spectral gaps that prevent the transmission of waves. This is analogous to the attenuation of electromagnetic waves in photonic crystals. Liu's team placed a sound source near the crystal and compared the amplitudes of sound waves at the surface of the crystal and at the centre of the crystal. They found distinct gaps in the range of frequencies transmitted through the crystal. The missing frequencies are absorbed by certain oscillations of the coated spheres, which are like vibrations in molecular crystals.
But to attenuate sound, the lattice spacing inside the crystal must usually be of the same order as the sound's wavelength, and for environmental noise the crystals would need to be metres across. Liu's team overcame this problem by creating disordered composites of the crystals. The local resonant properties of the disordered composite give it a negative elastic constant so that the absorption of sound increases exponentially with the thickness of the material.
The size and geometry of the crystals can be tuned to absorb different wavelengths. Future development of the sonic crystals to extend their frequency range may lead to applications in seismic wave reflection and ultrasonics.