Jens M. Hovem

Oscillations of polymeric microbubbles: Effect of the encapsulating shell

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.

Viscous attenuation of sound in saturated sand

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.

Optimal Modal Beamforming for Spherical Microphone Arrays

An approach to optimal array pattern synthesis based on spherical harmonics is presented. The array processing problem in the spherical harmonics domain is expressed with a matrix formulation. The beamformer weight vector design problem is written as a multiply constrained problem, so that the resulting beamformer can provide a suitable trade-off among multiple conflicting performance measures such as directivity index, robustness, array gain, sidelobe level, mainlobe width, and so on. The multiply constrained problem is formulated as a convex form of second-order cone programming which is computationally tractable. We show that the pure phase-mode spherical microphone array can be viewed as a minimum variance distortionless response (MVDR) beamformer in the spherical harmonics domain for the case of spherically isotropic noise. It is shown that our approach includes the delay-and-sum beamformer and a pure phase-mode beamformer as special cases, which leads to very flexible designs. Results of simulations and experimental data processing show good performance of the proposed array pattern synthesis approach. To simplify the analysis, the assumption of equidistant spatial sampling of the wavefield by microphones on a spherical surface is used and the aliasing effects due to noncontinuous spatial sampling are neglected.

Experimental observation of subharmonic oscillations in Infoson bubbles

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.

An experimental investigation of swimbladder resonance in fishes

An investigation of the resonant behavior of the swimbladder for different species and sizes has been performed. Most measurements have been done using the ring‐hydrophone method, but also a few normal echo measurements have been done. The results show the resonance frequency to increase with depth, but also to depend on what degree the fish is adapted to the depth. After a sudden transfer to new depth, the resonance frequency will first oscillate and then gradually adjust to a new resonance frequency given by the necessary swimbladder volume to retain neutral buoyancy. The time required to adapt to a deeper depth is longer than for a transfer in the opposite direction suggesting that gas production takes longer time than letting out gas. The observed Q values are also dependent on adaption, but appears to increase almost linearly with the resonance frequency at least for low Q values. The measured values are compared with a model for scattering from swimbladder fish and values for the swimbladder tension...

Viscous attenuation of sound in suspensions and high‐porosity marine sediments

The Biot theory for sound propagation in porous media has been specialized to suspensions of particles and compared with an earlier model based on scattering theory, for sound velocity and viscous attenuation in suspensions. It is shown that the Biot theory gives fairly correct predictions of attenuation as a function of concentration, whereas the suspension model fails when the concentration exceeds a few percent. The reason for this is discussed and a modification to the suspension theory is proposed. This theory has been applied to study viscous attenuation as a function of frequency and it is shown that viscous attenuation may have an almost linear dependence on frequency when account is taken of distributed grain sizes.

Acoustic waves in finely layered media

The 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.

Array Pattern Synthesis With Robustness Against Manifold Vectors Uncertainty

The directivity pattern of an array is known to degrade in the presence of errors in the array manifolds, with respect to the desired nominal array pattern. This paper describes a new robust pattern synthesis approach to arrays with manifold vectors perturbation. This synthesis technique optimizes the worst case performance by minimizing the worst case sidelobe level while maintaining a distortionless respect to the worst case signal steering vector. The possible values of the manifold are covered by an ellipsoid that describes the uncertainty in terms of errors in element gains and phase angles. The pattern synthesis parameters can be optimally chosen based on known levels of uncertainty in the manifold vectors. Two optimization criteria, l 2 regularization and l 1 regularization, of a robust beamformer are proposed. Both criteria of the robust beamformer problem can be reformulated in a convex form of second-order cone programming, which is computationally tractable. A simple lower bound on the difference between the worse case sidelobe level of the robust beamformer and the sidelobe level of the nominal optimal beamformer with no array manifold uncertainty is derived. This robust approach is applicable to arrays with arbitrary geometry. Its effectiveness is illustrated through its application to a circular hydrophone array. An experiment is performed to measure the manifold vectors uncertainty set of hydrophone arrays. Results of applying the algorithms to both simulated and experimental data are presented and they show good performance of the proposed robust pattern synthesis approach.

Linking Acoustic Communications and Network Performance: Integration and Experimentation of an Underwater Acoustic Network

Underwater acoustic networks (UANs) are an emerging technology for a number of oceanic applications, ranging from oceanographic data collection to surveillance applications. However, their reliable usage in the field is still an open research problem, due to the challenges posed by the oceanic environment. The UAN project, a European-Union-funded initiative, moved along these lines, and it was one of the first cases of successful deployment of a mobile underwater sensor network integrated within a wide-area network, which included above water and underwater sensors. This contribution, together with a description of the underwater network, aims at evaluating the communication performance, and correlating the variation of the acoustic channel to the behavior of the entire network stack. Results are given based on the data collected during the UAN11 (May 2011, Trondheim Fjord area, Norway) sea trial. During the experimental activities, the network was in operation for five continuous days and was composed of up to four Fixed NOdes (FNOs), two autonomous underwater vehicles (AUVs), and one mobile node mounted on the supporting research vessel. Results from the experimentation at sea are reported in terms of channel impulse response (CIR) and signal-to-interference-plus-noise ratio (SINR) as measured by the acoustic modems during the sea tests. The performance of the upper network levels is measured in terms of round trip time (RTT) and probability of packet loss (PL). The analysis shows how the communication performance was dominated by variations in signal-to-noise ratio, and how this impacted the behavior of the whole network. Qualitative explanation of communication performance variations can be accounted, at least in the UAN11 experiment, by standard computation of the CIR and transmission loss estimate.

Reflection loss at a bottom with a fluid sediment layer over a hard solid half‐space

Numerical modeling and experiments have shown that for acoustic propagation in shallow water where the bottom has a layer of relatively soft sediment over a hard rock subbottom, high losses may occur for certain frequencies and angles. These high losses have been attributed to shear waves in the sediment, but this paper demonstrates that the high losses also can occur without shear waves in the sedimentary layer and can be attributed to the excitation of interface waves of the Stoneley type at the fluid/solid interface between the sediment and the substrate. These waves are generated by the evanescent waves in the sedimentary layer for incoming plane waves in the water at grazing angles smaller than critical. The role of the sediment layer is to produce the inhomogeneous wave that is required to excite the interface wave. From an analysis of the dispersion equations, one can predict the necessary conditions for the high losses to occur.