MOST INTENSE MANMADE SOUND . The production of sound waves with 1600 times more energy per unit volume than previously achieved has been announced by researchers at this week's meeting of the Acoustical Society of America in San Diego, opening up possible new uses for sound in science and technology. Sound waves, patterns of compression and expansion in a gas such as air, are often created and studied in closed or semi-closed containers called cavities. In the past, attempts to make such sound waves louder (by adding more sound energy into the cavity) would fail beyond a certain point because additional energy would merely lead to the formation of a shock wave which would quickly dissipate the energy as heat. Until the late 1980s, researchers thought shock-wave formation was inevitable. In a new technique called "resonant macrosonic synthesis," Tim Lucas and colleagues at MacroSonix Corporation in Virginia have built cavities with special shapes (horns, bulbs, cones) each tailored to promote certain distinct modes of sound vibration which combine in such a way as to inhibit the creation of shock waves, allowing sound waves of unprecedented energy density to build up. Filling the containers with gas, and vibrating them to generate sound waves inside, the researchers produced sound waves with oscillating pressures up to 500 pounds per square inch. The first technological application for these powerful sound waves will be in an "acoustic compressor" which uses sound rather than moving parts to compress gas inside refrigerators and air conditioners. (Images at Physics News Graphics.)

A PHOTONIC HALL EFFECT AND PHOTONIC MAGNETORESISTANCE, the optical analogs of phenomena usually associated with electrons moving in solids, have been observed in an experiment involving light beams diffusing through powders (Physics World, November 1997). When electrons flowing through a material are subjected to a magnetic field, the electrons will feel a new force (the Lorentz force) and be deflected in a direction perpendicular both to their original direction and to the field. Photons are not charged and so do not feel the Lorentz force directly. But the field can establish a nonuniform index of refraction in a powdery medium consisting of cerium-fluoride particles with a definite handedness. When circularly polarized light enters this medium it gets deflected. This magnetically induced transverse diffusion of light was observed by scientists in Grenoble, France (Nature, 2 May 1996). A year later the same scientists reported that the transmission of light through a powder of europium-fluoride particles was proportional to the strength of an applied magnetic field---in effect the photonic equivalent of magnetoresistance (Sparenberg et al., Physical Review Letters, 28 July 1997).

SULPHUR SUPERCONDUCTIVITY . Squeezed in a diamond anvil press, sulphur undergoes a number of changes, including a transition from insulator to conductor at a pressure of 90 giga-Pascals (1 GPa is about 10,000 atmospheres). Scientists from the Institute of High Pressure Physics in Troitsk, Russia and the Carnegie Institution in Washington, DC have squeezed harder still and made sulphur into a superconductor. Above 162 GPa the superconducting transition temperature went up to 17 K, the highest for any elemental solid. (Struzhkin et al., Nature, 27 November 1997.)