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Journal of the Acoustical Society of America

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Jun 2010

Volume 127, Issue 6, pp. EL235-3881

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The scattering of the fundamental torsional mode from axi-symmetric defects with varying depth profile in pipes

R. Carandente, J. Ma, and P. Cawley

J. Acoust. Soc. Am. Volume 127, Issue 6, pp. 3440-3448 (2010); (9 pages)

Online Publication Date: 09 Jun 2010

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Guided ultrasonic waves have been successfully applied to detect defects and corrosion in plates and pipes; however, defect sizing remains challenging due to the complex profiles of the defects encountered in practice. A study of the reflection of the fundamental torsional mode T(0,1) from an axi-symmetric defect with varying depth profile has been carried out via finite element modeling and experimental validation. Defects with gradually varying depth are represented by a series of tapered steps. It is revealed that, for both tapered up- and down-steps, the wave packet is reflected only at the start and end of the steps. The reflections from the start and end of the step have been predicted, and the frequency dependence of the overall reflection from the step has been studied. The study shows that the reflection coefficient varies as a function of the ratio of the average length of the tapered defect to the wavelength due to the interference between the waves reflected from the two ends of the defect. The reflection coefficient maxima decrease as the slope of the taper becomes more gradual, this effect being more pronounced when the ratio of the average defect length to the wavelength increases.
Show PACS
43.35.Cg Ultrasonic velocity, dispersion, scattering, diffraction, and attenuation in solids; elastic constants
43.35.Zc Use of ultrasonics in nondestructive testing, industrial processes, and industrial products
43.20.Mv Waveguides, wave propagation in tubes and ducts

Determination of postexcitation thresholds for single ultrasound contrast agent microbubbles using double passive cavitation detection

Daniel A. King, Michael J. Malloy, Alayna C. Roberts, Alexander Haak, Christian C. Yoder, and William D. O’Brien, Jr.

J. Acoust. Soc. Am. Volume 127, Issue 6, pp. 3449-3455 (2010); (7 pages) | Cited 1 time

Online Publication Date: 09 Jun 2010

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This work presents experimental responses of single ultrasound contrast agents to short, large amplitude pulses, characterized using double passive cavitation detection. In this technique, two matched, focused receive transducers were aligned orthogonally to capture the acoustic response of a microbubble from within the overlapping confocal region. The microbubbles were categorized according to a classification scheme based on the presence or absence of postexcitation signals, which are secondary broadband spikes following the principle oscillatory response of the ultrasound contrast agent and are indicative of the transient collapse of the microbubble. Experiments were conducted varying insonifying frequencies (0.9, 2.8, 4.6, and 7.1 MHz) and peak rarefactional pressures (200 kPa to 6.2 MPa) for two types of contrast agents (Definity® and Optison™). Results were fit using logistic regression analysis to define pressure thresholds where at least 5% and 50% of the microbubble populations collapsed for each frequency. These thresholds were found to occur at lower pressures for Definity than for Optison over the range of frequencies studied; additionally, the thresholds occurred at lower pressures with lower frequencies for both microbubble types in most cases, though this trend did not follow a mechanical index scaling.
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43.35.Ei Acoustic cavitation in liquids

Transient cavitation in high-quality-factor resonators at high static pressures

D. Felipe Gaitan, Ross A. Tessien, Robert A. Hiller, Joel Gutierrez, Corey Scott, Henry Tardif, Brant Callahan, Thomas J. Matula, Lawrence A. Crum, R. Glynn Holt, Charles C. Church, and Jason L. Raymond

J. Acoust. Soc. Am. Volume 127, Issue 6, pp. 3456-3465 (2010); (10 pages) | Cited 6 times

Online Publication Date: 09 Jun 2010

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It is well known that cavitation collapse can generate intense concentrations of mechanical energy, sufficient to erode even the hardest metals and to generate light emissions visible to the naked eye [sonoluminescence (SL)]. Considerable attention has been devoted to the phenomenon of “single bubble sonoluminescence” (SBSL) in which a single stable cavitation bubble radiates light flashes each and every acoustic cycle. Most of these studies involve acoustic resonators in which the ambient pressure is near 0.1 MPa (1 bar), and with acoustic driving pressures on the order of 0.1 MPa. This study describes a high-quality factor, spherical resonator capable of achieving acoustic cavitation at ambient pressures in excess of 30 MPa (300 bars). This system generates bursts of violent inertial cavitation events lasting only a few milliseconds (hundreds of acoustic cycles), in contrast with the repetitive cavitation events (lasting several minutes) observed in SBSL; accordingly, these events are described as “inertial transient cavitation.” Cavitation observed in this high pressure resonator is characterized by flashes of light with intensities up to 1000 times brighter than SBSL flashes, as well as spherical shock waves with amplitudes exceeding 30 MPa at the resonator wall. Both SL and shock amplitudes increase with static pressure.
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43.35.Ei Acoustic cavitation in liquids
43.35.Hl Sonoluminescence

Optical Bragg imaging of acoustic fields after reflection

Nico F. Declercq, Michael S. McPherson, Mack A. Breazeale, and Alem A. Teklu

J. Acoust. Soc. Am. Volume 127, Issue 6, pp. 3466-3469 (2010); (4 pages) | Cited 1 time

Online Publication Date: 09 Jun 2010

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Bragg diffraction of x-rays occurs when the rays interact with a crystalline lattice at the appropriate angle. Bragg diffraction of visible light occurs when the light interacts at the Bragg angle with an ultrasonic field of the appropriate frequency. (The spacing between acoustic condensations and rarefactions acts like the planes in an atomic lattice.) If a beam of light is Bragg diffracted by an ultrasonic beam that previously has passed through an object, an image of the structure of the object is made visible in the diffraction field of the optical beam since there is a one-to-one mapping of the ultrasonic field onto the diffraction order. In many acoustic Bragg imaging applications, the sound field must pass through the object which is to be imaged. Ultrasonic attenuation at the very high acoustic frequencies needed for Bragg imaging (typically ∼ 25–30 MHz) severely limits the nondestructive testing (NDT) applications of traditional acoustic Bragg imaging. In this paper, a reflection-based application of acoustic Bragg imaging is discussed which may have useful industrial and biomedical NDT applications.
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43.35.Sx Acoustooptical effects, optoacoustics, acoustical visualization, acoustical microscopy, and acoustical holography
43.20.El Reflection, refraction, diffraction of acoustic waves
43.20.Fn Scattering of acoustic waves

Thermoacoustic properties of fibrous materials

Carl Jensen and Richard Raspet

J. Acoust. Soc. Am. Volume 127, Issue 6, pp. 3470-3484 (2010); (15 pages)

Online Publication Date: 09 Jun 2010

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The thermoacoustic properties of fibrous materials are studied using a computational fluid simulation as a test of proposed analytical models for propagation in porous materials with an ambient temperature gradient. The acoustic properties of porous materials have been understood in terms of microstructural models that approximate the material as an array of pores with empirical shape factors used to fit the pore theory to the material. An extension of these theories of acoustics to the thermoacoustic case with an ambient temperature gradient has been proposed by Roh et al. [J. Acoust. Soc. Am. 121, 1413–1422 (2007)] and a model based on Wilson’s relaxation approximation for porous acoustics [ J. Acoust. Soc. Am. 94, 1136–1145 (1993) ] is proposed herein, but the predictions of these analytical models have not been tested successfully against measurements. Accurately characterizing the effects of the applied temperature gradient in a wide bandwidth laboratory setup have proven difficult; as a result, the authors conducted a numerical simulation of propagation within a fibrous geometry in order to test the predictions of the analytical models. The results for several fibrous samples show that the models yield a reliable prediction of thermoacoustic performance from the shape factors and relaxation times.
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43.35.Ud Thermoacoustics, high temperature acoustics, photoacoustic effect
43.20.Bi Mathematical theory of wave propagation
43.20.Mv Waveguides, wave propagation in tubes and ducts

Low-frequency pulse echo reflection of the fundamental shear horizontal mode from part-thickness elliptical defects in plates

J. Ma and P. Cawley

J. Acoust. Soc. Am. Volume 127, Issue 6, pp. 3485-3493 (2010); (9 pages)

Online Publication Date: 09 Jun 2010

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Defect characterization using guided ultrasonic waves remains a challenging subject and requires a full understanding of the interaction of guided waves with a realistic representation of the defect. The characteristics of pulse echo reflection of the SH0 mode from part-thickness elliptical defects in plates is studied via finite element analysis and experimental measurements. The study shows that the reflection ratio spectrum of the SH0 mode from an elliptical defect exhibits periodic pattern due to interference between reflections from the two edges of the defect. The pattern of the reflection ratio spectrum is determined by the ratio of defect length in the incidence direction to wavelength, while the magnitude is affected by the maximum depth and the effective aspect ratio of the defect. Both the pattern and magnitude of the reflection ratio spectrum are found to be highly sensitive to the incidence angle, and the form of the variation of the reflection with angle is a strong function of the defect shape. In addition, a study of circular defects with tapered depth profiles reveals that the reflection is a function of average length of the tapered defect to wavelength ratio, and the magnitude of the reflection diminishes as the ratio increases.
Show PACS
43.35.Zc Use of ultrasonics in nondestructive testing, industrial processes, and industrial products
43.20.Mv Waveguides, wave propagation in tubes and ducts
43.35.Cg Ultrasonic velocity, dispersion, scattering, diffraction, and attenuation in solids; elastic constants
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