The rise time of N waves produced by sparks

Abstract : The rise time of spark produced N waves in air has been measured with a microphone of very wide bandwidth. The purpose was to determine the extent to which vibrational relaxation affects the rise time. Electric sparks having energies between 0.06 J and 26.5 J were used. The N waves had pressure amplitudes in the range 0.15-15.0 mbar, half durations 7.5-41.5 microsec, and rise times 0.65-7.0 microsec. The measurements were taken at source-receiver distances of 0.1-5.5 m. Data were taken under three different conditions: constant spark energy, constant half duration, and constant amplitude. Theoretical predictions in common use for the rise time of a plane step shock in a thermoviscous gas and in a relaxing gas were reviewed. The measurements show, however, that these predictions are not really applicable to the head shock of our spherical N waves. An alternative theoretical prediction was obtained by using a computer algorithm to predict the waveform of a propagating N wave. The algorithm includes finite amplitude distortion, spherical spreading, absorption based on the ANSI standard for still air, and dispersion appropriate for oxygen relaxation. The predicted waveforms, including amplitude, half duration, and rise time, are in good agreement with the measured waveforms. Further computer calculations show that the measured rise times cannot be attributed to relaxation (including dispersion) or thermoviscous absorption alone. Both mechanisms are important for the N waves in our experiment.