Volume 110, Issue 6, December 2001
Index of content:
- GENERAL LINEAR ACOUSTICS 
110(2001); http://dx.doi.org/10.1121/1.1413753View Description Hide Description
The use of piezoelectric transducer arrays has opened up the possibility of electronic steering and focusing of acoustic beams to track kidney stones. However, owing to the limited pressure delivered by each transducer (typically 10 bar), the number of transducers needed to reach an amplitude at the focus on the order of 1000 bars is typically of some hundreds of elements. We present here a new solution based on 1-bit time reversal in a solidwaveguide to obtain, with a small number of transducers, a very high amplitude pulse in tissues located in front of the waveguide. The idea is to take advantage of the temporal dispersion in the waveguide to create, after time reversal, a temporally recompressed pulse with a stronger amplitude. The aim of this work is threefold: first, we experimentally demonstrate 1-bit time reversal between a point source in water and several transducers fastened to one section of a finite-length cylindrical waveguide. Second, we numerically and experimentally study the temporal and spatial focusing at the source as a function of the characteristics of the “solid waveguide–time reversal mirror (TRM)” system: length and diameter of the guide, number of transducers of the TRM. Last, we show that the instantaneous power delivered in water at the focus of the solidwaveguide is much higher than the power directly transmitted into water from a classically focused transducer. The combination of 1-bit time reversal and a solidwaveguide leads to shock wave lithotripsy with low-power electronics.
110(2001); http://dx.doi.org/10.1121/1.1413997View Description Hide Description
Scattering from a ribbed finite cylindrical shell is investigated with a theoretical formalism valid for long cylinders. An analytic expression is derived for the scattered pressure and it is shown that the scattered far field can be expressed as the sum of three components associated with specular reflection, scattering from helical waves, and Bloch–Floquet waves. Approximate solutions are proposed to reduce the computation load and resonance scattering is analyzed from the derived formulas leading to a relatively simple equation for the calculation of the Bloch–Floquet dispersion curves. Simple equations are proposed to determine the locations of the Bloch–Floquet waves for weak interactions of the ribs and the surface elastic waves. In the first Brillouin zone, it is found that the maximum level of Bloch–Floquet wavescattering depends only on the length of the cylinder as for the resonant modes of uniform cylindrical shells. Finally the relevancy of the method is examined by comparing numerical results to experimental data available in the published literature.
110(2001); http://dx.doi.org/10.1121/1.1419085View Description Hide Description
In recent experiments and numerical studies, a leaky surface wave has been observed at the surface of an isotropic homogeneous elastic solid. This paper gives a detailed description of this leaky surface wave and explains its origin from the fundamental differential equations.Theoretically, the leaky surface wave arises from the complex conjugate roots of the Rayleigh equation. The complex conjugate roots give rise to a wave that propagates along the surface and is coupled to a plane shear wave in the medium. Due to the coupling, the surface wave leaks energy into the medium and is highly inhomogeneous. Its particle motion at the surface is prograde in nature, distinguishing it from the well-known Rayleigh surface wave which causes a retrograde particle motion.
110(2001); http://dx.doi.org/10.1121/1.1412444View Description Hide Description
In this article acoustic scattering by a random rough interface that separates a fluid incident medium from an underlying uniform scattering medium, either fluid or elasticsolid, in cases for which the Bragg scale lies within the power-law tail of the roughness spectrum is dealt with. The physical foundation is an inherently reciprocity-preserving, local small-slope theory. A fully bistatic formulation is developed for the scattering strength, together with a robust numerical implementation that allows a wide range of spectral exponent values. The practical result for ocean acoustics is a significantly improved description of the interface component of sea floor scattering. Calculations are presented to demonstrate the advantage of this approach over perturbation theory, and to illustrate its dependence on frequency and environmental parameters as well as its operation in bistatic geometries.
110(2001); http://dx.doi.org/10.1121/1.1413752View Description Hide Description
It has been noted that the absorption coefficient of a porous material sample placed in a standing wave tube is affected at low frequencies by the nature of the sample’s edge constraint. The edge constraint has the effect of inhibiting the motion of the solid phase of the material. The latter can be strongly coupled to the material’s fluid phase, and hence the incident sound field, by viscous means at low frequencies. Here the absorptioneffect noted earlier was demonstrated experimentally. The main focus of the work, however, was on a corresponding transmission loss effect. The material considered was aviation grade glass fiber in two densities. It was found that the edge constraint results in a shearing resonance of the sample at which frequency the transmission loss is a minimum: below that frequency the transmission loss increases with decreasing frequency to a finite low frequency limit proportional to the sample’s flow resistance. It was found that the constraint effect could be modeled by using a poroelastic finite element model. It was also found that the transmission loss of the edge-constrained samples approximated that of unconstrained samples at frequencies above approximately 100 Hz when measured in a 10-cm-diam tube.