Keith Attenborough

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 Forchheimers correction to Darcys 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.

Reflection of sound from random distributions of semi-cylinders on a hard plane : models and data

A new analytical theory for multiple scattering of cylindrical acoustic wave s by a n array of finite impedance semi -cylinders embedded in a smoot h 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 t he arrays and/or the frequency increases , t he theory offer s 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 t o the incid ent 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 L evel 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 .

Diffraction of sound due to moving sources by barriers and ground discontinuities

The solution for diffraction of sound by a wedge is extended to sources in uniform motion parallel to the obstacle. By means of an auxiliary transformation the problem is reduced to that of a set of stationary sources so that the resulting solution is in accordance with well-known models. Applications to prediction and abatement of transportation noise are considered. The formulation for the diffracted wave is combined with the solution for ground reflection of sound due to moving sources. New models for the sound field due to a source in motion along a barrier above the ground and for a source moving parallel to an impedance discontinuity are derived. In both situations, greater sensitivity to motion of the resulting sound pressure levels is found on source approach. However, attenuation of sound from moving sources by barriers is found to be so little affected by motion that design schemes for stationary source are relevant. On the other hand, it is noted that noise predictions must account for source m...

Sound absorption by clamped poroelastic plates

Measurements and predictions have been made of the absorption coefficient and the surface acoustic impedance of poroelastic plates clamped in a large impedance tube and separated from the rigid termination by an air gap. The measured and predicted absorption coefficient and surface impedance spectra exhibit low frequency peaks. The peak frequencies observed in the absorption coefficient are close to those predicted and measured in the deflection spectra of the clamped poroelastic plates. The influences of the rigidity of the clamping conditions and the width of the air gap have been investigated. Both influences are found to be important. Increasing the rigidity of clamping reduces the low frequency absorption peaks compared with those measured for simply supported plates or plates in an intermediate clamping condition. Results for a closed cell foam plate and for two open cell foam plates made from recycled materials are presented. For identical clamping conditions and width of air gap, the results for the different materials differ as a consequence mainly of their different elasticity, thickness, and cell structure.

Laboratory experiments to study N‐waves propagation: Effects of turbulence and/or ground roughness

Model experiments have done in the anechoic chamber of the Ecole Centrale de Lyon in order to investigate long range propagation of short duration N‐waves under various configurations. Measurements have been done in a free field configuration and in the shadow zone created by a curved surface. The surface was either smooth or rough.

Predicting noise from aircraft operated on the ground

Abstract Averaged data for sound levels due to aircraft engine testing out to a distance of 3 km obtained under low wind speed conditions are presented. Predictions of standard analytical approximations are compared with these averaged data. The measured average sound levels are shown to be consistent with the predicted influence of ground effect including impedance discontinuities. There is a noticeable influence of directionality also.

Laser-Doppler Vibrometer measurements of acoustic-to-seismic coupling in unconsolidated soils

Abstract Measurements of acoustic-to-seismic coupling ratio, i.e. the ratio of the pressure exerted by an acoustic wave at a point on the surface to the acoustic particle velocity generated at the surface at that point, may be used to determine both elastic and structural properties of poroelastic materials. The sound pressure is measured using a microphone and, usually, the velocities are measured using geophones. Problems with geophone sensors have been shown to include both mass loading of the soil and coupling resonances within the frequency range of interest. The latter can lead to inaccurate amplitude and phase measurements. In an attempt to overcome these problems, the use of a Laser-Doppler vibrometer (LDV) has been investigated. Previous work with compacted plane soil surfaces has been extended to loosely consolidated soils. Good agreement has been found between geophone and LDV measurements of vertical particle velocity for a continuous wave sound source. Problems with poor LDV signal-to-noise ratio in unconsolidated materials have been overcome using local ground treatment. Subsequent modelling shows reasonable agreement between the data and the predicted values of material properties.

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.