Effects of porous covering on sound attenuation by periodic arrays of cylinders

The acoustic transmission loss of a finite periodic array of long rigid cylinders, without and with porous absorbent covering, is studied both theoretically and in the laboratory. A multiple scattering model is extended to allow for the covering and its acoustical properties are described by a single parameter semi-empirical model. Data from laboratory measurements and numerical results are found to be in reasonable agreement. These data and predictions show that porous covering reduces the variation of transmission loss with frequency due to the stop/pass band structure observed with an array of rigid cylinders with similar overall radius and improves the overall attenuation in the higher frequency range. The predicted sensitivities to covering thickness and effective flow resistivity are explored. It is predicted that a random covered array also gives better attenuation than a random array of rigid cylinders with the same overall radius and volume fraction

Reflection of sound from random distributions of semicylinders on a hard plane: models and data

A new analytical theory for multiple scattering of cylindrical acoustic waves by an array of finite impedance semi-cylinders embedded in a smooth acoustically hard surface is derived by extending previous results for plane waves [Linton and Martin, J. Acoust. Soc. Am. 117 (6) 3413 – 3423 (2005)]. Although the computational demands of the new theory increase as the number of the semi-cylinders in the arrays and/or the frequency increases, the theory offers an improvement on analytical boss theories since the latter (i) are restricted to non-deterministic (infinite) random distributions of semi-cylinders with spacing/radii small compared to the incident wavelength and (ii) are derived only for plane waves. The influence on prediction accuracy of truncation of the infinite system of equations introduced by the new theory is explored empirically. Laboratory measurements have been made over deterministic random arrays of identical varnished wooden semi-cylinders on a glass plate. The agreement between predictions and measured relative sound pressure level spectra is very good both for single deterministic random distributions and for averages representing non-deterministic random distributions. The analytical theory is found to give identical results to a boundary element calculation but is much faster to compute

Generation of flexural waves in infinite plates by laser-initiated air shock waves

Controlled explosions that take place above ground surface during military testing generate not only shock waves in air but also strong ground vibrations. It is convenient and much less expensive to study the associated sound and vibration phenomena using reduced-scale laboratory simulations, with a laser as a source of air shock waves interacting with large elastic plates modelling the ground1,2. Earlier, a semi-analytical model describing interaction of air shock waves with an elastic half space has been suggested by one of the present authors to describe generation of Rayleigh surface waves by electric spark discharge near the surface3. The aim of the present paper is to further develop the above-mentioned semi-analytical model3 and to apply it to the interaction of laser-initiated air shock waves with an infinite elastic plate. The impact of the incident shock wave is to be approximated by an equivalent cylindrically diverging surface force resulting from the surface pressure of the incident and reflected shock waves. The well-known analytical expressions for air particle velocity and pressure in the front of a shock wave are used to describe this surface force as a function of time and distance from the epicentre. The problem is then solved using the Green’s function method applied to an infinite plate. The resulting frequency spectra and time shapes of the generated flexural wave pulses are calculated for different values of the height of the laser-generated spark above the plate surface. The obtained theoretical results for time histories and frequency spectra of generated flexural waves are compared with the results of the reduced-scale model experiments on shock wave interaction with a large plastic plate

Generation of rayleigh-type waves on plate edges by laser-initiated airborne shock waves

The present paper describes the results of the semi-analytical modelling of the interaction of laser-initiated air shock waves with the edge of an infinite vertically mounted elastic plate. The impact of the shock wave on the plate edge is approximated by an equivalent edge force resulting from the combined pressure of the incident and reflected shock waves. This force is then represented in the wavenumber-frequency domain by means of Fourier transforms that are carried out numerically. After that the problem is solved using the Green’s function method. The resulting frequency spectra and time histories of generated Rayleigh-type wave pulses propagating along the plate edge are calculated for different heights of the laser beam focusing above the plate edge. The obtained theoretical results are compared with the results of the laboratory experiments on laser-initiated air shock wave interaction with an edge of a large vertically mounted Perspex plate that is used for reduced-scale modelling of blast wave interaction with the ground surface. The resonant properties of the accelerometer have been taken into account to describe the received signals. The comparison shows that the obtained semi-analytical results are in good agreement with the experiments

Generation of Rayleigh-type edge waves by laser-initiated airborne shocks

This paper presents a semi-analytical model of the interaction of laser-initiated airborne shocks with the edge of an infinite vertically mounted elastic plate. The impact of the shock wave on the plate edge is approximated by an equivalent edge force resulting from the combined pressure of the incident and reflected shock waves. The problem is then solved using the Green’s function method. The resulting predictions are compared with the results of the laboratory experiments on laser-initiated air shock wave interaction with an edge of a large vertically mounted Perspex plate that is used for reduced-scale modelling of blast wave interaction with the ground surface. The resulting frequency spectra and time histories of generated Rayleigh-type wave pulses propagating along the plate edge are calculated for different heights of the laser beam focusing above the plate edge. The comparisons show that the obtained semi-analytical predictions are in good agreement with the experiments

Generation of flexural waves in plates by laser-initiated airborne shock waves

Results of the semi-analytical modelling of the interaction of laser-initiated airborne shock waves with an infinite horizontally positioned elastic plate are presented. The impact of the airborne shock waves on the plate is approximated by a cylindrically diverging surface force resulting from the pressure of the incident and reflected shock waves. This force is then represented in the wavenumber-frequency domain by means of Hankel and Fourier transforms that are carried out numerically so that the interaction problem can be solved using the Green’s function method. The resulting frequency spectra and time histories of generated 2 flexural wave pulses are calculated for different values of laser pulse energy and for different heights of the laser beam focusing above the plate surface. The theoretical results obtained are compared with the results of laboratory measurements of the interaction of laser-generated acoustic shocks with a large plastic plate. The comparison shows reasonably good agreement between the semi-analytical predictions and data

Semi-analytical modelling of plate flexural waves generated by laser-initiated air shock waves

Semi-analytical modelling of plate flexural waves generated by laser-initiated air shock wave