Index of content:
Volume 115, Issue 5, May 2004
- AEROACOUSTICS, ATMOSPHERIC SOUND 
Passive amplification and directivity from air-coupled surface waves generated above a structured ground115(2004); http://dx.doi.org/10.1121/1.1703535View Description Hide Description
An outdoor experiment is reported in which air-coupled surface waves are generated in the frequency range between 63 and 130 Hz above a strip of structured ground of finite width. Since the sound pressure above the structured ground can be greater than if the ground was rigid, passive amplification can be obtained. The experimental results are compared to theoretical calculations that use a boundary element method. Both predict gains of up to 6 dB, relative to the pressure on a rigid plane when the strip width is of the order of one wavelength. Due to their finite width, the surface wave strips have a directional response. For a source at a large distance from the strip, the sound pressure level is up to 6 dB greater for sound arriving along the axis of the strip than transverse. For sources near the strip, as might arise in a near-field beamforming application, this difference can be as much as 25 dB.
115(2004); http://dx.doi.org/10.1121/1.1691038View Description Hide Description
A model is developed to describe the response of the ground to airborne sound in the presence of a buried landmine. The model describes both the near-field acoustic wave phenomena as well as the mechanical dynamics of the mine itself. Since buried landmines are typically close to the surface of the soil, the induced ground vibration is the result of near-field scattering so that classical asymptotic scattering theory cannot be used. Instead, the near-field problem is solved in a right circular waveguide with rigid walls. The waveguide contains air in the upper half, soil in the lower half, and a buried mine placed concentrically on the waveguide’s axis. The advantage of a waveguide model over infinite space models is that the resulting computer models are much more straightforward to code and run much more quickly. As the radius of the waveguide increases, the results of the waveguide model converge to those of the infinite space model. For low frequencies (a few hundred Hz) this convergence is quite rapid: it is found that a waveguide radius of 10 times the mine radius is more than sufficient. The results obtained are found to qualitatively explain the phenomena observed in the field.
115(2004); http://dx.doi.org/10.1121/1.1699394View Description Hide Description
Bearing determinations for ground vehicles have been made, using a single buried three-axis seismometer. The method, based on Rayleigh wave detection, exploits the phase difference between measured wave components to obtain vehicle bearing estimates. It is referred to as a Rayleigh wave retention method and uses both prograde and retrograde motion. Determination of the propagation direction is based upon analysis of maximum correlation values obtained by cross-correlating the vertical wave components with horizontal beam components. Theoretical simulations demonstrate the technique. Methods for obtaining direction of travel and vehicle speed estimates are also discussed. Applications of the method to real data obtained from several vehicles at different test sites are presented.