Spikes on Sonic-Boom Pressure Waveforms

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 being 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 number 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. Estimates based on the data and the mechanism suggest that the dominant scattering layer should be of the order of 1–3 km above the ground. The mechanism appears capable of explaining the rounded category of sonic-boom waveforms as well as the peaked category.