Index of content:
Volume 83, Issue 4, April 1988

Acoustical Doppler effect analysis—Is it a valid method?
View Description Hide DescriptionThe Doppler effect approach is a powerful tool used in various areas of physics to analyze motion in a wave field, by observing the frequency shifts of scatteredwaves. In acoustics, Doppler effectanalysis is used for diagnostics in biomedical systems and for industrial applications, e.g., in cases where scatterers are present in moving fluids. The theoretical models used to analyze such systems usually ignore the fact that the moving ambient medium might produce effects of the same order of magnitude as the scatterers themselves. It has even been argued that in certain circumstances such effects should completely cancel the Doppler frequency shifts. A model is developed here that contributes to our understanding of the scattering in the presence of moving objects and space‐ and time‐dependent moving media. The model is restricted to irrotational flows, neglects velocity effects except of the first order in the Mach numberv/c, and assumes slow variations in the ambient medium. These restrictions facilitate the analytical discussion of specific canonical problems. The present study indicates that Doppler effects can be produced by moving scatterers in a medium at rest, scatterers at rest in a moving medium, and in configurations in which combined motion of scatterers and media take place. The Doppler effects are of the same order of magnitude in all cases. This vindicates the simple models used for research and applications, which assume that the moving objects produce Doppler effects but neglect the flow of the surrounding medium.

The scattering of ultrasonic waves by multiphase polycrystals
View Description Hide DescriptionThe theory of ultrasonic propagation in single‐phase polycrystals presented in previous articles [J. Acoust. Soc. Am. 7 2, 1021–1031 (1982); 7 3, 1160–1163 (1983)] is generalized to calculate the scattering coefficients and phase and group velocities of plane compressional and shear waves in multiphase polycrystals. The analytical calculation was done for macroscopically isotropic materials with phases of cubic, hexagonal, or orthorhombic single‐crystal symmetry in second‐order perturbation theory using the assumption that the relative mean quadratic deviation of the wavenumbers in the polycrystal considered from its homogeneous isotropic approximation is small. Numerical evaluation is carried out for a two‐phase material of α‐ and γ‐Fe.

The effect of windowing on spectral estimation of echoes scattered by a random medium
View Description Hide DescriptionIn executing spectral analysis of echoes scattered by a random medium, a window is generally applied to an echo signal. Spatial resolving power to characterize the random medium is improved by shortening the window length. But it is difficult to shorten the window length arbitrarily since, in the conventional method, the shorter the window length, the more severely the spectrum is distorted. This article shows that the spectrum distorted by the window can be expressed as a convolution of the spectrum of echoes that would be obtained from a spatially truncated thin layer of the random medium with the spectrum of the window function. Consequently, it can be corrected by a deconvolution operation, and by the fact that the spectral estimation over a wide frequency range can be carried out even using a window of shorter length. The effects of the deconvolution are first demonstrated by computer simulation where a random signal is used as the echo signal. The effects can be seen more clearly in the measurement of attenuation coefficient slope obtained by the ratio of two spectra that are subjected to different attenuation. Finally, the applicability of this method is shown by the experiments where the attenuation coefficient slope of an agar block is measured using the window of various lengths.

On the nonobservance of nonspecular bounded beam reflection effects of Lamb modes
View Description Hide DescriptionReflection and transmission effects are investigated for angles of excitation of Lamb waves that are near the longitudinal critical angle. It will be shown that, for this particular angle, the coupling at the interfaces liquid/solid/liquid is such that the leaky‐surface‐wave‐radiation effect does not occur because of the weak coupling between the two media. This result affects only the symmetrical Lamb modes S _{ i } (i≠0).

A reanalysis of the acoustic scattering from elastic spheroids
View Description Hide DescriptionThe elastic response of fluid‐loaded, elastic spheroids to an end‐on incident acoustic wave is analyzed for a variety of aspect ratios and material properties. The focus of this analysis is the nature of the elastic excitations of the target, and, to a lesser extent, the nature of the inelastic background. It is concluded that the low k L/2, resonant response of the target is closely related to a bar wave, propagating at nearly one and one‐half times the shear speed at low frequencies and that the rigid body solution with recoil is the appropriate background choice for all materials. Bistatic scattering calculations are also studied, and it is concluded that the lobe structure of the elastically reradiated signal cannot be used to characterize a resonance without a p r i o r i knowledge of the target. The results contradict recent, but more phenomenologically oriented, studies.

Reflection characteristics of an elastic slab containing a periodic array of circular elastic cylinders: P and S V wave analysis
View Description Hide DescriptionThe reflection and transmission characteristics of circular elastic cylinders arranged periodically in an elastic slab are examined. A simple approach synthesized from Fourier–Bessel expansions and the T‐matrix method has been used for the analysis of a two‐dimensional problem involving P and S Vwaves. The computed results are of importance in gratingtheory as well as in noise control, although the procedure given is restricted to only the low‐frequency regime.

Experimental study of the Stoneley wave at a plane liquid–solid interface
View Description Hide DescriptionSome of the properties of the Stoneley wave at a plane liquid–solid interface are reviewed after having deduced them from the general ones of evanescent plane waves. A liquid wedge method of generation of this wave is presented, and the predicted properties are verified.

Characterization of guided waves propagating in infinite cylinders for oblique insonification
View Description Hide DescriptionThe present study confirms that in oblique incidence, leaky guided waves propagate in isotropic elastic cylinders. The various longitudinal and flexural modes of propagation are characterized by phase velocity dispersion curves. From leaking properties and resonance effects, the frequency dependence of phase velocity is experimentally measured in case of an infinite stainless‐steel cylinder for values of the dimensionless parameter k a<15. The plots, which are limited to the flexural modes (n=1, m=1–6) and to the longitudinal modes (n=0, m=1–4), are compared to the theoretical calculations based on the Pochhammer–Chree frequency equation.

Spherical‐wave scattering by a finite‐thickness solid plate of infinite lateral extent, with some implications for panel measurements
View Description Hide DescriptionThe solution to the problem of the interaction of a spherical wave with a homogeneous and isotropic solid plate of infinite lateral extent, but finite thickness, is considered theoretically. Both the source and the plate are immersed in an infinite, inviscid fluid. Appropriate boundary conditions are imposed on the full three‐dimensional elasticityequations. The solution is evaluated numerically for a variety of materials for a 1‐kHz incident spherical wave and for a 5‐kHz incident spherical wave. For the numerical study, the fluid medium is taken to be water. Under certain conditions, ‘‘overpressures’’ are predicted for both the reflected and transmitted fields (i.e., the amplitude of the reflected pressure and/or the transmitted pressure can exceed the maximum value of the amplitude of the incident pressure on the plate surface). These overpressures are consistent with the law of conservation of energy in the sense that, for a plate composed of lossless material, the total incident power is found to be equal to the sum of the total reflected power plus the total transmitted power. An important conclusion of this research is that the practice of attempting to reduce the influence of edge diffraction in panel tests by using samples of increasingly larger lateral extent may result in measurements that are substantially corrupted by wave‐front curvature effects, particularly if the sample panel includes a steel backing plate.

Mathematical model for internal ultrasonic inspection of cylindrically layered structures
View Description Hide DescriptionAn efficient numerical model is described for predicting the pulse‐echo response of an ultrasonic transducer insonifying a cylindrically layered medium from the inside. A rigorous 3‐D formulation of the problem is first developed. Reduction to an approximate but computationally much more efficient form is accomplished based on several general assumptions regarding the geometrical properties of the transducer and the cylindrically symmetric layered medium. Within these constraints, the model accurately accounts for diffraction, attenuation, and dispersion in the interior immersing fluid and for reflection from the multilayered annular region that may contain strong internal resonances. Calibration of the temporal response of the transducer and associated measurement electronics is an integral part of the model, which is formulated using reciprocity relations for cylindrical coordinates. The principal application for the model is the ultrasonic internal inspection of pipes.

Active cancellation of noise in a liquid‐filled pipe using an adaptive filter
View Description Hide DescriptionAn experimental study of active attenuation of sound in a liquid‐filled pipe is reported. The pipe was connected to a ‘‘noise’’ source at one end and opened into a large liquid reservoir at the other. A sound source for active cancellation purposes was located midway between the two extremities. A cancellation signal to drive the active source was generated by a digital, adaptive filter from hydrophones located in the duct and reservoir. The reduction in noise level was measured at the exit of the pipe into the reservoir and at selected locations in the reservoir. Sound attenuations in the 20‐dB range were observed.

The propagation constant in lossy circular tubes near the cutoff frequencies of higher‐order modes
View Description Hide DescriptionThe propagation constants of higher‐order modes in lossy circular ducts are investigated with particular interest to the frequency range around the corresponding cutoff frequencies. The analytic expressions for the propagation constants published up to now contain poles at each cutoff frequency. In this article, it is shown that no poles exist. The transition between the frequency range of evanescent and propagating modes is rapid but continuous. An explicit formula for the propagation constant is derived. The theory is experimentally checked with an indirect measuring method that is able to give results in close vicinity to the cutoff frequencies. The measured attenuation due to the transmission line losses is partially somewhat higher than theoretically expected. The deviations are less than about 30%.

Some closed‐form solutions of the impedance inverse problem
View Description Hide DescriptionThe time‐domain modified Gelfand–Levitan (MGL) integral equation is solved in closed form for the recovery of acoustical impedance logs from a class of reflection seismograms. The MGL method is compared with usual Gelfand–Levitan (GL) inverse scattering method. A relationship between the kernels, considered to be the known data, of the MGL equation and the Gopinath–Sondhi (GS) equation is established.

Acoustic radiation force on a heated sphere including effects of heat transfer and acoustic streaming
View Description Hide DescriptionIn this investigation, our previous theoretical result on the subject of the acoustic radiation force on a heated sphere [Lee and Wang, J. Acoust. Soc. Am. 7 5, 88–96 (1984)] is reexamined. For a more complete understanding, effects of heat transfer and acoustic streaming are taken into consideration. Essentially, it was found that at high sound‐pressure levels in a steady situation, the force is not affected significantly by the temperature profile, consistent with the result of an experimental work [Leung and Wang, J. Acoust. Soc. Am. 7 7, 1686–1691 (1985)]. This resolves the earlier apparent contradiction between the theory and the experiment. But our previous result is reaffirmed that, if excessive hot air is accumulated around the sphere, which can happen in transient situations, the force can be weakened or reversed in sign. As a by‐product of our study, a heat transfermodel due to acoustic streaming was also found.

Theoretical estimates of sound scattering by atmospheric turbulence
View Description Hide DescriptionThe coefficients of sound scattered by the temperature and wind fluctuations and common normalized scattering phase functions are computed using the von Karman three‐dimensional spectrum of temperaturefluctuations and the von Karman three‐dimensional energy spectrum that enables one to take into account the outer scale of atmospheric turbulence. It is shown that both scattering coefficients have a quadratic frequency dependence and 1.67 power dependence upon the outer scale of turbulence. At the same time, the common scattering cross section is proportional to the cube root of the sound frequency that agrees with the previously obtained results. The obtained results are in a good agreement with the experimental data.

Transient Scholte wave transmission along rough liquid–solid interfaces
View Description Hide DescriptionTransmission characteristics of transient Scholte waves along periodic and random liquid–solid interfaces are demonstrated using laboratory ultrasonic models. These characteristics indicate that, for periodic interfaces, the transmitted Scholte wave is composed of an attenuated main pulse, which arrives first, followed by substantial energy at the Bragg frequency, which is delayed and spread over a wide time window. Random topographic variations in the liquid–solid interface eliminate the transmission of the late‐arriving components. The experiments are limited to a water ‘‘half‐space’’ over a high‐speed solid half‐space, in which case, most of the Scholte wave energy is in the water. The Scholte wavelength is comparable to the horizontal spatial wavelength in the surface but is much greater than the vertical scale of the spatial variations. Results from composite interfaces formed by placing thin parallel scatterers on a plane solid surface and from corrugated interfaces formed by directly machining grooves and ridges in the solid are presented. Field data on transient Scholte wave propagation in the shallow waters of Barrow Strait of the Canadian Arctic are also described. The field data have some similar characteristics to that obtained from the ultrasonic models, which may indicate that the propagation in Barrow Strait is strongly influenced by the bottom topography of the region.

Near‐surface ocean ambient noise measurements at very low frequencies
View Description Hide DescriptionAmbient noise in the frequency range 1–20 Hz was measured for 1‐day periods at each of four widely separated sites in the North Atlantic Ocean with freely drifting, surface‐suspended hydrophones. The acoustic data were recorded aboard an attending research vessel following transmission over a radio link. Narrow‐band spectralanalysis was performed at 1‐h intervals to form time series of noisespectral level for each site. Statistical analyses of the power‐spectral time series included the determination of cumulative probabilities, standard deviation, skew, kurtosis, decorrelation times, and the wind‐speed dependence of mean spectral level. Comparisons with data from bottom‐mounted hydrophones and studies of inter‐ and intra‐array coherence indicate that the data were, with few exceptions, uncontaminated by self‐noise down to 1 Hz. A significant dependence of noise spectrum level upon local wind speed was observed in the 1.5‐ to 3‐Hz frequency band at each site. At 4 Hz, the dependence upon wind speed was much weaker, suggesting that the noise sources at this frequency are more distant. Estimates of noise directionality and tests for normality were also made on the data between 1 and 5 Hz.

The sound fields of an infinite, fluid‐loaded plate excited by a point force
View Description Hide DescriptionThe acoustic fields of an infinite, fluid‐loaded plate that is excited by a source are considered. The plate is modeled exactly as a linear elastic, isotropic, homogeneous layer of constant thickness bounding two different homogeneous fluid half‐spaces. First, the complete acoustic field (pressure fluctuations in the fluids and displacements in the plate) of an harmonic monopole source in one of the fluids is derived. Then, by invoking reciprocity, the sound fields in the fluids of an harmonic point force applied somewhere within the plate are found. There are no restrictions upon the frequency or the orientation of the applied force. A standard asymptotic result for the sound fields in the fluids far from a source in the plate is given.

Exact and approximate theories for fluid‐loaded, infinite, elastic plates
View Description Hide DescriptionThe sound radiated by an infinite, elastic plate excited by a point force is considered. The plate is loaded by two fluid half‐spaces that may have different properties, and the force may have an arbitrary orientation. Results derived at low frequency show that the ‘‘exact’’ solution, when the plate is modeled as an elastic layer, concurs with ‘‘thin plate’’ theories. A demonstration of how a thin plate theory fails as frequency is increased is given. Particular attention is paid to the longitudinal wave in the plate (corresponding to the zero‐order symmetric Lamb mode), which is shown to be sensitive to the symmetry of the fluid loading. The importance of the position of the driving force within the plate’s thickness, symmetric waves, material absorption, and finite plate size is also studied to see when those factors should be included in the plate model. Results tend to confirm the common view that symmetric waves and the position of the force need not be considered when computing the field of a transverse force, at least at low frequencies and with a typical set of material parameters.

Time domain signal transmission and source location in a waveguide: Matched filter and deconvolution experiments
View Description Hide DescriptionThe experiments were made in an ideal air waveguide having Sheetrock (hard) boundaries. Transmissions of a signal were from a source to a pair of receivers. Matched filter and deconvolution procedures were tested. Matched filters were computed for transmissions from a set of trial source locations to the receivers. The trial matched filters were then applied to the signals. The filtered signals were cross correlated. A map of cross‐correlation absolute maxima for trial source locations gave the actual source location. Many smaller maxima indicated false locations. For the second method, the signals were deconvolved with estimates of the impulse transmission from trial locations. At the actual source location, deconvolution operations gave a diverging oscillatory remainder. The oscillations had a higher frequency than the signal. It was possible to low‐pass filter the deconvolved signals and recover good replicas of the transmitted signal. The map of cross correlations of the deconvolved and filtered signals gave the source location. The deconvolution–low‐pass filter method gave fewer and smaller indications of false locations than the matched filter method. However, the matched filter method was more tolerant of errors in the estimates of the impulse response.