Acoustic refraction by a spark discharge in air

It is observed that, when the acoustic impulse from an electric spark is reflected normally from a rigid plate and detected after passage through the remains of the spark region, a precursor component appears in the received waveform. To explain this phenomenon in terms of refraction, a two-spark experiment is used to examine the acoustic properties of the post-discharge region near a spark gap. The measured times of arrival of the precursor signal are fitted to a simple lens model consisting of two parameters: lens radius and acoustic index of refraction. The temporal evolution of these lens parameters is also measured and interpreted in terms of a diffusion model. The diffusion coefficient characterizing the process is found to be much larger than that for thermal diffusion in air at room temperature. This is consistent with the much higher temperatures corresponding to the low acoustic index of refraction of the measured lens models.