Problems in the ultrasonic characterisation of inhomogeneous materials due to scattering

SIGLEAvailable from British Library Document Supply Centre-DSC:DXN006110 / BLDSC - British Library Document Supply CentreGBUnited Kingdo

Problems in the ultrasonic characterisation of inhomogeneous materials due to scattering

SIGLEAvailable from British Library Document Supply Centre-DSC:DXN006110 / BLDSC - British Library Document Supply CentreGBUnited Kingdo

Problems in the ultrasonic characterisation of inhomogeneous materials due to scattering.

Ultrasound wavefronts travelling through inhomogeneous materials such as biological tissues and tissue mimics are distorted by scattering processes. It has been proposed that measurements of these transmitted wavefronts may be used to define the contribution of coherent and incoherent scattering to the attenuation coefficient of such materials, by means of a comparison between the outputs of phase sensitive and phase insensitive receivers. Measurements of the complex pressure fields transmitted by scattering specimens consisting of glass beads in silicone rubber and by specimens of ox liver, kidney and myocardium have been carried out by point- by-point mapping of the amplitude and phase of the fields using a new design of pvdf needle hydrophone in a high precision scanning tank. It is demonstrated that transmission measurements of the scattered fields are not independent of the size and location of the measurement plane, and the proposed method is unlikely to be helpful in quantifying material properties. Alternative techniques based on the measurement of the angular scattering pattern of such materials are shown to be able to characterise scattering specimens consisting of glass beads in agar and gelatin, and to be able to distinguish normal ox myocardium from ox liver and kidney

Acoustic characterisation of panel materials in simulated ocean conditions using the NPL acoustic pressure vessel

Acoustic characterisation of panel materials in simulated ocean conditions using the NPL Acoustic Pressure Vessel.A wide range of viscoelastic materials is used for encapsulation, housing and coating/lining in underwater acoustics. The acoustic properties of these materials can vary dramatically with frequency, temperature and depth, in particular when one is close to the glass-rubber transition for the material. Accurate characterisation of the acoustic properties is therefore of crucial importance when selecting materials and when assessing the performance of transducers. Techniques exist to determine properties such as echo reduction and transmission loss from measurements made on panels of the material, but these are typically undertaken in open laboratory tanks. This limits the range of testing of acoustic devices, since such tanks cannot provide the range of environmental conditions that exist during deployment in the sea. Sea-trials are themselves expensive and one is limited by the prevailing temperature conditions in the sea where the measurements are carried out. An alternative to sea-trials is now available using the Acoustic Pressure Vessel (APV), recently commissioned on site at the UK's National Physical Laboratory (NPL) to provide the capability to undertake acoustic measurements under simulated ocean conditions. The environmental conditions within the APV can be modified to simulate ocean depths down to 700 m (hydrostatic pressure from atmospheric to 7 MPa) and temperatures from 2 oC to 35 oC.To meet the need for measurements of panel materials in ocean conditions, NPL is establishing a measurement facility in the APV, based on parametric array techniques, to allow the measurement of transmission loss, echo reduction and sound speed as functions of pressure, temperature and frequency.The problems of establishing a parametric array in the closed, confined space of the APV are examined in detail. Apparently simple tasks, such as mounting acoustic filter materials, and supporting and aligning the test panels themselves, can present substantial engineering challenges. Finally, test results of measurements of transmission loss and echo reduction are analysed and compared with parametric array measurements in an open tank