Measurements of spark‐generated N‐waves in air using a combination of acoustical and optical methods.

Abstract

Accurate measurement of broadband acoustic signals in air, particularly N‐waves, remains a challenge. Bandwidth of existing microphones typically does not exceed 150 kHz, which results in significant overestimations of the shock rise time. To better resolve the shock thickness, it is proposed to use a focused optical shadowgraphy technique. The approach is tested experimentally. A spark source is used to generate high amplitude N‐waves in air. Acoustic measurements are performed using conventional microphones (3 mm diameter), and optical shadowgrams are made using a collimated light beam from a 20‐ns flash source. The results of modeling based on the generalized Burgers equation are in a good agreement with the microphone measurements in respect to the wave peak pressure and duration. However, the measured rise time of the front shock is ten times longer than the calculated one, which is attributed to the limited bandwidth of the microphone. The recorded optical shadowgrams in the vicinity of the shock front are compared with shadow patterns predicted theoretically. It is shown that a combination of microphone measurements and focused optical shadowgraphy is a reliable way of studying evolution of spark‐generated shock waves in air. [Work supported by RFBR, French Government, and CNRS PICS 5603.]