Page 50 - Fall 2006

Scanning the Journals
detectors is allowing researchers to tune in to our atmos- phere.” Infrasound has been a hot topic in ASA recently, as readers of JASA, Acoustics Today, and ECHOES are well aware. “Infrasound” was the lead article in the January 2006 issue of Acoustics Today, as it was in the Fall 2001 and Fall 2005 issues of ECHOES. Although much of the same material is covered in this article, two items that caught my eye were the “mystery of the Earth’s low-frequency hum,” due to standing Rayleigh waves driven by atmospheric tur- bulence; and the new optical fiber infrasound sensors that get around the use of mechanical filters to reduce back- ground noise. Unlike audible sound, infrasound can travel thousands of kilometers through the atmosphere and is used by some animals as a form of communication.
􏰀 According to a paper entitled “The failure of the Tacoma Bridge: A physical model” in the August issue of the American Journal of Physics, “one of the most surprising of physical phenomena is the conversion of a steady state con- dition into oscillations.” Other examples of self-excited oscillations include the blowing of air through the reed of a clarinet, the flow of air over the embouchure hole of a flute, and the conversion of the steady pull of a violin bow into oscillation of the string. The paper addresses historical mis- conceptions of the 1941 Tacoma Narrows Bridge collapse, a computational model of vortex behavior, and evidence for the correctness of this model. Fluid mechanics has typical- ly been the domain of experimentalists because the govern- ing equations are difficult to solve. Only now, they point out, is computer power becoming equal to the task.
􏰀 “Zap-while-you-scan therapies set sights on cancer” is the title of an article in the August 12 issue of New Scientist describing the use of high-intensity ultrasound to kill tumors. The idea is to use magnetic resonance imaging (MRI) or computed tomography (CT) to create three- dimensional images precise enough to guide a tumor-zap- ping beam of ultrasound to its target. Surgeons can be con- fident they have hit all of the tumor and spared the sur- rounding tissue according to surgeons at the Ontario Cancer Institute in Toronto. The scanner can also measure real- time tissue temperature, so the doctor knows exactly when the target has received a sufficient dose.
􏰀 “Hearing colours, seeing sounds” in the 3 August issue of Nature is a commentary on Kandinsky’s synaesthetic paintings. Reacting to a performance of Wagner’s opera “Lohengrin,” the Russian painter conceived a kind of paint- ing that might aspire to the abstract condition of music. Attempts to devise color notations for music date back to the sixteenth century, and the earliest attempt to construct a color organ was made as long ago as the early eighteenth century. Kandinsky considered color as the “keyboard, the eyes are the hammers, the soul is the piano with many strings.” In 1914 he painted “Fugue,” whose interwoven sequences of colored patterns are explicit representations of musical motifs.
􏰀 Physicists in the US and Japan have found strong evi- dence that phonons play a key role in high-temperature superconductivity, according to a letter in the 3 August issue of Nature. The phonons allow electrons with opposite spins to pair up, which is widely believe to be what happens in conventional low-temperature superconductors, but until now no boson-mediated electron pairing has been observed in high-temperature superconductors. The paired electrons form bosons as they collapse into a single quantum state via a process called Bose-Einstein condensation. By placing the tiny metal tip of a scanning tunneling microscope above the surface of a bismuth strontium calcium copper-oxide (BSCCO) sample, the researchers measured the energy states in the superconductor on an atomic scale. Changes in current as the tip moved just a few nanometers across the sample indicate that the electron-pairing mechanism varies on these tiny scales, and that there is interplay between the paired up electrons and the crystal lattice.
􏰀 A discussion of vibration isolation of precision objects appears in the July issue of Sound and Vbration. The use of “smart” constant natural frequency (CNF) isolators sub- stantially widens the application range of inexpensive pas- sive isolators, which are generally less expensive than active isolation systems. CNF isolators have their stiffness pro- portional to the applied weight load in order to automatical- ly satisfy decoupling conditions.
􏰀 According to an article in the September-October issue of American Scientist, some tadpoles hatch prematurely upon sensing a predator’s vibrations. The red-eyed tree frog, an inhabitant of Central American tropical forests, lays clutch- es of eggs on leaves overhanging ponds, so that when the tadpoles hatch they drop into the water. The usual gestation period is six to eight days, but after four days of gestation the tadpoles will start to hatch if the clutch is attacked by a predator. The eggs apparently sense vibrations as a snake or other predator tears into the clutch. In the laboratory, the highest rate of hatching was induced by signals that had a half-second duration and intervals of 1.5 to 2.5 seconds, which are consistent with snake attacks.
􏰀 At terahertz frequencies, light couples with periodic lat- tice distortions resulting in phonon-polaritons, according to a letter in the 10 August issue of issue of Nature. Polaritons are electro-mechanical excitations in condensed matter that describe light propagation near resonances. In ferroelectric materials, terahertz radiation propagates by driving infrared-active lattice vibrations, resulting in phonon-polaritron waves. The resulting fast motion of these charges has been observed by using femtosecond X- ray diffraction.
􏰀 An ultrasonic device could help athletes who have had their teeth broken while playing high impact sports, according to a note in the 8 July issue of New Scientist. A piezolectric crystal generates low-power ultrasound at about 20 kHz applied to a tooth brace attached to the damaged tooth.
48 Acoustics Today, October 2006