Acoustic radiation

About: Acoustic radiation is a research topic. Over the lifetime, 1954 publications have been published within this topic receiving 26931 citations. The topic is also known as: geniculotemporal fibres & geniculotemporal fibers.

On the Acoustic Radiation Pressure on Spheres

Abstract: Although frequent reference is made to acoustic radiation pressure in treatises and memoirs on sound, there appears to be no systematic theoretical development of the subject enabling actual pressures on obstacles of simple geometrical form to be calculated. In the audible range of acoustic frequencies, it is possible to devise, in a number of ways, means of measuring pressure amplitudes in sound waves as first order effects. At supersonic frequencies, however, these methods are no longer serviceable. When the dimensions of resonators of diaphragms become comparable with the wave-length, the physical effects which enable the pressure amplitude to be measured involve intractable diffraction problems, while the extremely high frequencies and small amplitudes involved make the employment of stroboscopic methods of observation extremely difficult. It has been shown, however, that at supersonic frequencies the acoustic radiation pressures on spheres and discs become sufficiently large to be measured easily, at any rate, in liquids. The mean pressure is generally assumed to be proportional to the energy density in the neighbourhood of the obstacle, and on this basis relative measurements can be made, for instance, in the radiation field of a supersonic oscillator. Such formulae may be obtained without restriction as to wave-length, for spheres in plane progressive and stationary radiation fields, and the magnitude of the pressure is found to be of entirely different orders of magnitude in the two cases.

A numerical study of microparticle acoustophoresis driven by acoustic radiation forces and streaming-induced drag forces

Abstract: We present a numerical study of the transient acoustophoretic motion of microparticles suspended in a liquid-filled microchannel and driven by the acoustic forces arising from an imposed standing ultrasound wave: the acoustic radiation force from the scattering of sound waves on the particles and the Stokes drag force from the induced acoustic streaming flow. These forces are calculated numerically in two steps. First, the thermoacoustic equations are solved to first order in the imposed ultrasound field taking into account the micrometer-thin but crucial thermoviscous boundary layer near the rigid walls. Second, the products of the resulting first-order fields are used as source terms in the time-averaged second-order equations, from which the net acoustic forces acting on the particles are determined. The resulting acoustophoretic particle velocities are quantified for experimentally relevant parameters using a numerical particle-tracking scheme. The model shows the transition in the acoustophoretic particle motion from being dominated by streaming-induced drag to being dominated by radiation forces as a function of particle size, channel geometry, and material properties.

Radiation and outflow boundary conditions for direct computation of acoustic and flow disturbances in a nonuniform mean flow

Abstract: It is well known that Euler equations support small amplitude acoustic, vorticity and entropy waves. To perform high quality direct numerical simulations of flow generated noise problems, acoustic radiation boundary conditions are required along inflow boundaries. Along boundaries where the mean flow leaves the computation domain, outflow boundary conditions are needed to allow the acoustic, vorticity and entropy disturbances to exit the computation domain without significant reflection. A set of radiation and outflow boundary conditions for problems with nonuniform mean flows are developed in this work. Flow generated acoustic disturbances are usually many orders of magnitude smaller than that of the mean flow. To capture weak acoustic waves by direct computation (without first separating out the mean flow), the intensity of numerical noise generated by the numerical algorithm and the radiation and outflow boundary conditions (and the computer) must be extremely low. It is demonstrated by a test problem ...

Measurements of bubble-enhanced heating from focused, MHz-frequency ultrasound in a tissue-mimicking material.

Abstract: Time-resolved measurements of the temperature field in an agar-based tissue-mimicking phantom insonated with a large aperture 1-MHz focused acoustic transducer are reported. The acoustic pressure amplitude and insonation duration were varied. Above a critical threshold acoustic pressure, a large increase in the temperature rise during insonation was observed. Evidence for the hypothesis that cavitation bubble activity in the focal zone is the cause of enhanced heating is presented and discussed. Mechanisms for bubble-assisted heating are presented and modeled, and quantitative estimates for the thermal power generated by viscous dissipation and bubble acoustic radiation are given.

A finite-element method model of soft tissue response to impulsive acoustic radiation force

Abstract: Several groups are studying acoustic radiation force and its ability to image the mechanical properties of tissue. Acoustic radiation force impulse (ARFI) imaging is one modality using standard diagnostic ultrasound scanners to generate localized, impulsive, acoustic radiation forces in tissue. The dynamic response of tissue is measured via conventional ultrasonic speckle-tracking methods and provides information about the mechanical properties of tissue. A finite-element method (FEM) model has been developed that simulates the dynamic response of tissues, with and without spherical inclusions, to an impulsive acoustic radiation force excitation from a linear array transducer. These FEM models were validated with calibrated phantoms. Shear wave speed, and therefore elasticity, dictates tissue relaxation following ARFI excitation, but Poisson's ratio and density do not significantly alter tissue relaxation rates. Increased acoustic attenuation in tissue increases the relative amount of tissue displacement in the near field compared with the focal depth, but relaxation rates are not altered. Applications of this model include improving image quality, and distilling material and structural information from tissue's dynamic response to ARFI excitation. Future work on these models includes incorporation of viscous material properties and modeling the ultrasonic tracking of displaced scatterers.