Scaling relations for sound scattering by a lattice of hard inclusions in a soft mediuma).

Soft elastic materials embedded with resonant inclusions are widely used as acoustic coatings for maritime applications. A versatile analytical framework for resonance scattering of sound waves in a soft material by a lattice of hard inclusions of complex shape is presented. Analogies from hydrodynamics and electrostatics are employed to derive universal scaling relations for a small number of well-known lumped parameters that map resonant scattering of a complex-shaped hard inclusion to that of a sphere. Multiple scattering of waves between inclusions in proximity is also considered. The problem is then treated using an effective medium theory, viz, a layer of hard inclusions is modeled as a homogenized layer with some effective properties. The acoustic performance of hard inclusions for a range of shapes with spheres of the same volume are compared. Results obtained using this approach are in good agreement with finite element simulations.

Sound absorption by a metasurface comprising hard spheres in a soft medium.

We present a theoretical framework for acoustic wave propagation in a metasurface comprising a hexagonal lattice of hard spherical inclusions embedded in a soft elastic medium. Each layer of inclusions in the direction of sound propagation is approximated as a homogenized layer with effective geometric and material properties. To account for multiple scattering effects in the lattice of resonant inclusions, an analogy between the fluid dynamics of creeping flows and elastodynamics of soft materials is implemented. Results obtained analytically are in excellent agreement with numerical simulations that exactly model the geometric and material properties of the metasurface.

Evaluating diffusion resistance of a constriction in a membrane channel by the method of boundary homogenization.

In this paper we analyze diffusive transport of noninteracting electrically uncharged solute molecules through a cylindrical membrane channel with a constriction located in the middle of the channel. The constriction is modeled by an infinitely thin partition with a circular hole in its center. The focus is on how the presence of the partition slows down the transport governed by the difference in the solute concentrations in the two reservoirs separated by the membrane. It is assumed that the solutions in both reservoirs are well stirred. To quantify the effect of the constriction we use the notion of diffusion resistance defined as the ratio of the concentration difference to the steady-state flux. We show that when the channel length exceeds its radius, the diffusion resistance is the sum of the diffusion resistance of the cylindrical channel without a partition and an additional diffusion resistance due to the presence of the partition. We derive an expression for the additional diffusion resistance as a function of the tube radius and that of the hole in the partition. The derivation involves the replacement of the nonpermeable partition with the hole by an effective uniform semipermeable partition with a properly chosen permeability. Such a replacement makes it possible to reduce the initial three-dimensional diffusion problem to a one-dimensional one that can be easily solved. To determine the permeability of the effective partition, we take advantage of the results found earlier for trapping of diffusing particles by inhomogeneous surfaces, which were obtained with the method of boundary homogenization. Brownian dynamics simulations are used to corroborate our approximate analytical results and to establish the range of their applicability.

Acoustic radiation from a cylindrical shell with a voided soft elastic coating.

An analytical framework for sound radiation from a fluid-loaded cylindrical shell covered with an acoustic coating is presented. The coating is composed of a soft elastic material embedded with a circumferential layer of equispaced voids. The layer of voids is modeled as an effective fluid medium sandwiched between two layers of the host material. Expressions for the effective impedance of the coating, radial displacement of the elastic shell, and the structure-borne radiated pressure for harmonic excitation of the shell are derived. Results show that the coating design can be tuned to reduce the radiated sound in a broad frequency range.

Sound scattering by a lattice of resonant inclusions in a soft medium.

We present a generalized analytical model to investigate acoustic scattering by a lattice of voids of arbitrary shape in a viscoelastic matrix. To this end, we represent the lattice of voids using effective boundary conditions that incorporate multiple scattering effects. Applying analogies between acoustics and electrostatics, the model is derived for voids of nonspherical shape and different lattice arrangements. Our analytical results are compared with those from numerical simulations as well as experimental results from the literature.

Acoustic performance of gratings of cylindrical voids in a soft elastic medium with a steel backing.

An approximate analytical model is presented to investigate sound transmission, reflection and absorption of a rubber-like medium comprising a single layer of periodic cylindrical voids attached to a steel backing. The layer of voids is modelled as a homogeneous medium with effective material and geometric properties. A numerical model based on the finite element method is developed to validate results from the homogenization model, as well as to show further insights into the physical mechanisms associated with the system acoustic performance. Monopole resonance of the voids is shown to reduce sound transmission through the voided medium due to increased reflection, resulting in poor sound absorption around this frequency. Peaks of high sound absorption are attributed to Fabry-Pérot resonance with the frequency of the first peak derivable by a lumped spring-mass analogy. Sound absorption for a single layer of voids in a soft elastic medium with a steel backing is shown to be similar to the sound absorption in the same elastic medium but without the steel backing, for a single layer of voids and its mirror image in the direction of sound propagation.

International variation in the incidence of hypertension in pregnancy among primiparae: the Jamaican experience - abstract

The study was undertaken to determine the natural history of blood pressure measurements during pregnancy in a geographical population of primiparae in the Caribbean, and to compare this with similarly measured populations. It used data collected during a randomized controlled trial of low dose aspirin. Serial measurements of blood pressure, proteinuria and maternal weight were obtained at each antentalepisode and inpatient admission. The population comprised 6043 primiprae resident in the parishes of Kingston and St. Andrew, Jamaica, with singleton pregnancies and no indications for exculsion from randomization in the low dose aspirin trial. The mean systolic and diastolic blood pressure at 20 - 24 weeks gestation (105mm Hg and 63mm, respectively) was substantially lower than that found in four different population in south-east Asia or in the United Kingdom (UK). During the latter half of pregnancy, there was a rise of 4 mm Hg systolic and 5 mm Hg diastolic, substantially less than that found among primiparae in China (9mm) or the UK (9mm). There were, however, high prevalences of eclampsia and proteinuria, and maternal and perinatal mortality ascribed to maternal hypertension is substantial. Proteinuria prevalence started to increase with diastoblic blood pressures of 80 mm. In conclusion, there is evidence that definition of hypertension in pregnancy in Jamaica should involve lower levels of diastoblic blood pressure than those used in many countries. No epidemiological studies are yet available to ascertain difference between European countries or American populations. These should help formulate coherent theories as to the aetiology of the hypertensive disorders of pregnancy(AU)

Low dose aspirin in pregnancy are there any benefits

Although the question of major prophylactic benefits of low-dose aspirin in preventing pre-eclampsia or intrauterine growth retardation in pregnancies of mothers who had had such a history has been convincingly negated by the CLASP study, there has remained a question whether pregnancies of primiparae (the group most at risk of such outcomes) might benefit. This study was designed as a randomized controlled trial to address this possibility and other hypotheses arising from the CLASP study. The hypotheses addressed concerns whether low-dose aspirin prevented in or delayed the onset of the hypertensive disorders of pregnancy, including pre-eclampsia, and eclampsia, and whether preterm delivery or low birthweight rates were reduced. In addition, the study was designed to assess whether enrolment, early rather than late, had the most beneficial effect. In parallel, possible adverse effects on mother and infant were monitored. A total of 6275 primiparae were enrolled from 12 weeks gestation in community clinics in Kingston and St. Andrew in Jamaica; 97 percent of them were followed throughout pregnancy. There were no differences in the development of markers of hypertensive disorders, the prevalence of pre-eclampsia or eclampsia except for oedema, which was more prevalent in those on placebo. Mothers on the drug showed no differences in mean diastolic bloodpressure at any stage of pregnancy - they were not less likely to deliver preterm or have a larger foetus. They were, however, significantly more likely to suffer from bleeding disorders antenatally, intrapartum and postpartum. It is concluded that low-dose aspirin has no place in the management of normal pregnancy (AU)