Abstract
The cause of the spikes or peaks frequently observed on sonic-boom pressure waveforms is generally attributed to small-scale atmospheric variations representing deviations of the atmosphere from a stratified medium. A detailed mechanism consistent with this theory is proposed that interprets the spikes as due to the simultaneous focusing and diffraction of a nearly planar N wave by an inhomogeneous layer in the atmosphere. Spikes appear on the waveform in regions where a ray analysis indicates a magnification due to focusing. That the mechanism does lead to spiked waveforms is exemplified quantitatively by analysis of a simple model. Examination of a limited amount of data showing very pronounced spikes at one site and showing gradually vanishing spikes at adjacent sites during a given experiment suggests that the scale of the atmospheric irregularities should be of the order of 200 m, which is compatible with turbulence correlation coefficients derived by Panofsky for unstable air. The width of the spikes is related somewhat simply to the travel delay time of waves diffracted around the “edges” of irregularities.
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