Number 169 (Story #2), March 17, 1994 by Phillip F. Schewe and Ben Stein PHONONIC CRYSTALS would be to sound waves what photonic crystals are to light waves or what semiconductors are to electrons: they would exclude sound waves with a certain range (band gap) of acoustic energies. Scientists at Puebla University in Mexico have proposed ideas on how to engineer such a material from long elastic metal (nickel, say) cylinders arranged in a hexagonal lattice and embedded in a second material (aluminum, say) with different elastic properties. Phononic crystals would be of use in sound transducers and in creating vibrationless environments. (M.S. Kushwaha and P. Halevi, Applied Physics Letters, 28 Feb. 1994.)