Page 45 - Spring 2006

Acoustics in the seventies
Thomas D. Rossing
As I write this, I am packing to move to Stanford, and by the time you read it, I will be a Californian. Although packing up an office is a chore, it is also sort of fun to go through 35 years of papers and mementos.
I just came across the little booklets that the American Institute of Physics used to distribute annually to the media: “Physics in 1970,” etc. The sub-fields of physics were listed alphabetically, and so Acoustics came first.
Big news in 1970 was the use of light to amplify sound waves. Edward Cassedy and Martin Piltch flashed the light from a ruby laser into a birefringent quartz crys- tal and amplified an ultrasonic pulse about ten times. The amplification is similar to an effect known as stimulated Brillouin scattering. Another bit of news was that Robert Beyer and Joie Jones showed that two beams of sound interact only when they are collinear, thus substantiating the theory of Peter Westervelt.
In 1971 William Rhode and Brian Johnstone inde- pendently used the Mössbauer effect (gamma-ray reso- nance spectroscopy) to observe the motion of the inner ear and showed that the motion of the basilar membrane is not linear. Elsewhere researchers used liquid-surface ultrasonic holography to produce acoustic holograms of living as well as preserved specimens.
Two top stories in 1972 dealt with aids for the hearing impaired. Researchers at the Callier Hearing and Speech Center in Dallas have experimented with a system that replaces the headphone of a hearing aid with a coil of wire and a permanent magnet glued to the eardrum. Magnetically coupled hearing aids permit placement of the entire device at ear level without danger of acoustic feed- back and preserve sound localization ability. The other newsworthy device was the artificial cochlea developed by Martin Sonn, Wolfgang Feiset and Geza Jako. A 35-mil- limeter-long array of 37 electrodes is implanted in the cochlea to set up electric fields in close proximity to viable nerve fibers along the basilar membrane.
Acoustics does not appear in the 1973 booklet, but in the 1974 edition, infrasound and scattering of sound on
Members of the Technical Program Organizing Committee for the Providence meeting.
sound are discussed. Westervelt showed that it is possible to use nonlinear sound interactions to produce the absorption of sound by sound and the displacement of a liquid-gas sur- face under the influence of acoustic radiation pressure, sur- face tension, and gravity. One application of this is the attenuation of sound in superfluid helium.
Applications of acoustic holography in the medical field were big news in 1975. Single-frequency ultrasonic waves transmitted through the body can be made to interact, at a liquid surface, with a holographic reference wave. Laser light reflected off the rippled surface recon- structs the transmitted image. Certain tumors are more readily detected by ultrasonic holography than with x- rays. Also in 1975, Floyd Dunn and John Brady showed how the ultrasonic absorption coefficient increases with increasing temperature, and Thomas Muir and Charles Culbertson showed that modulated neodymium glass lasers fired into a freshwater lake generated highly direc- tive pure-tone sounds.
In 1976 terahertz phonons were generated by two dif- ferent methods: using super-conducting film tunnel junc- tions; and applying short infrared laser pulses to a piezo- electric crystal. The latter method produced phonons with frequencies up to 2.5 THz. These match some of the lower frequency properties of crystal lattices. Also in 1976, Ochs, Snowdon, and Kerlin studied the effect of ribs on the vibra- tion modes and the transmissibility of flat plates.
In 1978 Philip Marston extended Robert Apfel’s methods for acoustically levitating liquid drops to observe shape oscillations in drops. Modulating the acoustic waves at a frequency near the natural frequency of quadrupole shape oscillations (typically a few hundred hertz) significantly enhances the amplitude of oscillation. Drop oscillations were observed by using “rainbow inter- ferometry” which makes use of light reflected and refracted by the drops. Also in 1978 Lawrence Sulak and colleagues detected sound generated thermally by proton pulses in water. Such thermooptic techniques are useful for detecting muons and neutrinos deep under water.
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