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Journal of the Acoustical Society of America

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Jun 2011

Volume 129, Issue 6, pp. EL217-4101

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A k-space Green’s function solution for acoustic initial value problems in homogeneous media with power law absorption

Bradley E. Treeby and B. T. Cox

J. Acoust. Soc. Am. Volume 129, Issue 6, pp. 3652-3660 (2011); (9 pages) | Cited 1 time

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An efficient Green’s function solution for acoustic initial value problems in homogeneous media with power law absorption is derived. The solution is based on the homogeneous wave equation for lossless media with two additional terms. These terms are dependent on the fractional Laplacian and separately account for power law absorption and dispersion. Given initial conditions for the pressure and its temporal derivative, the solution allows the pressure field for any time t>0 to be calculated in a single step using the Fourier transform and an exact k-space time propagator. For regularly spaced Cartesian grids, the former can be computed efficiently using the fast Fourier transform. Because no time stepping is required, the solution facilitates the efficient computation of the pressure field in one, two, or three dimensions without stability constraints. Several computational aspects of the solution are discussed, including the effect of using a truncated Fourier series to represent discrete initial conditions, the use of smoothing, and the properties of the encapsulated absorption and dispersion.
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43.35.Bf Ultrasonic velocity, dispersion, scattering, diffraction, and attenuation in liquids, liquid crystals, suspensions, and emulsions
43.20.Bi Mathematical theory of wave propagation
43.20.Hq Velocity and attenuation of acoustic waves
43.80.Cs Acoustical characteristics of biological media: molecular species, cellular level tissues

Scattering of obliquely incident shear waves from a cylindrical cavity

John C. Aldrin, Mark P. Blodgett, Eric A. Lindgren, Gary J. Steffes, and Jeremy S. Knopp

J. Acoust. Soc. Am. Volume 129, Issue 6, pp. 3661-3675 (2011); (15 pages) | Cited 1 time

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Prior work has proposed the use of ultrasonic angle-beam shear wave techniques to detect cracks of varying angular location around fastener sites by generating and detecting creeping waves. To better understand the nature of the scattering problem and quantify the role of creeping waves in fastener site inspections, a 3D analytical model was developed for the propagation and scattering of an obliquely incident plane shear wave from a cylindrical cavity with arbitrary shear wave polarization. The generation and decay of the spiral creeping waves was found to be dependent on both the angle of incidence and polarization of the plane shear wave. A difference between the angle of displacement in 3D and the direction of propagation for the spiral creeping wave was observed and attributed to differences in the curvature of the cavity surface for the tangential and vertical (z) directions. Using the model, practical insight was presented on measuring the displacement response in the far-field from the hole. Both analytical and experimental results highlighted the value of the diffracted and leaky spiral creeping wave signals for nondestructive evaluation of a crack located on the cavity. Last, array and signal processing methods are discussed to improve the resolution of the weaker creeping wave signals in the presence of noise.
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43.35.Cg Ultrasonic velocity, dispersion, scattering, diffraction, and attenuation in solids; elastic constants
43.35.Yb Ultrasonic instrumentation and measurement techniques
43.25.Zx Measurement methods and instrumentation for nonlinear acoustics
43.25.Dc Nonlinear acoustics of solids

Characterization of a fiber-optic displacement sensor for measurements in high-intensity focused ultrasound fields

Julian Haller, Volker Wilkens, Klaus-Vitold Jenderka, and Christian Koch

J. Acoust. Soc. Am. Volume 129, Issue 6, pp. 3676-3681 (2011); (6 pages) | Cited 2 times

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A fiber-optic sensor is presented that is capable of measuring the particle displacement in high-intensity focused ultrasound (HIFU) fields. For this probe, a secondary calibration was performed, and the resulting complex frequency response is discussed. As a first practical application, the setup was used to measure the pressure in the field of a weakly focusing ultrasound transducer. The result is compared with that of a membrane hydrophone measurement. The feasibility of measurements in HIFU fields is demonstrated by means of measurements of the spatial distribution of the peak particle velocity within the focus of a HIFU transducer and of the dependence of the peak values on the acoustical power level.
Show PACS
43.35.Yb Ultrasonic instrumentation and measurement techniques
43.35.Sx Acoustooptical effects, optoacoustics, acoustical visualization, acoustical microscopy, and acoustical holography
43.58.Fm Sound level meters, level recorders, sound pressure, particle velocity, and sound intensity measurements, meters, and controllers
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