"When light passes between areas of different air density, it bends. You've probably noticed the way distant pavement seems to shimmer on a hot day, or the way stars appear to twinkle. You're seeing light that has been distorted as it passes through varying air densities, which are in turn created by varying temperatures and pressures.
In the mid-19th century, German physicist August Toepler invented a photography technique called Schlieren Flow Visualization to visually capture these changes in density. The setup is a bit hard to explain in words (watch the video above for a full explanation) but it allows scientists and engineers to see things that are normally invisible: the rising heat from a candle, the turbulence around an airplane wing, the plume of a sneeze.
It can also be used to see sound. Sound, after all, is just another change in air density — a traveling compression wave. A speaker pushes on the surrounding air, creating a wave that travels outward until it encounters the ear drum."
High Speed Schlieren Video of Premixed Flame, Spark Ignition