Page 49 - Spring 2006

Scanning the Journals
quencies that are up to 2 times higher than phonon frequen- cies. Observation of these peaks requires that there be spec- tral components of atom velocities at these higher frequen- cies around the shock front, which requires that the shock front be very sharp.
􏰀Phononic crystals are the subject of a feature review in the December issue of Physics World, Phononic crystals are novel materials that offer exceptional control over sound and other mechanical waves. The writers characterize them as “a revolution in acoustics.” When a wave passes through a phononic crystal, interference leads to the formation of “band gaps” that prevent waves with certain frequencies from traveling through the structure. The periodic variation in the density and/or speed of sound that is needed to make a phononic crystal can be achieved by making air holes in an otherwise solid structure. This leads to the phenomenon of “negative refraction” which can be exploited to make super lenses that can beat the diffraction limit. Controlling the dis- persion relation for phonons both inside and outside the band gap could lead to breakthroughs in both fundamental research and applications.
􏰀 Sound waves in a fluid behave much like light waves in space. Black holes even have acoustic counterparts, accord- ing to an article in the December issue of Scientific American. Like spacetime, condensed matter looks like a continuum when viewed at large scales, but unlike spacetime it has a well-understood microscopic structure governed by quantum mechanics. Moreover, the propagation of sound in an uneven fluid flow is closely analogous to the propagation of light in a curved spacetime. By studying a model of a black hole using sound waves, physicists are attempting to gain insight into the possible microscopic workings of spacetime. Spacetime, like a material fluid, may be granular and possess a preferred frame of reference that manifests itself on fine scales, contrary to Einstein’s assumptions.
􏰀 Researchers have linked the hormone aldosterone to the quality of hearing as we age, according to a paper in the November issue of Hearing Research. The more of the hor-
mone that older people have in their bloodstream, the better their hearing is. The hormone is known to also regulate kid- ney function and plays a role in controlling levels of two cru- cial chemicals in the nervous system, potassium and sodium. For nerves to send signals crisply and work properly, these two chemicals must be in precise proportion. Levels of potas- sium are particularly crucial in the inner ear. Of particular interest to ASA members is the fact that two of the authors of the paper are the father and son team D. Robert Frisina and Robert D. Frisina who are both members of the ASA.
􏰀 A new report on bubble fusion appears in the 21 January issue of New Scientist. Rusi Taleyarkhan and his colleagues at Purdue University report on new experiments in which the external neutron beam has been eliminated and bubbles are induced in dissolved uranyl nitrate by alpha particles from the radioactive decay of uranium. The researchers got the same results with deuterated water, acetone, and benzene. To answer critics, the researchers used four different types of particle detectors and reported seeing 5000 to 7000 neutrons per second at the energy levels expected for deuterium-deu- terium fusion.
􏰀 Gaze control circuitry in the forebrain of barn owls medi- ates spatial attention for audition as well as for vision, according to a paper in the January issue of Nature. When the forebrain circuit was activated, midbrain responses to auditory stimuli at the location encoded by the forebrain site were enhanced and spatial selectivity was sharpened. The sharpening of auditory tuning curves that resulted from gaze fields activation in barn owls is considered quite remarkable, because auditory tuning curves are the sharpest observed in any part of the brain in any species. The sharpening is caused by an enhancement of responses specifically to interaural timing differences.
􏰀 The January/February issue of Acta Acustica/Acustica includes a review paper of 15 pages entitled “A Tutorial on “Scattering and Diffusion Coefficients for Room Acoustic Surfaces” by T. J. Cox, B.-I. L. Dalenback, P.D'Antonio, J. J. Embrechts, J. Y. Jeon, E. Mommertz, and M. Vorländer.
Acoustics in the news
􏰀 A sonic device known as a Long Range Acoustic Device (LRAD) helped ward off a pirate attack on the cruise ship Seabourn Spirit off the coast of Somalia, according to a story in the November 8 issue of the Chicago Tribune. The device, developed for the military after the 2000 attack on the USS Cole in Yemen, uses a narrow intense beam of sound in order to keep small boats from approaching U.S. warships. Although within the beam sounds can reach 150 dB, neither the LRAD operators or others in the immediate area are in danger.
􏰀 The ability of some large animals to hear infrasound may allow them to anticipate natural disasters such as tsunamis before humans can, according to a recent television program
on PBS. This ability may give elephants and other animals enough time to react and flee to safety. On the morning of December 26, 2004, for example, workers at an elephant camp in Thailand were awakened by the trumpeting and wailing of elephants. The huge animals broke their chains and stampeded up a nearby hill. Moments later a terrifying sound overtook them: the sound of a towering wave of water crash- ing ashore and overwhelming everything in its path.
􏰀 People who have a difficult time hearing conversations in the midst of cocktail party chatter may benefit from an ear training program developed at the University of California, San Francisco, according to a story in the November issue of
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