Low Sound Speed Research Articles

Broadband transducers for underwater communications

Properties of underwater acoustic cylindrical piezoelectric (PZT) transducers currently used for broadband underwater communications with unmanned underwater vehicles (UUV) and new transducers under development utilizing single crystal relaxor ferroelectrics (PMN‐PT) with significantly higher material coupling coefficients and lower sound speed are presented. Characteristics including resonance frequency, coupling coefficient, transmit (TVR) and receive (OCVS) frequency response, maximum drive and achievable source levels, tuned and untuned power factor, and directivity factors are discussed. Results for experimental calibrations and field tests will be compared with detailed equivalent electrical circuit models of the transducer and transducer channel. Work supported in part by BTech Acoustics and Office of Naval Research.

The Viscous-Grain-Shearing (VGS) theory of wave propagation in marine sediments

The grain-shearing (GS) theory of wave propagation in a marine sediment is based upon the idea that, as grains slide against one another, the dissipation becomes progressively stronger, a phenomenon known as strain hardening. The GS theory predicts a sound speed showing weak, logarithmic dispersion and an attenuation that scales linearly with frequency. At frequencies above about 10 kHz, such behavior matches the sound speed and attenuation data obtained during the SAX99 experiment in the Gulf of Mexico. But at lower frequencies, the measured dispersion curves show a lower sound speed and higher attenuation than predicted by the GS theory. A generalized version of the GS theory, designated the VGS theory, takes account of the effective viscosity of the molecularly thin layer of pore fluid between contiguous grains, which limits the degree of strain hardening that can occur during sliding. The VGS dispersion expressions are the same as those of the GS theory, except for the appearance of a simple algebraic function that has an effect only at low frequencies, below 10 kHz. The resultant VGS curves match the sound speed and attenuation measurements from SAX99 over the measurement frequency band from 1 to 400 kHz. (Research supported by ONR.)

Real-Gas Effects in Organic Rankine Cycle Turbine Nozzles

Organic Rankine cycle turbogenerators are a viable option as stationary energy converters for external heat sources, in the low power range (from a few kW up to a few MW). The fluid-dynamic design of organic Rankine cycle turbines can benefit from computational fluid dynamics tools which are capable of properly taking into account real-gas effects occurring in the turbine, which typically expands in the nonideal-gas thermodynamic region. In addition, the potential efficiency increase offered by supercritical organic Rankine cycles, which entails even stronger real-gas effects, has not yet been exploited in current practice. In this paper, real-gas effects occurring in subcritical and supercritical organic Rankine cycle nozzles have been investigated. Two-dimensional Euler simulations of an existing axial organic Rankine cycle stator nozzle are carried out using a computational fluid dynamics code, which is linked to an accurate thermodynamic model for the working fluid (octamethyltrisiloxane C 8 H 28 O 2 Si 3 ). The cases analyzed include the expansions starting from actual subcritical conditions, that is, the design point and part-load operation, and three expansions starting from supercritical conditions. Results of the simulations of the existing nozzle for current operating conditions can be used to refine its design. Moreover, the simulations of the nozzle expansions starting from supercritical conditions show that a nozzle geometry with a much higher exit-to-throat area ratio is required to obtain an efficient expansion. Other peculiar characteristics of supercritical expansions such as low sound speed and velocity, high density, and mass flow rate, are discussed.

Effects of Fine Metal Oxide Particle Dopant on the Acoustic Properties of Silicone Rubber Lens for Medical Array Probe

The effects of fine metal oxide particles, particularly those of high-density elements (7.7 to 9.7 x 10(3) kg/m3), on the acoustic properties of silicone rubber have been investigated in order to develop an acoustic lens with a low acoustic attenuation. Silicone rubber doped with Yb2O3 powder having nanoparticle size of 16 nm showed a lower acoustic attenuation than silicone rubber doped with powders of CeO2, Bi2O3, Lu2O3 and HfO2. The silicone rubber doped with Yb2O3 powder showed a sound speed of 0.88 km/s, an acoustic impedance of 1.35 x 10(6) kg/m2s, an acoustic attenuation of 0.93 dB/mmMHz, and a Shore A hardness of 55 at 37 degrees C. Although typical silicone rubber doped with SiO2 (2.6 x 10(3) kg/m3) shows a sound speed of about 1.00 km/s, heavy metal oxide particles decreased the sound velocities to lower than 0.93 km/s. Therefore, an acoustic lens of silicone rubber doped with Yb2O3 powder provides increased sensitivity because it realizes a thinner acoustic lens than is conventionally used due to its low sound speed. Moreover, it has an advantage in that a focus point is not changed when the acoustic lens is pressed to a human body due to its reasonable hardness.

Mechanics and refractive power optimization of tunable acoustic gradient lenses

Tunable acoustic gradient index (TAG) lenses create tunable multiscale Bessel beams. These lenses are fluid-filled cylindrical cavities within which an acoustic radial standing wave is excited. This standing wave modulates the density, and thereby the refractive index within the lens. Spatial gradients in the refractive index can be used for lensing. A predictive model for the steady-state fluid mechanics behind TAG lenses driven with a sinusoidal voltage signal is presented here. The model covers inviscid and viscous regimes in both the resonant and off-resonant cases. The density fluctuations from the fluidic model are related to refractive index fluctuations. The entire model is then analyzed to determine the optimal values of lens design parameters for greatest lens refractive power. These design parameters include lens length, radius, static refractive index, fluid viscosity, sound speed, and driving frequency and amplitude. It is found that long lenses filled with a fluid of high refractive index and driven with large amplitude signals form the most effective lenses. When dealing with resonant driving conditions, low driving frequencies, smaller lens radii, and fluids with larger sound speeds are optimal. At nonresonant driving conditions, the opposite is true: High driving frequencies, larger radius lenses, and fluids with low sound speeds are beneficial. The ease of tunability of the TAG lens through modifying the driving signal is discussed, as are limitations of the model including cavitation and nonlinearities within the lens.

Normal DXA bone mineral density but frail cortical bone in Turner's syndrome

Patients with Turner's syndrome have normal bone mineral density by dual energy X-ray absorptiometry (DXA), but a predisposition for fractures. Quantitative ultrasonography (QUS) measures cortical bone strength. To compare QUS with DXA in patients with Turner's syndrome. Twenty-seven Turner's syndrome patients, aged 21.1 +/- 6.3 years (mean +/- SD), were evaluated by DXA, measuring two-dimensional bone mineral density (BMD), and QUS, measuring speed of sound (SOS) of the radius and tibia. The results were compared to sex- and age-matched (Ctr A, n = 53) and height-matched (Ctr B, n = 34) control groups. Fracture incidence per 1000 women years was 4.76 in Ctr A, 5 in Ctr B and 7.69 in Turner's patients. In Turner's syndrome patients, QUS results were significantly lower than in controls, whereas DXA Z-scores were not different from reference values. Correlation between tibia and radius SOS and height and age in controls (P < 0.0001) was not evident in Turner's syndrome. Oestrogen or growth hormone therapy had no effect on either QUS or DXA parameters. Bone fragility in Turner's syndrome is reflected by low SOS but not by DXA BMD. Low QUS, which assesses the cortical bone only, supports a defect in cortical bone in Turner's syndrome. Lack of SOS correlation with age, height and hormonal therapy in Turner's syndrome suggests a primary bone defect, rather than enhanced resorption of endocrine origin.

Constraining the nature of dark energy using the Square Kilometer Array Telescope

We investigate the potential of the Square Kilometer Array Telescope (SKA) to constrain the sound speed of dark energy. The Integrated Sachs Wolfe (ISW) effect results in a significant power spectrum signal when Cosmic Microwave Background (CMB) temperature anisotropies are cross-correlated with galaxies detectable with the SKA in H i. We consider using this measurement, the autocorrelation of H i galaxies and the CMB temperature power spectrum to derive constraints on the sound speed. We study the contributions to the cross-correlation signal made by galaxies at different redshifts and use redshift tomography to improve the signal-to-noise. We use a χ2 analysis to estimate the significance of detecting a sound speed different from that expected in quintessence models, finding that there is potential to distinguish very low sound speeds from the quintessence value.

Extension of the grain-shearing theory of wave propagation in marine sediments to include pore-fluid viscosity

Buckingham’s grain‐shearing (GS) theory of wave propagation in saturated granular materials predicts a sound speed showing weak dispersion and an attenuation that scales essentially linearly with the frequency. Measurements of the dispersion curves made during the SAX99 experiment in the Gulf of Mexico match the GS theory at higher frequencies, above 10 kHz, but show a lower sound speed and a higher attenuation than predicted by the GS theory at lower frequencies. A generalized version of the GS theory has now been developed, designated the VGS theory, which takes into account the effective viscosity of the molecularly thin layer of pore fluid between contiguous grains. In effect, the viscosity of the pore fluid limits the degree strain‐hardening that occurs as grains slide against one another. The resultant VGS dispersion curves are the same as those of the GS theory, except for the appearance of a simple algebraic function that has an effect only at frequencies below about 10 kHz. This low‐frequency function gives rise to VGS compressional‐wave dispersion curves that match the sound speed and attenuation measurements made during SAX99 across the frequency band from 1 kHz to 400 kHz. [Research supported by ONR.]

Investigation of media with large, negative parameters of nonlinearity and their application to the enhancement of a compact, omnidirectional, parametric source

This paper reports on two media whose acoustic properties are highly pressure dependent and, thus, have large nonlinear parameters (B/A∼−3000 to −5000). One is Xanthan gelatin filled with, 10%, Expancel microspheres. The other is a polyurethane rubber, Polytek 74, with 20% Expancel microspheres. The microspheres have a high bulk modulus at ambient pressure due to the hoop stiffness of the spherical shell. As pressure is increased some of the microspheres buckle, leading to significantly decreased bulk modulus. The decreasing bulk modulus with increasing hydrostatic pressure (negative dc/dp) results in both negative B/A and low sound speed (150 m/s at the minimum), which further enhances nonlinear distortion. These media are used to enhance the efficiency of an underwater parametric source. Experiments on a spherical source with a thin layer (2 cm thick) of the Xanthan gelatin material resulted in a generated difference frequency source level 16 dB higher at 300 Hz, than would be obtained driving the transducer directly at the difference frequency. Experiments on the same source with a thin layer of the microvoided, polyurethane rubber are in progress. This material has a smaller coefficient of nonlinearity but operates over a broader range of hydrostatic pressures.

Closed Form Bistatic Reverberation and Target Echoes With Variable Bathymetry and Sound Speed

Some existing closed-form expressions for target echo and reverberation, assuming classical mode-stripping and Lambert's law in isovelocity water, are extended to the case of uniform slope bathymetry combined with range-independent linear sound speed and bistatic geometry. The analytical, or extremely fast numerical, calculation of signal-to-reverberation-ratio provides insight useful for sonar design, and operational research. There are three types of path, each of which may be important at some range: Those that only interact with one boundary, those that always interact with both boundaries and those that sometimes interact with both. Each provides a separate closed-form contribution. The earlier finding, that with Lambert's law the reverberation tends to follow the same range trend as the target echo, is slightly modified by the addition of refraction. The stronger reverberation from the low sound speed side of the duct tends to fall off slightly less slowly than the target echo according to the strength of the refraction. Some graphical examples show that the very large parameter space can be handled very easily

Head waves, diving waves, and interface waves at the seafloor

Brekhovskikh (1960) summarizes the system of waves that arises from reflection and refraction of spherical waves at the interface between homogeneous solid half-spaces. By eliminating the shear wave potential in one half-space, the system for fluid–solid half-spaces like the seafloor is obtained. There are two cases: one where the shear speed in the bottom is less than the compressional speed in the fluid (soft sediments), and the other where the shear speed in the bottom is greater than the compressional speed in the fluid (hard volcanic basement). This model is the basis for defining interface phenomena such as evanescent waves, head waves, pseudo-Rayleigh waves, and Stoneley/Scholte waves. If a positive gradient is introduced into the compressional and shear sound speeds in the bottom, one obtains diving waves and interference head waves (Cerveny and Ravindra, 1971). There are two types of interface waves: pseudo-Rayleigh waves that are evanescent in the bottom but propagate in the water, and Stoneley/Scholte waves that are evanescent in both media. In multi-interface models there are of course normal modes. In actual seafloors, low speed layers, sound and shear speed gradients, and interface and volume lateral heterogeneities affect the characteristics of propagation and scattering. [Work supported by ONR.]

Sound speed scanning acoustic microscopy for biomedical applications

Since 1985, we have been developing a scanning acoustic microscope (SAM) system for biomedical use and have been investigating the acoustic properties of various organs and disease states by using this SAM system. In biomedicine, SAM is useful for intraoperative pathological examination, study of low-frequency ultrasonic images, and assessment of biomechanics at a microscopic level. Recently, we have proposed a new concept -- acoustic microscopy -- using a single pulsed wave instead of continuous waves used in conventional SAM systems. In the present study, we compared two systems by measuring the same biological material. The sound speed image obtained by sound speed microscopy corresponded well to that obtained using a conventional SAM system. Lesions with hyaline degeneration showed a lower sound speed when compared with that of normal myocardium. Frequency domain analysis of amplitude and phase by both methods also showed similar characteristics. Although the data acquisition time of one frame was greater than that in conventional SAM, the total time required for calculation was significantly shorter. The SAM system can be applied to intraoperative pathological examination.

The Risk of Colles’ Fracture is Associated with the Collagen I Alpha1 Sp1 Polymorphism and Ultrasound Transmission Velocity in the Calcaneus Only in Heavier Postmenopausal Women

To compare the ability of the bone mineral density (BMD) at the distal forearm, collagen I alpha 1 (COLIA1) polymorphism, and ultrasound stiffness to identify individuals with increased risk of wrist fracture, we studied 183 postmenopausal Czech women with a wrist fracture and 178 postmenopausal controls, ages 45-70 years. The genotypes "Ss" and "ss" were significantly overrepresented among fracture cases. The BMD measurements at the femoral neck, total femur, and distal forearm as well as ultrasound stiffness of the heel, broadband ultrasound attenuation (BUA), and speed of sound (SOS) were significantly lower in the fracture cohort. BMD of the distal forearm was the main determinant of susceptibility to the wrist fracture. Weight, the COLIA1 genotype, and ultrasound SOS further strengthened the predictive value of BMD. However, we found interaction between weight and both the COLIA1 Sp1 polymorphism and ultrasound parameters. Presence of the "s" allele as well as low SOS acted as significant predictors of wrist fracture only in heavier women, (> or =62 kg) but not in women with a body weight of less than 62 kg. In heavier women, both the COLIA1 Sp1 polymorphism and ultrasound parameters acted as independent markers that contributed to BMD to enhance fracture prediction. However, the COLIA1 enabled a higher specificity (specificity 72.4%, sensitivity 44.2%), whereas SOS enabled a higher sensitivity (sensitivity 73.9%, specificity, 45.7%). We conclude that BMD at total forearm, the COLIA1 polymorphism, and ultrasound SOS are independent predictors of wrist fracture in postmenopausal women. The effect of the COLIA1 Sp1 polymorphism and SOS on wrist fracture risk is more pronounced in patients with a higher body weight.

Subphotospheric Structure of Sunspots and Active Regions

New methods of local area helioseismology provide three-dimensional maps of sound-speed variations and mass flows in the upper convection zone, giving important insight into the internal structure and dynamics of sunspots and active regions. Most of these results are obtained from SOHO/MDI data using the method of time-distance helioseismology (or acoustic tomography). Robustness of this method has been significantly improved by incorporating most important wave propagation effects and stochastic properties of solar oscillations. Time-distance helioseismology reveals that developed sunspots have a two layer structure: a relatively thin sub-photospheric layer of lower sound speed, and a deeper layer of higher sound speed. The mass flows in the upper layer are typically converging and directed downward, while in the deeper interior the flows are mostly diverging. These results support the cluster model of sunspots suggested by Parker. New observations also provide interesting information about emerging magnetic flux, formation and evolution of active regions and complexes of activity, and allow us to investigate effects of sub-photospheric dynamics of active regions on the global circulation of the Sun and also small-scale rapid shear flows associated with flares and CME.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

MATERIAL PROPERTIES OF THE SUPRASPINATUS TENDON AT ITS INSERTION — A MEASUREMENT WITH THE SCANNING ACOUSTIC MICROSCOPY

The acoustic properties of the supraspinatus tendon insertions of three adult Japanese white rabbits (male, 3.2–3.5 kg) were measured by scanning acoustic microscopy (SAM). After cutting parallel to the supraspinatus tendon fibers, specimens were fixed with 10% neutralized formalin, embedded in paraffin and sectioned at 5 μm thickness. The sound speed and the attenuation constant along the articular surface of the tendon from the insertion site to the musculotendinous junction were measured by SAM. The two-dimensional distribution of the acoustic properties was displayed with color-coded scales and was compared to the histologic structure. Toluidine-blue staining and immunohistochemical staining against type II collagen were performed to determine the presence of fibrocartilage besides a standard Elastica–Masson staining to assess the overall histologic structure. The distribution of the sound speed and the attenuation constant showed an almost identical pattern. Both parameters were lower in the non-mineralized fibrocartilage than those in the tendon proper. The area indicating low sound speed corresponded well to the area of metachromasia and type II collagen. The non-mineralized fibrocartilage indicated softer material properties than the tendon proper, which could be interpreted as an adaptation for the various biomechanical stresses at the insertion site.

The influence of large-scale seafloor slope and average bottom sound speed on low-grazing-angle monostatic acoustic scattering.

Variations in large-scale seafloor slope and average seabed sound speed account for a significant portion of the variations in scattering intensity observed in low-grazing-angle monostatic reverberation. Numerical modeling using a finite-difference solution to the elastic wave equation is used to quantify the effect of these large-scale parameters in interpretations of reverberation data. For hard rough seafloor (e.g., basalt), the results of the modeling suggest that the monostatic backscattering strength increases with increasing large-scale seafloor slope up to a slope of about 15 degrees dipping toward the incident direction. Once the grazing angle of the incident wavefield exceeds the critical grazing angle for the flat reference seafloor the backscattering intensity increases only slowly with increasing grazing angle. Similarly, average subseafloor sound speed has a significant effect. Seafloor with low sound speeds characteristic of soft bottoms (e.g., sediment) generate significantly weaker backscatter signals than seafloor with sound speeds characteristic of hard bottoms (e.g., basalt). The difference is that the shear waves can always be passed into soft bottoms because even for a flat seafloor there is no shear wave critical grazing angle.

Effects of nonlinearity on the propagation of acoustic pulses in random media

We conducted a numerical investigation into the propagation of finite-amplitude pulses in media with inhomogeneous random sound speed. An N wave (idealized sonic boom) was used as the pulse shape. Initial simulations considered a medium with a single spherical scattering object with a slow sound speed. This object acted as a focusing lens. As the amplitude of the N wave was increased nonlinear effects initially led to enhancement of focusing, reduction in shock risetime, and a shift of the peak away from the object. However, for high amplitude, energy loss at the shock led to a dramatic reduction in the amplitude of the focus and a shift towards the object. Simulations were then carried out in a two-dimensional random media. The sound speed in the random media was constructed using a Fourier mode decomposition with parameters appropriate for turbulence in the atmospheric boundary layer. For low amplitude waves the N wave was focused and defocused by regions of low and high sound speed, respectively. However, the presence of multiple paths means that the wave form no longer resembled an N-wave after propagating about 10 wavelengths. As the amplitude was increased the focusing was enhanced and more localized.

Subsurface structure of sunspots

The subsurface structure of sunspots is probed by measuring travel-time delays of acoustic waves propagating beneath the spots, and using a helioseismic inversion method to infer the internal sound-speed perturbations and mass flows. The initial results obtained from the MDI instrument on SOHO reveal under sunspots zones of the relatively low sound speed, extended to a depth of approximately 4 Mm. These zones are associated with cool areas of sunspots. In the deeper interior of sunspots the sound speed is higher than in the surrounding plasma. The regions of the higher sound speed are at least 60 Mm deep. These observations also show complicated flow patterns that include converging vortex flows in the upper 4 Mm deep layer. These flows play important role for maintaining the structure of sunspots. Developing active regions are associated with complicated evolving sound-speed perturbations beneath the surface, which are probably caused by multiple flux tubes emerging from the deep interior. A study of a rapidly rotating sunspot revealed strong shear flows beneath the spot.

Thermodynamics of Geysers: Application to Titan

General constraints on geyser phenomena are developed and applied to speculative methane geysers on Titan. Variation of boiling point with depth of ethane–methane–nitrogen fluids is found to be of order 0.3 K m −1, in contrast with around 2 K m −1 for water on Earth. It is found that geysers are possible on Titan but require enhancements of ∼100 above global-average geothermal heat flux, a factor similar to that required on Earth. Eruption velocities of order 25 m s −1 appear to be typical for 10-m-deep geyser vents on both Earth and Titan. While eruption velocities on Earth are usually limited by the low sound speed in water–steam mixtures, sound speeds in Titan fluids are higher and the Titan limit is imposed by the available energy in the boiling fluid. Eruption intervals should be nearly the same for geysers with equivalent plumbing. There is an interesting symmetry between geysers on Titan and those on Earth: The volatility of the relevant fluid scales with the available heat flow.

Early postmenopausal bone loss in hyperthyroidism

Objectives: To evaluate the effect of hyperthyroidism on bone in relation to the menopausal state. Methods: Fifty-nine hyperthyroid (HYPER), 40 hypothyroid (HYPO), and 51 control euthyroid (EUTH) women were studied. Bone mineral density (BMD) was assessed by dual X-rays absorptiometry (DXA) at the lumbar spine, and at the femoral neck. A multi-site QUS device evaluated speed of sound (SOS) at the radius (RAD), tibia (TIB), metatarsus (MTR), and phalanx (PLX). Bone markers used were serum bone specific alkaline phosphatase (BSAP) and urinary deoxypyridinoline (DPD). Results: At all sites, SOS was lower in HYPER than in EUTH (RAD P<0.05, TIB P<0.01, MTR P<0.05, PLX P=0.01). The low SOS was only noted at the early postmenopausal period. BMD at the femoral neck but not at the lumbar spine was lower in HYPER as compared to EUTH ( P<0.05). Both femoral neck and tibia were the sites with the highest odds ratio for being hyperthyroid (2.3 and 2.04, respectively). There was no correlation between BMD or SOS and FT 4, TT 3 or duration of hyperthyroidism. BSAP and DPD positively correlated with FT 4 and TT 3 ( P<0.05). Conclusions: This study suggests that hyperthyroidism affects bone mineralization especially during the early postmenopausal period, and the effect is mainly at the cortical bone.