A model for the oscillation of gas bubbles encapsulated in a thin shell has been developed. The model depends on viscous and elastic properties of the shell, described by thickness, shear modulus, and shear viscosity. This theory was used to describe an experimental ultrasound contrast agent from Nycomed, composed of air bubbles encapsulated in a polymer shell. Theoretical calculations were compared with measurements of acoustic attenuation at amplitudes where bubble oscillations are linear. A good fit between measured and calculated results was obtained. The results were used to estimate the viscoelastic properties of the shell material. The shell shear modulus was estimated to between 10.6 and 12.9 MPa, the shell viscosity was estimated to between 0.39 and 0.49 Pas. The shell thickness was 5% of the particle radius. These results imply that the particles are around 20 times more rigid than free air bubbles, and that the oscillations are heavily damped, corresponding to Q-values around 1. We conclude that the shell strongly alters the acoustic behavior of the bubbles: The stiffness and viscosity of the particles are mainly determined by the encapsulating shell, not by the air inside.

Based on Biot’s theory for the propagation of sound in a fluid-saturated porous medium, the viscous attenuation of sound has been studied both theoretically and experimentally. It is shown that the important dynamic parameters can be derived from a knowledge of the permeability, grain size, and porosity. Measurements in uniform sized beads show attenuation and velocity dispersion as a function of frequency in agreement with the theory. A comparison between theoretical viscous attenuation and earlier reported measurements indicates that viscous losses may be of significant importance for higher frequencies in sands of high permeability.

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computed synthetic seismograms for different dominant frequencies and determined in this way the ratio Aid for several different cases. For equal thicknesses of epoxy and glass, they found the ratio to be about 8, and for equal thickness sandstone and limestone, they found values between 6 and 7. From theoretical calculation, Helbig (1984)found the ratio to be about 3. The significant variation in the values found for the transitional frequency was the motivation for this study.The two velocities for a two-component medium where the constituents have sound velocities c 1 and c z, densities PI and Pz, and relative thicknesses d 1 and d z with d = d 1 + d z, are defined as time average velocity: ABSTRACTThe propagation of acoustic waves through a periodically stratified medium is examined theoretically and experimentally with the purpose of determining how the velocity of the composite material depends on the periodicity structure, the material properties, and frequency. A numerical simulation of a recently published experiment shows that the propagator method gives results in close agreement with the experimental observations. Using eigenvalue analysis, an expression for the sound velocity and scattering loss is calculated for all frequencies. The results show that, for frequencies lower than a certain critical (or limiting) frequency, the propagation is dispersive and no loss occurs. Above this frequency the waves are evanescent and suffer scattering loss at each interface. An expression for the limiting frequency is derived which takes into account the contrast in impedance between the two media.

This paper reports the result of an experimental study of nonlinear emission from gas-filled bubbles with particular emphasis on the subharmonic emission. The gas bubbles are Infoson, a contrast agent for use in echocardiology consisting of small gas-filled microspheres with a mean diameter of approximately 4 μm. Pulsed signals with center frequencies of 3.5 and 4 MHz were transmitted through a cloud of bubbles and the level of the subharmonic component was measured as a function of the level of the exciting signal. No sharp threshold, as expected from theory, is found. The amplitude of the subharmonic response increases, however, approximately as the cube of the driving pressure when the driving pressure is in the region of 50–100 kPa.