Making sound waves visible with a lock-in amplifier, microphone, and LEDs

https://www.youtube.com/watch?v=25wlW0otiGg

This video focused more on a mathematical approach to recording and visualising sound. His method also allows you to measure the speed of sound which shows the level of accuracy. However the way in which he visualises the sound is very irregular. He has achieved this effect by using two different colour lights connected to a microphone, and then captured by doing a long exposure on a camera.