Precise measurements of bulk-wave ultrasonic velocity dispersion and attenuation in solid materials in the VHF range

A general method was established for precisely measuring velocity dispersion and attenuation in solid specimens with acoustic losses in the very high frequency (VHF) range, using the complex-mode measurement method and the diffraction correction method. Experimental procedures were presented for implementing such a method and demonstrated this measurement method in the frequency range of 50-230 MHz, using borosilicate glass (C-7740) as a dispersive specimen and synthetic silica glass (C-7980) as a nondispersive standard specimen. C-7980 exhibited no velocity dispersion; velocity was constant at 5929.14 +/- 0.03 m/s. C-7740 exhibited velocity dispersion, from 5542.27 m/s at 50 MHz to 5544.47 m/s at 230 MHz with an increase of about 2 m/s in the measured frequency range. When frequency dependence of attenuation was expressed as alpha = alpha(0)f(beta), the results were as follows: alpha0 = 1.07 x 10(-16) s2/m and beta = 2 for C-7980 and alpha0 = 5.16 x 10(-9) s(1.25)/m and beta = 1.25 for C-7740.     

The effect of bone structure on ultrasonic attenuation and velocity

The relationship between the structure of bovine cancellous bone, and its ultrasonic propagation parameters is investigated by means of a novel technique involving the application of large static loads, thereby changing the porosity in a controlled manner. The results show that for frequencies in the range 0.4 to 1 MHz, porosity decreases up to 35% are associated with a reduction in attenuation of up to 500%, whereas the velocity increases by roughly 35% for the same changes. The data taken overall suggest that in determining the ultrasonic attenuation coefficient at these frequencies, the amount of material in a given bone section is significantly less important than the distribution of that material.     

Nonlinear propagation effects on broadband attenuation measurements and its implications for ultrasonic tissue characterization.

A study is presented in which the influence of the pressure amplitude of the incident pulse on the estimated frequency dependency of the attenuation coefficient is shown. First, the effect is demonstrated with a simple theoretical model for both transmission and reflection measurements. Simulations show that for both measurement techniques a high-amplitude incident pulse results in a biased estimate of the attenuation coefficient due to nonlinear interaction of the different frequency components of the incident pulse. It is shown that in transmission and reflection measurements the biases have opposite signs. The effect of bandwidth, central frequency, and phase of the incident pulse on this bias is investigated. Second, the effect is demonstrated both in vitro, using a broadband through-transmission substitution technique on a tissue mimicking gelatine phantom, and in vivo, using reflection measurements with standard clinical equipment. The experimental results agree well with the theoretical model. The relevance of this study for ultrasonic tissue characterization is shown.     

Long-path measurements of ultrasonic attenuation and velocity for very dilute slurries and liquids and detection of contaminates

The objective was to use multiple paths through the slurry to determine the lowest concentration that provided accurate attenuation measurements and to measure the velocity of sound through an effective long path. Ultrasonic attenuation measurements were obtained for slurries of silica (10 microm diameter) in water for concentrations of 0.1%, 0.25%, 0.5%, 0.75% and 1% silica by weight. Attenuation measurements for concentrations less than 0.1% may prove useful for process control to detect contaminants. A long path is obtained due to multiple reflections occurring within the stainless steel (SS) vessel used; broad-band transducers are affixed on the outside of the thick-walled vessel. The signal in the receive transducer permits the measurement of the attenuation and also the velocity by measuring the time-of-flight. The FFT of the appropriate signal for each echo was obtained and compared with that for water to yield the attenuation as a function of frequency. The attenuation measurements are self-calibrating because they are not affected by changes in the pulser voltage. The data show the feasibility for measuring a concentration of 0.025 wt% silica, which is equivalent to 0.25 g of silica in 1 l of water. Therefore, such measurements can prove useful for detecting contaminants in liquid. The velocity of sound measurements for solutions of hydrogen peroxide in water were obtained and accurate to about 0.3m/s, or 0.02% uncertainty.     

Effect of moisture content on ultrasonic velocity and attenuation in woods

Ultrasonic velocity and attenuation are measured at 1 MHz in several kinds of wood as a function of the moisture content U. The results are interpreted in terms of water absorption mechanism of woods. The velocity decreases rapidly with increasing U up to a critical value U₁ at which the curve shows a sharp bend, and the decrease is very slow for U>U₁. This critical point U₁ is in the range 20–50% depending on the species of wood and is regarded as the fibre saturation point of the wood, the moisture content at which the wood substance is saturated with absorbed water. Ultrasonic measurements thus provide us with an accurate way to determine the fibre saturation point. The attenuation curve also shows a critical change at U = U₂, which is slightly lower than U₁. The attenuation shows no change from U = 0 up to U₂, but begins to increase rapidly thereafter. This point U₂ indicates the moisture content at which free water begins to enter the vacant space of wood cells.     

A study of ultrasonic velocity and attenuation on nanocystalline MgCuZn ferrites

The nanocrystalline MgCuZn ferrites with particle size (∼30 nm) have been synthesized by microwave-hydrothermal (M-H) method at 160 °C/45 min. The powders were densified at 750-900 °C/30 min using microwave sintering method. The sintered samples were characterized using X-ray diffraction and scanning electron microscope. The grain sizes of the sintered samples are in the range of 60-80 nm. The ultrasonic velocities have been measured on MgCuZn ferrites using the pulse transmission method at 1 MHz. The ultrasonic velocity is found to decrease with an increase of temperature. A small anomaly is observed around the Curie temperature, 520 K. The anomaly observed in the thermal variation of longitudinal velocity and attenuation is explained with the help of magneto-crystalline anisotropy constant.     

Parametric estimation of ultrasonic phase velocity and attenuation in dispersive media

In ultrasonic characterization of liquids, gases, and solids, accurate estimation of frequency dependent attenuation and phase velocity is of great importance. Non-parametric methods, such as Fourier analysis, suffers from noise sensitivity, and the variance of the estimated quantities is limited by the signal-to-noise ratio. In this paper we present a parametric method for estimation of these properties. Pulse echo experiments in ethane, oxygen and mixtures of the two show that the proposed method can estimate phase velocity and attenuation with up to 50 times lower variance than standard non-parametric methods.     

Assessment of broadband ultrasonic attenuation measurements in inhomogeneous media

In this paper, we address the problem of evaluating the acoustic attenuation of "difficult" media, i.e. highly attenuating and scattering media. In a broadband, through transmission setup, the signals acquired from such media are characterized by a poor signal-to-noise ratio. Therefore, an accurate estimate of attenuation cannot be obtained from a single measurement, but multiple measurements must be combined. Two methods are considered to yield a single estimate of attenuation from multiple measurements. The first one, the "average attenuation" (AA) method, consists in a simple average of individual attenuation estimates. The second one, the "cross spectrum" (CS) method, is based on a system identification approach. In order to evaluate the estimation errors for these methods, ultrasonic signals transmitted through a material of known attenuation were simulated and mixed with both coherent and incoherent noise. In all tests performed, the "CS" method was found to yield the most accurate estimate. This method, combined time delay compensation, is then applied to real signals measured from a concrete slab. A valid frequency band for the attenuation estimate can be defined based on the coherence function. Results from this research are being applied to characterize the degradation of concrete structures using high-frequency ultrasound.     

Energy and phase velocity considerations required for attenuation and velocity measurements of anisotropic composites

Viscoelastic fibre-reinforced composite materials have a number of possible advantages for use in underwater acoustic applications. In order to exploit these materials it is important to be able to measure their complex stiffness matrix in order to determine their acoustic response. Ultrasonic transmission measurements on parallel-sided samples, employing broadband pulsed transducers at 2.25 MHz and an immersion method, have been used to determine the viscoelastic properties of a glass-reinforced composite with uniaxially aligned fibres. The composite measured was constructed from Cytecfiberite's CYCOM 919 E-glass. The theory of acoustic propagation in anisotropic materials shows that the direction of energy propagation is, in general, different from that given by Snell's Law. At 15 degrees incidence, Snell's Law implies a refracted angle of 40 +/- 2 degrees, whereas the energy direction is observed to be 70 +/- 2 degrees. Despite this, the experimental data indicates that the position of the receiving transducer has relatively little effect on the apparent phase velocity measured. The phase velocities measured at positions determined from the refracted angle and energy direction are 3647 and 3652 +/- 50 m s(-1), respectively. However, the amplitude of the received signal, and hence estimate of attenuation, is highly sensitive to the receiver position. This indicates that the acoustic Poynting vector must be considered in order to precisely determine the correct position of the receiving transducer for attenuation measurements. The beam displacement for a 17.6 mm sample at 15 degrees incidence is 9.5 and 40 mm by Snell's Law and Poynting's Theorem, respectively. Measured beam displacements have been compared with predictions derived from material stiffness coefficients. These considerations are important in recovering the complex stiffness matrix.     

Phase velocity and normalized broadband ultrasonic attenuation in Polyacetal cuboid bone-mimicking phantoms

The variations of phase velocity and normalized broadband ultrasonic attenuation (nBUA) with porosity were investigated in Polyacetal cuboid bone-mimicking phantoms with circular cylindrical pores running normal to the surface along the three orthogonal axes. The frequency-dependent phase velocity and attenuation coefficient in the phantoms with porosities from 0% to 65.9% were measured from 0.65 to 1.10 MHz. The results showed that the phase velocity at 880 kHz decreased linearly with porosity, whereas the nBUA increased linearly with porosity. This study provides a useful insight into the relationships between ultrasonic properties and porosity in bone at porosities lower than 70%.     

Anomalous ultrasonic attenuation and velocity change in LaAl2

The attenuation and velocity of longitudinal waves has been measured in superconducting and normal LaAl₂ in the frequency range from 15 to 225 MHz. Both in the superconducting and in the normal state an unusually large damping has been found superimposed on the relatively small attenuation due to the well known electron-phonon interaction. On cooling into the superconducting state the attenuation starts to drop only at temperatures well below T_c and — as in insulating glasses — it rises again below 0.8 K. In our view this indicates that two-level systems similar to the ones found in amorphous superconductors may be present in our crystalline LaAl₂ sample and are responsible for the observed acoustic anomalies.     

Measurements of velocity and attenuation of leaky waves using an ultrasonic array

A new method of measuring velocity and attenuation of leaky surface waves is presented. A single focused transmitting transducer and linear receiving array in a pitch-catch arrangement are used in the proposed system. The spatial distribution of the acoustic field in the leaky wave is recorded by the array, and the parameters of the leaky wave can be obtained by processing the output waveforms. In comparison with existing material characterization systems, the mechanical scanning of the transducers is not used any more, and the measurement time is only limited by the time of the wave propagation and speed of the electronic data acquisition system.     

Measurement of ultrasonic velocity and attenuation in bone tumors, in vitro

Ultrasonic velocity and attenuation are determined in different types of bone tumors by using a double-probe-through-transmission ultrasonic technique. The average values of propagation velocity in different types of tumors are found as 2106, 2304, 2677.5, and 3586 m/s with 1.49%, 1.04%, and 0.74% standard deviation in Giant Cell, Lymphoma, Chondro Sarcoma, and Osteogenic Sarcoma, respectively. Absorption coefficient of ultrasound is found to be minimum as 19.7 dB/cm with 0.002% standard deviation in Lymphoma, and high in Osteochondroma and not measurable with the present setup. A direct technique for the diagnosis and differentiation of various types of tumors can be developed by standardization of the data.     

声速和衰减系数与含蜡原油特性的关系

当含蜡原油的温度低于蜡的析出温度后,蜡晶将不断析出并相互交联成网状结构,若网状结构达到一定强度,原油将失去流动而凝结。在该过程中不仅含蜡原油的流动特性发生变化,原油的声速和衰减系数也发生变化。本研究结果表明：含蜡原油的超速及衰减系数均随温度降低而增大;含蜡原油在凝结温度前后表现出不同的声速与温度的对应关系;含蜡原油在凝结后的声速的温度的关系同原油的含蜡量有关;含蜡原油的衰减系数在接近凝结温度时急剧     

Piezoelectric constant and conductivity relaxation in n-type InP from ultrasonic attenuation measurements

Measurements have been made of the attenuation of 30, 90 and 150 MHz {111}; longitudinal ultrasonic waves and of the d.c. resistivity in chromium-doped, n-type InP from -50 to +150°C. From the attenuation maximum observed for 30 MHz waves we find γe₁₄γ = 0.040 C/m².     

Impact of localized inhomogeneity on the surface-wave velocity and bulk-wave reflection in solids

The effect of a weak surface, near-surface and interfacial inhomogeneity on the frequency dependence of the surface wave velocity and of the SH (shear horizontal) wave reflectivity in isotropic elastic media is studied analytically and numerically. The inhomogeneity is modeled as an infinite planar layer with continuously varying properties. Weak inhomogeneity may markedly affect the dispersion of the Rayleigh velocity and especially of the reflectivity. It is demonstrated how this effect, particularly pronounced at high frequency, depends on the extent of inhomogeneity. The material data for damaged and ideal concrete and several simple examples of inhomogeneity profiles are utilized for the numerical calculations based on the Peano expansion. The use of explicit low- and high-frequency approximations is also exemplified. Among these, simple WKB asymptotics are shown to be particularly helpful for the Rayleigh velocity in the case of a prominent inhomogeneity attached to the surface and for the reflection on weak interfaces.     

An apparatus for high precision measurements of ultrasonic wave velocity

An automatic gated-carrier pulse superposition system for high precision ultrasonic velocity measurements is described, having better stability and lower cost than previous equipment. The necessary new integrated circuitry, and the method of operation, are described in detail. The principal application is to the measurement of small changes in the velocity of MHz ultrasonic waves, such as occur when materials are subjected to stress. Results are presented confirming a detection sensitivity of 1 part in 10.     

Anisotropy of the ultrasonic attenuation in soft tissues: measurements in vitro

This study was designed to measure the ultrasonic attenuation within phantoms and tissue samples over a broad bandwidth and at many angles of incidence with respect to intrinsic orientations in order to elucidate both the frequency and angular dependence of the attenuation coefficient. Significant angular dependence, or anisotropy, of the attenuation was observed in canine myocardium (maximum to minimum ratio: 2.2 to 1) and a tissue mimicking phantom of oriented graphite fibers in gelatin (max to min: 2 to 1). In control studies, insignificant anisotropy was observed in the attenuation in canine liver samples and phantoms with graphite powder suspended in gelatin. Comparisons of the magnitude of variations of the oriented-fiber phantom to that predicted by a viscous relative motion model are presented.     

Finite-bandwidth effects on the causal prediction of ultrasonic attenuation of the power-law form

Kramers-Kronig (K-K) relations exist as a consequence of causality, placing nonlocal constraints on the relationship between dispersion and absorption. The finite-bandwidth method of applying these relations is examined where the K-K integrals are restricted to the spectrum of the experimental data. These finite-bandwidth K-K relations are known to work with resonant-type data and here are applied to dispersion data consistent with a power-law attenuation coefficient (exponent from 1 to 2). Bandwidth-restricted forms of the zero and once-subtracted K-K relations are used to determine the attenuation coefficient from phase velocity. Analytically, it is shown that these transforms produce the proper power-law form of the attenuation coefficient as a stand-alone term summed with artifacts that are dependent on the integration limits. Calculations are performed to demonstrate how these finite-bandwidth artifacts affect the K-K predictions under a variety of conditions. The predictions are studied in a local context as a function of subtraction frequency, bandwidth, and power-law exponent. The K-K predictions of the power-law exponent within various decades of the spectrum are also examined. In general, the agreement between finite-bandwidth K-K predictions and exact values grows as the power-law exponent approaches 1 and with increasing bandwidth.