Research Fellow Guillaume de Sercey used the Schlieren technique to capture the sonic boom of a whipcrack for a feature on BBC1's The One Show, broadcast on 8 June 2016. To achieve this feat, the setup used for the BBC was extremely simple yet very effective.
It started with a point light source (a blue LED in this case) that was placed at the focal point of a telescope mirror resulting in a cone of light directed towards the mirror. The mirror reflected that light back to a point. A knife-edge obstructing part of the light was placed at that point and past that point there was a high-speed video camera focussed on the mirror. In the absence of disturbance, the knife edge only effect was to dim the image.
However, any change in density in the air in front of the mirror caused the light rays to bend slightly (the shimmering effect you see on a hot road) and some of them were be blocked by the knife-edge causing dark areas in the image, while others that were previously blocked were visible causing brighter areas.
See Dr Guillaume De Sercey on BBC’s The One Show to demonstrate what a sonic boom looks like (starts 24.44).