Schlieren photography of 40 kHz leaky Lamb waves in air

Schlieren photography is widely used for visualization of acoustic or shock waves, e.g. in ballistics and aerodynamics. Due to the low molar refractivity of air the visualization of acoustic waves requires sensitive equipment or high sound pressure levels. We present a Schlieren system utilized in a nondestructive test setup, that is capable to visualize leaky Lamb waves, radiating from a steel sheet specimen. The Lamb wave is excited with our air-coupled ultrasonic phased array. Image processing includes averaging, substraction of a phase-shifted image and filtering in the frequency domain. We analyze the detection limit of our setup and discuss measurements with two different specimen. Leaky Lamb waves with SPLs down to 101.5 dB can be visualized. The exit angles above and beneath the specimen, measured in the Schlieren images are in good agreement with the excitation angles for 0.5 mm and 0.8 mm thick steel sheets.