Studies of blast sound propagation and passive absorption using laser-generated shock waves and a semi-analytical method

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Previous field work has shown several phenomena in outdoor blast sound propagation near to the ground surface that may be related to acoustic-to-seismic coupling but as yet are unexplained. Moreover there remains a need for further work on passive methods for absorbing sound generated near to airborne blasts. However field experiments with explosions are expensive and difficult. Experience has been gained by the applicants and coworkers in the use of acoustic shocks, either electrically-produced or caused by focussing the beam of a high powered pulse YAG laser through a lens, in laboratory simulations of blast sound propagation. The laser-generated shocks have been found to have higher intensity and repeatability than those from an electrical sparker source. Based on this previous experience at Hull, resulting from the EPSRC laser loan scheme, it is proposed to carry out a systematic programme of research on the effects of ground elasticity, roughness and curvature on linear and nonlinear near-surface propagation. This will involve use of specially-constructed wide-band microphones, design and development of microphone positioning systems for detailed sound field mapping and microphone baffles to reduce the effects of the interference between sound reflected around the microphone and the incident signals of interest. Rather than further development of completely-numerical methods for modelling blast sound propagation near the source, in conjunction with Loughborough University it is proposed to investigate and validate the application of a semi-analytical method. Apart from being less computationally-expensive than a completely numerical method, the resulting model should improve understanding of the parameters and geometry associated with creation of acoustically-induced Rayleigh waves. The high intensity and good repeatability of laser-generated shocks also make them a useful source for laboratory investigations of transmission through thick rigid- or elastic-porous layers composed of relatively large grains or fibres that are potentially useful for passive absorption of low-frequency acoustic shocks. Consequently a systematic experimental investigation of the linear and nonlinear acoustrical properties of such materials is proposed also.