Theory Of Acoustic Propagation Research Articles

Study of Near-field Acoustic Pressure in Near-coastal Ice-water Media Based on Angular Spectrum Method

For seawater detection, acoustic means is currently one of the most promising technical means of development. According to the physical properties of acoustic waves, they can be effectively propagated in seawater and ice, and at the interface of different media, there will be reverse acoustic scattering. Using the theory of acoustic propagation in solids, the propagation velocity and refractive index of transverse and vertical, and taking the longitudinal wave as an example, we use the angular spectrum method to solve the fluctuation equation and obtain the beam angle spectrum. Numerical software simulation calculates the time domain characteristics of acoustic scattering at different incidence angles. The effect of evanescent waves on solid sound transmission is also considered. Considering the effect of apocalyptic waves on thin ice on the ocean surface, it provides a theoretical basis for further research on detecting offshore ice using acoustic methods.

Weld quality inspection of small-diameter thin-walled pipes by a laser ultrasonic method.

Defect inspection of small-diameter thin-walled pipes is a difficult problem in the field of nondestructive testing. In this paper, a new detection method based on laser ultrasonics and guided circumferential wave technology is proposed and used to inspect the defects in pipes. First, a theoretical model based on the theory of acoustic propagation in solids is proposed for the small-diameter thin-walled pipes. The dispersion characteristics of the guided circumferential waves of thin-walled pipes are obtained by numerical simulation. Second, a laser ultrasonic system is constructed and used to detect the flaws in the small-diameter thin-walled pipes. Finally, the laser ultrasonic system is used to analyze the welding quality of stainless steel pipes with a diameter of 4.20 mm and a wall thickness of 0.30 mm. The experimental results are in agreement with the theoretical analysis, which demonstrates the reliability and practicability of the laser ultrasonic method in the weld quality inspection of small-diameter thin-walled pipes.

On geoacoustic inversions, sediment acoustics, and deep ocean ambient noise

During the period of 2005–2011, three topics in acoustical oceanography were actively investigated in the Buckingham Lab which combined field, lab, and theoretical work. Geoacoustic inversions in the low frequency (<500 Hz) band were carried out using the noise generated by a light aircraft off the coast of La Jolla, California, and during the MAKAI experiment in Kauai, Hawaii. The inversion relied on a comparison of data to an analytical model of a moving airborne source over a two-layer shallow water waveguide. In an effort to investigate the link between grain roughness and porosity to support the development of a theory of acoustic propagation in sediments, a statistical model for the two-dimensional shape of sand grains was developed from the digitized outlines of grains from a dozen locations, including deserts, beaches, and seabeds. Lastly, a family of autonomous free-falling ambient noise profilers known as Deep Sound, were designed, manufactured, and deployed, returning measurements of ambient noise from below the critical (or reciprocal) depth in the deep ocean. Field experiments were carried out in the San Diego trough, the Philippine Sea, the Tonga Trench, and the Sirena Deep and Challenger Deep in the Mariana Trench.

Seabed reflection measurement results shed light on theories of acoustic propagation in marine sediments

Marine sediments are inherently a complex assemblage of solid particles (of size ranging over five orders of magnitude) with fluid-filled interstices sometimes containing gas. The mechanisms that govern the dispersive wave speeds and concomitant attenuation are still hotly debated. In order to shed light on the mechanisms and current theoretical approaches to approximating them, measurements of in-situ sediments are desirable since they provide the natural complexity desirable for testing theories against realistic sediment structures. On the other hand, in-situmeasurements present significant challenges since seabed effects must be separated from other ocean processes, e.g., water column variability, biologics and sea surface roughness, and bubbles. Seabed reflection measurements offer one way to mitigate unwanted ocean processes. Reflection-derived observations are presented that shed light on current sediment acoustic models. In addition, experiments are proposed which will further test and help guide sediment acoustics theoretical developments. [Work supported by the ONR Ocean Acoustics Program.]Marine sediments are inherently a complex assemblage of solid particles (of size ranging over five orders of magnitude) with fluid-filled interstices sometimes containing gas. The mechanisms that govern the dispersive wave speeds and concomitant attenuation are still hotly debated. In order to shed light on the mechanisms and current theoretical approaches to approximating them, measurements of in-situ sediments are desirable since they provide the natural complexity desirable for testing theories against realistic sediment structures. On the other hand, in-situmeasurements present significant challenges since seabed effects must be separated from other ocean processes, e.g., water column variability, biologics and sea surface roughness, and bubbles. Seabed reflection measurements offer one way to mitigate unwanted ocean processes. Reflection-derived observations are presented that shed light on current sediment acoustic models. In addition, experiments are proposed which will further test and help guide ...

Broadband noise suppression for turbofan inlet applications

Broadband noise in turbofan inlets is represented in a frequency domain analysis by a statistical distribution of propagating duct modes. The required number of modes is estimated by Rice’s approximation method. A brief overview of essential results from the theory of acoustic propagation in ducts is given, providing a starting point for Rice’s estimation of the distribution of propagating modes in a uniform unlined duct required to represent a broadband noise source. The relationship between mode cut-off ratio and attenuation in lined ducts is examined to connect cut-off ratio to modal attenuation, and it is shown by example that for arbitrary impedance at a specified frequency, modes with similar cut-off ratios have similar attenuation. It is then shown by example that the attenuation of a low-order circumferential mode, specifically the axially symmetric circumferential mode, with a large range of propagating radial mode cut-off ratios, is insensitive to impedance variations near the optimum impedance. Two models for estimating achievable attenuation for broadband noise attenuation, based on equipartition of acoustic power among propagating modes, are considered. One uses only the axially symmetric circumferential mode, and the second uses all propagating circumferential modes and their associated propagating radial modes. A new parallel architecture version of an established propagation code, designed to include all propagating modes on a statistical basis, is described. Examples for prediction of acoustic power attenuation of broadband noise at low frequency and at moderate frequency are used to compare the performance of the axially symmetric circumferential mode and the all circumferential mode models. It is shown by example that a broadband noise model based on the axially symmetric circumferential mode predicts lower attenuation than a model with all propagating circumferential modes. Rice’s correlation of mode cut-off ratio with attenuation justifies this observation.

Design of Longitudinal-Torsional Vibration Stepped Horn with Multiple Diagonal Slits

Based on the theory of acoustic propagation, the realization mechanism of longitudinal-torsional vibration (LTV) by converter of cylinder with multiple diagonal slits (MDS) under single excitation was studied. The influences of composite stepped horns structural parameters on resonant frequency were studied by numerical analysis method, and the horn (resonant frequency about 20kHz) with LTV was designed. The results show that LTV could be realized under single excitation of longitudinal vibration when resonant frequency is 20498Hz, and the amplitude varied periodically. The results can be applied in the design of acoustic system in ultrasonic machining with LTV.

Variability of parameter estimates used in the Buckingham viscous grain shearing model.

A compilation of 54 sets of geoacoustic measurements of marine sediments was reviewed in order to bound estimates of parameters for use in Buckingham’s grain shearing (GS) theory of acoustic propagation. These data, for unconsolidated sands (siliciclastic and carbonate) with grain sizes between 0.0156 and 0.57 mm, were all made in shallow water (maximum depth of 60 m) sites at diverse locales. In each data set, measurements of the speed and attenuation of the compressional wave at high frequency, the shear wave speed at 1 kHz, porosity, and bulk density are sufficient to calculate the three free parameters in GS theory (a material exponent, a compressional coefficient, and a shear coefficient). The spread of the values calculated for GS parameters, combined with a sensitivity analysis, does not support the use of a single material exponent value for all sediments. Shear wave attenuation measured at 18 sites was compared to the predictions of Buckingham’s (2010) most recent version of the VGS model. In this model the shear viscoelastic time constant is based on the wavelength of the propagating shear wave rather than shear speed.

Theory for acoustic propagation in marine sediment containing gas bubbles which may pulsate in a non‐stationary nonlinear manner

Whilst there is a considerable body of work in the literature on the theory of acoustic propagation in marine sediments, the incorporation of gas bubbles into such theories is done with the inclusion of assumptions which severely limit the applicability of those models to practical gas‐laden marine sediments. This paper provides a theory which does not assume that the bubble dynamics are linear, steady‐state and monochromatic. Such assumptions would be incompatible with many of the practical acoustic fields with which sediments are insonified today. These fields are necessarily high amplitude to provide adequate signal‐to‐noise ratios, given the high attenuation in gassy marine sediments; and often they utilise short pulses to obtain range resolution. This paper provides a theory appropriate for predicting the acoustically‐driven non‐stationary nonlinear dynamics of spherical gas bubbles embedded in a lossy elastic solid, and discusses how this could be incorporated into a nonlinear, time‐dependent propagation model.

Ultrasonic wave propagation and scattering in cancellous bone

Theoretical models and experimental data describing the interaction between ultrasound and cancellous bone will be discussed. Ultrasonic attenuation in cancellous bone is much greater than that for soft tissues and varies approximately linearly with frequency between 400 kHz and 1.7 MHz. Speed of sound in cancellous bone is slightly higher than that for soft tissues and decreases gradually with frequency at diagnostic frequencies (between 300 and 700 kHz). The dependence of phase velocity on porosity may be predicted from theory of acoustic propagation in fluid-filled porous solids. The negative dispersion can be explained using a stratified two-component model. At diagnostic frequencies, scattering varies approximately as frequency to the nth power where 3<n<3.5. This may be explained either by a continuous or discrete scattering model. The latter represents trabeculae as small finite-length cylindrical scatterers. The frequency dependence of scattering, along with the anisotropy of scattering and attenuation, implies that absorption is a greater source of attenuation than scattering. [Funding from the FDA Office of Women’s Health is gratefully acknowledged.]

Theory of inert gas-condensing vapor thermoacoustics: propagation equation.

The theory of acoustic propagation in an inert gas-condensing vapor mixture contained in a cylindrical pore with wet walls and an imposed temperature gradient is developed. It is shown that the vapor diffusion effects in the mixture are analogous to the heat diffusion effects in the thermoacoustics of inert gases, and that these effects occur in parallel with the heat diffusion effects in the wet system. The vapor diffusion effects can be expressed in terms of the thermoviscous function F(lambda) used in the theory of sound propagation of constant cross-section tubes. As such, these results can be extended to any shape parallel-walled tube. The propagation equations predict that the temperature gradient required for onset of sound amplification in a wet-walled prime mover is much lower than the corresponding temperature gradient for an inert gas prime mover. The results of a measurement of the onset temperature of a simple demonstration prime mover in air with a dry stack and with a stack wetted with water provide a qualitative verification of the theory.

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° incidence, Snell's Law implies a refracted angle of 40±2°, whereas the energy direction is observed to be 70±2°. 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° 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.

Theory of acoustic propagation in a multi-phase stratified liquid flowing within an elastic-walled conduit of varying cross-sectional area

A theoretical solution is derived for the sound field in an arbitrarily layered viscous fluid moving with the non-negligible Mach number within a duct with elastic walls and varying cross-sectional area. The solution is applied to the interpretation of infrasonic and seismic signals preceding and accompanying volcanic eruptions, and can be used to study the acoustic response of various types of volcanic fluids, including magma–gas mixtures, ash–gas mixtures, and bubble-rich liquids. The acoustic field in a magma conduit can be propagated into the atmosphere through an open vent, and coupled into the ground through the displacement of the magma conduit walls. The theoretical solutions predict that fluids moving with the non-negligible Mach number will exhibit significant attenuation in the upstream direction, thereby reducing the quality factor of the conduit resonance. Thus, acoustic energy generated during an eruption may be preferentially radiated downstream, exacerbating the acoustic decoupling between the upper and lower parts of a stratified magma column. A source model for a repeated cavitation process is introduced as a possible excitation mechanism for tremor signals.

Wave diffraction at reflection and propagation in layered media

In his theory of waves in layered media, L. M. Brekhovskikh has systematically employed integral representations of the field in terms of plane or quasi-plane waves, with all the other useful representations of the field derived from this exact solution. It is this approach that enabled him to efficiently study, and for the first time gain a deep insight into and a quantitative description of the fundamental diffraction phenomena accompanying total reflection, guided propagation, and formation of caustics in inhomogeneous media. In this presentation, L. M. Brekhovskikh’s work on wave diffraction in layered media, including his concept of diffracted rays, study of ray and beam displacement at reflection, investigation of ray theory and the WKBJ approximation domains of validity, and an exact solution for a range-dependent benchmark problem, are reviewed. Recent developments in the theory of acoustic propagation in inhomogeneous media are traced back to the ideas first introduced and explored by L. M. Brekhovskikh.

Biot's theory of acoustic propagation in porous media applied to aerogels and alcogels

Comparison of acoustic propagation in alcogels and aerogels presents an interesting difference for the high porosity: in alcogels, longitudinal wave velocity remains around the velocity in alcohol while, in aerogels, it is significantly lower than the velocity in air. In this paper, Biot's model of acoustic propagation in porous media is applied. We find a good agreement with measurements in terms of velocity that validates this theory for both alcogel and aerogel cases. Qualitative agreement is obtained for absorption but further comparisons are needed to test quantitative agreement. The theory allows explanation of behaviour differences observed by distinguishing the influence of the solid and fluid phases from that of the gel network.

Chemical and Structural Evolution of Silica Alcogels During their Formation: Acoustical Study

Silica aerogels are materials of very high porosity obtained through a sol-gel process. Since their structural properties greatly influence their acoustical and mechanical properties, it is important to follow how the different chemical reactions occur during this process. We present a study of these evolutions for two different types of alcogels through acoustical measurements in the ultrasonic range. These samples have been chosen because of the significantly different behavior of the derived aerogels. The data obtained during gelation show fluctuations of the velocity that can be interpreted in terms of chemical changes. The absence of major changes in the velocity before and after gelation is explained with the aid of Biot's theory of acoustic propagation in porous media.

Acoustic bottom penetration in a shallow water site from a parametric projector

Acoustic penetration of a sandy ocean sediment by a narrow beam signal was investigated. A parametric source, the Simrad TOPAS, projected acoustic pulses at a shallow grazing angle into a sandy silt sediment in which a sparse 3-D array of acoustic sensors was buried, in a site off La Spezia, Italy. Although the experiment was conducted in shallow waters, the received signals were free of surface-bounce multipaths, due to the narrowness of sound beam, and excellent penetration was obtained. The collected signals were processed for direction and speed of the sediment acoustic waves. From a theoretical point of view, the medium is treated as a poroelastic solid governed by Biot’s theory of acoustic propagation. Comparisons are made between theory and experiment. [Work supported by Office of Naval Research, Ocean Acoustics Program, Code 321.]

Comparison of theory and experiment of acoustic bottom penetration in a shallow water site

Signals from a spark source in water and received by acoustic sensors buried in a sandy sediment in a site off La Spezia, Italy, are compared to theoretical model predictions from a fast field model. The model is a modification of OASES 1.6 that includes Biot’s poroelastic wave propagation model. The received acoustic pulses are compared to determine the transfer function from the in‐water source to the buried acoustic sensors. From a theoretical point of view, the sediment is treated as a poroelastic solid governed by Biot’s theory of acoustic propagation. It is predicted to support two acoustic waves. Comparisons are made between theory and experiment. [Work supported by Office of Naval Research, Ocean Acoustics Program, code 321 OA, under the initiative for basic research in the physics of moderate to high frequency acoustics.]

Biot model of sound propagation in water-saturated sand

Elastic theory of wave propagation and the measured speed of sound in sandy ocean sediments indicate that such sediments are impenetrable to high-frequency sound at shallow grazing angles. The speed of sound in water-saturated, unconsolidated sand is in the region of 1700 m/s which, under the elastic theory of wave propagation, gives it a critical grazing angle in the region of 28°. At shallower grazing angles, refraction is not permitted, and total internal reflection is predicted. Recent experimental measurements contradict this view. Biot’s theory of acoustic propagation in porous sediments is the most likely explanation. Biot’s theory of acoustic propagation, as it applies to water-saturated sand, is reviewed. The speed of the slow wave is found to be higher than previously predicted. New input parameter values are deduced.

High-frequency acoustic penetration of ocean sediments

Two experiments were conducted to investigate acoustic penetration of ocean sediments: one in a muddy site in the Baltic Sea and the other in a sandy site in the Gulf of Mexico. In both experiments, acoustic sensors were placed in the sediment to measure the acoustic signals. The signals are processed coherently to give direction and speed of the acoustic waves. The data from the two sites give very different results. In the muddy site, the sediment sound speed is very close to that of the water, the situation in the sandy site is more complicated. The medium is treated as a poro-elastic solid governed by Biot’s theory of acoustic propagation. For the sandy site, the theory predicts two acoustic waves, one faster than the sound speed in water, and the other slower. Comparisons are made between theory and experiment. [Work supported by Office of Naval Research, under the Coastal Benthic Boundary Layer (CBBL) Special Research Program.]

Moderate frequency acoustic penetration of a sandy shallow water sediment

An experiment was conducted to investigate acoustic penetration of a sandy shallow water sediment, in the band 500 Hz to 2 kHz, in a site off LaSpezia, Italy. Acoustic sensors were buried in the sediment, forming a sparse three-dimensional array, to measure the sediment penetrating signals. The sound source was a sparker. The collected signals are processed coherently to give direction and speed of the sediment acoustic waves. From a theoretical point of view, the medium is treated as a poro-elastic solid governed by Biot’s theory of acoustic propagation. It is predicted to support two acoustic waves. Comparisons are made between theory and experiment. [Work supported by Office of Naval Research, Ocean Acoustics Program, Code 11250A, under the initiative for basic research in the physics of moderate to high frequency acoustics.]