Acoustic Radiation from a Finite Line Source with N-Wave Excitation

A shock wave with an N-shaped pressure profile is propagated in the equatorial plane of a weak spark in air. In the present work, the directional dependence of the acoustic waveform is examined for 0.5- and 1.0-cm sparks having discharge energies between 0.01 and 0.1 J. The transient signals are detected with a wide-band capacitor microphone that has a rise time of less than 1 μsec. As the spark axis is approached, an interval with no acoustic disturbance appears near the center of the waveform. The angular dependence of both the shock amplitude and the length of this central interval is found to be in qualitative agreement with the results of a linear analysis in which it is assumed that each point along the spark acts as a source of N waves. [Research supported in part by the U. S. Office of Naval Research and by the National Science Foundation.]