Propagation in air of n waves produced by sparks

Abstract

Weak sparks, of length 0.5-1.0 cm and energy per discharge 0.01-0.1J, served to produce intense acoustic transients resembling N waves. Amplitude decay and waveform elongation were studied, for propagation distance up to 2 m, through the use of a wideband capacitor microphone with essentially uniform response from dc to 1 MHz. Within the range of propagation distances for which the first (compression) phase of the N wave was completely formed) the duration of this compression phase T and its amplitude ps were found to agree with the theoretical relations T=T0[l + lt;σ0 ln(r/r0)]1/2 and ps= (r0ps0/r)[\\\\\\\\ + σ0ln(r/r0)]-1/2, where σ0 is a parameter that depends upon the values ofps and T at a reference distance from the source r0. The time required for the amplitude of the head shock to increase from 5% to 95% of peak value was observed to vary from 0.45 μs (imposed by the microphone response) to greater than 2.0 us as the wave traveled outward and as its amplitude decreased. Finally, the microphone was calibrated through use of the variation with distance of measured values of T; this new method has led to calculation of a free-field sensitivity that agrees within + 1 dB with the results of other calibrations. © 1983, Acoustical Society of America. All rights reserved.