Schlieren Imaging of Air-coupled Ultrasound

Schlieren imaging uses changes in the refractive index of a medium to permit visualisation of pressure changes in the medium. Schlieren images have been used to observe turbulent convection flow off heat sources, shock waves from bullets, jets and rockets. For a while, in nondestrructive testing it was a popular option to see beam shapes associated with immersion ultrasonic probes as well as pulse-interactions in immersion test conditions. From the perspective of sound transmission, air can be considered another form of a fluid. Although schlieren imaging has been used to illustrate shockwaves in air, the challenge of imaging ultrasound sound in air is significant, primarily due to the extremely high attenuation. This article provides a description of the techniques used to image ultrasound in air using several air-coupled ultrasonic transducers.