stroboscopic Schlieren videos

In an attempt to image the acoustic fields emerging from ultrasound transducers we've built a small Schlieren setup with stroboscopic LED illumination. There's a 10-cycle 7.5 MHz sound-wave coming in from the top at a velocity of around 1500 m/s, and if you illuminate it at just the right time with a 500 ns pulse of light you see the change in refractive index due to the pressure change. In the videos we are adjusting the delay between the acoustic pulse and the light-pulse from about 0 us up to 10 us to see how the sound propagates through the ~15 mm field of view.

Here's another one with a reflecting metal piece at the bottom. The reflected pulse shows quite nicely! If you look carefully between 3 and 4 s you can see an interference pattern between the incoming and reflected pulse.