Schlieren imaging normally shows heat plumes and airflow by amplifying refraction differences with a concave mirror. PlasmatronX, covered here by Hackaday, points the same setup at the far smaller density changes caused by sound, and captures constructive and destructive interference in mid-air for under two hundred dollars.
The trick is timing rather than optics. Rather than buy a camera fast enough, the build synchronises the audio and the optical capture, which meant modifying the audio circuit to remove high-pass filters and overvolting an LED so it can strobe briefly and brightly enough to freeze the wave. The project files are on GitHub.
This is the closest thing in the feed to what we do with strobes phased against sound, and the failure modes are the same ones: jitter between the light and the signal shows up immediately as a smeared or drifting image.