Physicists in Hong Kong have come up with a new material for soundproofing. Their "sonic crystals" are made of lead balls, about 1 cm in diameter, coated in silicone rubber and mounted in an epoxy matrix. They say a layer of this material only 2 cm thick could block out everyday noise. The periodic variation of density inside the crystal means certain frequencies of sound can't propagate through it - exactly like the blocking of electromagnetic radiation inside photonic crystals (May p9). The researchers built up disordered composites of the crystals so that local resonant properties could even block long-wavelength environmental noise within a small thickness.