Page 48 - Spring 2006

Scanning the Journals
􏰀 Physicist Seth Putterman, well known to ASA members, is the subject of a biographical feature in the 27 October issue of Nature. “Ignoring the mainstream of physics,” says the article, “Seth Putterman has a knack for bringing long-for- gotten mysteries back to the fore.” A case in point is sonolu- minescence, light generated by sound, which was known as long as 60 years ago but has recently become a “hot topic” in physics (see ECHOES Winter 1993, Spring 1997, Fall 1997, Winter 1998, Spring 2002, and Summer 2003, for example). Putterman firmly believes that the flash seen at the center of the bubble is created by electrons being shaken out of their atomic orbits, whereas others suspect more conventional chemistry is the culprit.
􏰀 Micromachined fluid-filled variable impedance wave- guides intended to mimic the mechanics of the passive mammalian cochlea have been fabricated, according to a paper in the January 21 issue of Proceedings of the National Academy of Sciences. Experimental tests demonstrate acoustically excited traveling fluid-structure waves with phase accumulations between 1.5 and 3 π radians at the loca- tion of maximum response. The achieved orthotropy ratio of 8:1 in tension is insufficient to produce the sharp filtering observed in animal experiments and many computational models that use higher ratios. A mathematical model incor- porating a thin-layer viscous, compressible fluid coupled to an orthotropic membrane model is validated.
􏰀 Bright and responsive “ultralight” violins may be the instruments of the future, according to an article in the 2 December issue of Science. The article reports mainly on the 33rd annual convention of the Violin Society of America held in King of Prussia, PA in November. Joseph Curtin, an Ann Arbor violin maker who recently won a MacArthur Foundation Fellowship (see Winter 2006 issue of ECHOES), is one of the makers featured in the article. Curtin was a pre- senter at the special session and workshop on Design and Construction of String Instruments at the ASA meeting in Vancouver (see Fall 2005 issue of ECHOES). Balsa wood and carbon-fiber composites are materials that have been used for experimental ultralight violins. Although few people will agree with Fan-Chia Tao that “Within a generation, the wood violin will be as obsolete as the wooden tennis racket or the wooden golf club,” makers such as Curtin feel that some things in the traditional violin design can be improved. ASA members Carleen Hutchins, Gabriel Weinreich, and Norman Pickering are quoted in the article.
􏰀 Crowded footbridges can exhibit lateral synchronous excitation, according to an article in the 3 November issue of Nature. This is not unlike the collective synchronization of biological oscillators such as neurons and fireflies. One example was observed on London’s Millennium Bridge on opening day, when it began to sway from side to side. Pedestrians fell spontaneously into step with the bridge’s vibrations, inadvertently amplifying them. Generalizing the ideas developed in mathematical biology can provide a uni-
Thomas D. Rossing
fied picture of what happened on the Millennium bridge five years ago, both for the bridge vibrations and the crowd dynamics.
􏰀 Physicists in France have developed a new form of “touch- screen” technology that relies on detecting sound waves and locating their source by time reversal, according to a paper in the 14 November issue of Applied Physics Letters. Time reversal in acoustics (see Winter 2002 issue of ECHOES) is an efficient way to focus sound back to its source in a wide range of materials including reverberating media. A wave still has the memory of its source location. The technique has been demonstrated in a glass plate 400x300x5 mm. Tapping the plate at various positions and detecting the resulting sound waves with a simple sensor connected to a personal computer transforms the plate into an interactive surface. The number of possible touch locations is shown to be directly related to the mean wavelength of the detected acoustic wave.
􏰀 Congenital deafness results in abnormal synaptic structure in auditory nerve endings. If these abnormalities persist after restoration of auditory nerve activity by a cochlear implant, the processing of speech signals would likely be impaired. A research report in the 2 December issue of Science describes experiments in which deaf cats were stimulated for 3 months with a six-channel cochlear implant. Auditory nerve fibers exhibited a recovery of normal synaptic structure in these cats. This rescue of synapses is attributed to a return of spike activity in the auditory nerve and may help explain cochlear implant benefits in childhood deafness.
􏰀 “Singing icebergs” is the subject of an article in the 25 November issue of Science. Seismic tremors were recorded near the continental margin of Dronning Maud Land, Antarctica whose spectra consisted of narrow peaks with a fundamental frequency around 0.5 Hz and more than 30 harmonic overtones. The spectral peaks varied slightly with time (frequency gliding), and amplitude was inversely pro- portional to frequency. The tremor signals change from har- monic to non-harmonic and vice versa. It is proposed that the iceberg tremor signals represent elastic vibrations of the iceberg produced by the flow of water through tunnels and crevasses.
􏰀 People regularly exposed to loud noise over several years are more likely to develop a benign tumor called an acoustic neuroma that causes hearing loss according to a paper in the February 15 issue of the American Journal of Epidemiology. People exposed to loud music were most likely to develop acoustic neuroma, while those exposed to machines, power tools and construction were slightly less likely, according to a study at Ohio State University.
􏰀 It should be possible to produce coherent light with fre- quencies of 20 THz or more by subjecting crystals to shock waves, according to a paper in the 13 January issue of Physical Review Letters. Emission peaks would have fre-
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46 Acoustics Today, April 2006