Behavior of rigid-porous layers at high levels of continuous acoustic excitation: theory and experiment

A model for the propagation of high amplitude continuous sound through hard-backed rigid-porous layers has been developed which allows for Forchheimer's correction to Darcy's law. The nonlinearity associated with this is shown to be particularly important in the range of frequencies around layer resonance. The model is based on the introduction of particle velocity dependent flow resistivity into the equivalent fluid model expression for complex tortuosity. Thermal effects are accounted for by means of a linear complex compressibility function. The model has been used to derive analytical expressions for surface impedance and reflection coefficient as a function of incident pressure amplitude. Depending on the material parameters, sample thickness, and frequency range the model predicts either growth or decrease of reflection coefficient with sound amplitude. Good agreement between model predictions and data for three rigid-porous materials is demonstrated.     

Comparison of the biological activity of phytochemical compositions in native and liposomal forms

The biological activities of phytochemical compositions in native and liposomal forms were compared, and membrane-stabilizing and antioxidant properties of phytocompositions, which have an anti-inflammatory and wound-healing action on the cell walls of protozoa, were studied.     

Cell model calculations of dynamic drag parameters in packings of spheres

An external flow approach is used to predict the viscous drag due to oscillating flow in an air-filled stack of fixed identical rigid spheres. Analytical expressions for dynamic and direct current (dc) permeability, high-frequency limit of tortuosity, and the characteristic viscous dimension are derived using a cell model with an adjustable cell radius which allows for hydrodynamic interactions between the spherical particles. The resulting theory requires knowledge of two fixed parameters: the volume porosity and the particle radius. The theory also requires a value for the cell radius. Use of the cell radius corresponding to that of the sphere circumscribing a unit cell of a cubic lattice arrangement is proposed. This is found to enable good agreement between predictions of the new theory and both published data and numerical results for simple cubic and random spherical packings.     

Low frequency sound propagation in activated carbon

Activated carbon can adsorb and desorb gas molecules onto and off its surface. Research has examined whether this sorption affects low frequency sound waves, with pressures typical of audible sound, interacting with granular activated carbon. Impedance tube measurements were undertaken examining the resonant frequencies of Helmholtz resonators with different backing materials. It was found that the addition of activated carbon increased the compliance of the backing volume. The effect was observed up to the highest frequency measured (500 Hz), but was most significant at lower frequencies (at higher frequencies another phenomenon can explain the behavior). An apparatus was constructed to measure the effective porosity of the activated carbon as well as the number of moles adsorbed at sound pressures between 104 and 118 dB and low frequencies between 20 and 55 Hz. Whilst the results were consistent with adsorption affecting sound propagation, other phenomena cannot be ruled out. Measurements of sorption isotherms showed that additional energy losses can be caused by water vapor condensing onto and then evaporating from the surface of the material. However, the excess absorption measured for low frequency sound waves is primarily caused by decreases in surface reactance rather than changes in surface resistance.     

Development of a new generation of drug dosage forms by construction of liposomal nanocontainers

Various methods of construction of liposomal nanocontainers are studied. Parameters of liposomes are determined. Advantages of phase transformation method are demonstrated. The routes of liposome delivery to the organism are determined.     

Predictions and measurements of sound transmission through a periodic array of elastic shells in air

Analytical and numerical approaches have been made to the problems of (a) propagation through a doubly periodic array of elastic shells in air, (b) scattering by a single elastic shell in air, and (c) scattering by a finite periodic array of elastic shells in air. Using the Rayleigh identity and the Kirchhoff-Love approximations, a relationship is found between the elastic material parameters and the size of the bandgap below the first Bragg frequency, which results from the axisymmetric resonance of the shells in an array. Predictions and laboratory data confirm that use of a suitably "soft" non-vulcanized rubber results in substantial insertion loss peaks related to the resonances of the shells. Inclusion of viscoelasticity is found to improve the correspondence between predictions and data. In addition the possible influences of inhomogeneity due to the manufacturing of the elastic shells (i.e., the effects of gluing sheet edges together) and of departures from circular cylindrical cross-sections are considered by means of numerical methods.     

Acoustical properties of double porosity granular materials

Granular materials have been conventionally used for acoustic treatment due to their sound absorptive and sound insulating properties. An emerging field is the study of the acoustical properties of multiscale porous materials. An example of these is a granular material in which the particles are porous. In this paper, analytical and hybrid analytical-numerical models describing the acoustical properties of these materials are introduced. Image processing techniques have been employed to estimate characteristic dimensions of the materials. The model predictions are compared with measurements on expanded perlite and activated carbon showing satisfactory agreement. It is concluded that a double porosity granular material exhibits greater low-frequency sound absorption at reduced weight compared to a solid-grain granular material with similar mesoscopic characteristics.     

Specific features of magnetic properties of “thick” microwires produced by the Ulitovsky-Taylor method

The magnetic properties of initial and heat-treated Co₆₉Fe₄Cr₄Si₁₂B₁₁ microwires in a glass shell with the diameter D = 125 μm and the diameter of the amorphous metallic core d = 90 μm produced by the Ulitovsky-Taylor method have been studied. It has been found that the magnetic characteristics, in particular, the saturation field H _S and the coercive force H _C of the samples annealed at a temperature T < 300°C do not differ from H _S and H _C of the initial microwire, and those of the samples annealed at T ≥ 400°C increase by almost one order of magnitude. The obtained experimental data have been explained by the structural features of the microwires. The near-surface values of H _S and H _C at T < 300°C are found to be larger than the bulk values by a factor of 5–10. These experimental data have been explained by the existence of structural and chemical ingomogeneities in the near-surface layer, which are inherent in amorphous materials. This difference decreases with a further increase in the annealing temperature, but H _S and H _C increase substantially. This fact has been explained by the beginning of the microwire crystallization.     

Deduction of tortuosity and porosity from acoustic reflection and transmission measurements on thick samples of rigid-porous materials

An acoustic method for obtaining the tortuosity, and porosity of thick samples of rigid porous materials consisting of large (>1 mm) grains or fibres is proposed. The method uses pulses with central frequencies close to 12 kHz and an approximate bandwidth of between 3 and 20 kHz. In this frequency range, inertial rather than viscous or scattering effects dominate sound propagation in large pores. This allows application of the high frequency limit of the “equivalent fluid” model. Both reflected and transmitted signals are used in the measurements. Tortuosity is deduced from the high frequency limit of the phase speed (obtained from transmission data) and porosity is obtained from the high frequency limit of the reflection coefficient once the tortuosity is known. The method is shown to give good results in the cases where significant scattering does not occur until frequencies much higher than the upper limit of the pulse bandwidth. Apart from its applicability to samples with several centimetres thickness, the method needs only one set of measurements with the sample to deduce both tortuosity and porosity. In principle the method can be used also to estimate characteristic lengths. However, the errors are found to be larger and the results less consistent than for tortuosity.     

Mathematics summer schools for acoustics research training

Mathematical methods are important for research in many aspects of acoustics. Most researchers in acoustics in the United Kingdom do not have access to master level courses to broaden their postgraduate study, so they advance their fundamental mathematical methodologies taught at undergraduate level through independent learning. They develop their mathematical skills as appropriate rather than being made aware of the potential of advanced mathematical tools at the onset of their research career. Attempts to improve this situation were made through summer schools held in 2003 and 2005 at Southampton University and in 2007 at Salford University. The background to these Summer Schools, their content and structure, recruitment figures and student feedback are reported together with conclusions about their performance and role particularly in respect of PhD completion.