Index of content:
Volume 103, Issue 5, May 1998
- ULTRASONICS, QUANTUM ACOUSTICS, AND PHYSICAL EFFECTS OF SOUND 
103(1998); http://dx.doi.org/10.1121/1.422757View Description Hide Description
The pressure dependence of the resonance frequency of several resonant ultrasound spectroscopy modes in a sample of fused silica has been measured at UCLA in atmospheres of air, helium, and argon near ambient temperature. For both compressional and torsional modes, the radiation resistance is linearly dependent upon pressure and increases with the molecular mass of the surrounding gas. The effects are larger for breathing modes than for torsional modes. They also increase with the molecular mass of the gas. A radiation impedance model is presented which explains some of these data qualitatively and quantitatively.
103(1998); http://dx.doi.org/10.1121/1.422758View Description Hide Description
In many cases, it is convenient to have approximate relations to evaluate thermoacoustic gain and efficiency. G. W. Swift [J. Acoust. Soc. Am. 84, 1145–1180 (1988)] has provided such relations for short stacks in a standing wave. This paper describes an approximation method based on a finite difference form of the thermoacousticequations and provides results for thermoacoustic engines or refrigerators for both standing and traveling waves. The short stack assumption is retained. The method is limited at present to linear temperature gradients across the stack. The calculations are compared to measurements and in the appropriate limits to Rott’s boundary conditions for a temperature discontinuity [N. Rott, Adv. Appl. Mech. 20, 135–175 (1980)]. The approximation is applied to both thermoacoustic prime movers and thermoacousticrefrigerators.
Focusing and steering through absorbing and aberrating layers: Application to ultrasonic propagation through the skull103(1998); http://dx.doi.org/10.1121/1.422759View Description Hide Description
The time-reversal process is applied to focus pulsed ultrasonicwaves through the human skull bone. The aim here is to treat braintumors, which are difficult to reach with classical surgery means. Such a surgical application requires precise control of the size and location of the therapeutic focal beam. The severe ultrasonic attenuation in the skull reduces the efficiency of the time reversal process. Nevertheless, an improvement of the time reversal process in absorbing media has been investigated and applied to the focusing through the skull [J.-L. Thomas and M. Fink, IEEE Trans. Ultrason. Ferroelectr. Freq. Control 43, 1122–1129 (1996)]. Here an extension of this technique is presented in order to focus on a set of points surrounding an initial artificial source implanted in the tissue volume to treat. From the knowledge of the Green’s function matched to this initial source location a new Green’s function matched to various points of interest is deduced in order to treat the whole volume. In a homogeneous medium, conventional steering consists of tilting the wave front focused on the acoustical source. In a heterogeneous medium, this process is only valid for small angles or when aberrations are located in a layer close to the array. It is shown here how to extend this method to aberrating and absorbing layers, like the skull bone, located at any distance from the array of transducers.
103(1998); http://dx.doi.org/10.1121/1.422760View Description Hide Description
An electromagneticacoustic transducer has been developed for line-focusing the shear-vertical wave in a metal. The EMAT consists of a permanent magnet to supply the bias magnetic field normal to the surface, and a meanderline coil to induce the dynamic field and eddy currents in the surface region of the sample. The meanderline spacing is continuously changed so that the generated waves from all segment sources become coherent on the focal line after traveling oblique paths. The printed circuit technique enables the fabrication of such a functionally spaced meanderline coil within 1-μm accuracy. The directivity is measured using a half-cylindrical sample of an aluminum alloy, which shows a much sharper radiation pattern of the focusing EMAT than the EMAT having a meanderline coil of constant spacings. This EMAT is then tested through detecting a shallow notch. The scattered signal from the notch is received by the same EMAT, which shows enough strength even for the notch of 0.2-mm depth.
103(1998); http://dx.doi.org/10.1121/1.422761View Description Hide Description
Among the various techniques available, ultrasonicLamb waves offer a convenient method of evaluating composite materials. Since the Lamb wavevelocity depends on the elastic properties of a structure, an effective tool exists to monitor damage in composites by measuring the velocity of these waves.Lamb wavemeasurements can propagate over long distances and are sensitive to the desired in-plane elastic properties of the material. This paper describes two studies which monitor fatigue damage and two studies which monitor thermal damage in composites using Lamb waves. In the fatigue studies, the Lamb wavevelocity is compared to modulusmeasurements obtained using strain gage measurements in the first experiment and the velocity is monitored along with the crack density in the second. In the thermal damage studies, one examines samples which were exposed to varying temperatures for a three minute duration and the second includes rapid thermal damage in composites by intense laser beams. In all studies, the Lamb wavevelocity is demonstrated to be an excellent method to monitor damage in composites.