Ice thickness estimation using low frequencies, and an investigation of diffraction of sound in samples with micro structures using ultrasound
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In the first section, the thickness of ice on the lakes and canals is estimated by analyzing the sound spectrum generated by dispersion of Lamb type waves propagating in ice. In winters when the lakes and canals freeze, it is important to know the thickness of the ice layer before setting foot on it. When a stone is thrown on the ice layer, a fluting sound can be heard. This is recorded for different thicknesses of ice, and the sound spectrum is compared with the results simulated using a parameterized model. This model is created using a combination of plane waves for different incident angles and frequencies to generate dispersion curves for different thicknesses of ice. The frequencies of the reflected sound are then compared with the frequencies of musical instruments in order to assign different musical notes to different thicknesses of ice. The technique enables thickness estimation without the use of specialized equipment or time consuming drilling and may therefore be of practical value in the preservation of the lives of ice skaters and playing children. In the second half of the study, high frequencies (400 MHz and 1 GHz) are used to investigate samples with micro structures. Acoustic microscopy is a well established technique as far as smooth surfaces are concerned. V (z) curves are obtained from which, through surface wave generation, important features concerning elasticity and related properties can be extracted. Recently, high resolution imaging using high frequency focused transducers, based on acoustic microscopy has appeared. The surface profiles of the samples used in this study, have periodic structures but lack smoothness. The periodicity causes sound diffraction and the roughness influences the acoustic microscopic investigation. The small acoustic contrast between the substrate and the periodic corrugation on the material, gives us information about the additional stresses which develop and affect the bonding between the two materials. In this study, experiments are conducted using samples with corrugations of different periodicity, and a comparison is made between the results for smoother surfaces and results for the periodic structures of the same material. An attempt is made to analyse the effects described above.