Neon lamp used to track physical movement of sound.
Neon Lamp Traces Sound Wave, Bell Labs, 1950. Link
Neon Lamp Traces Sound Wave’s Picture
That’s a sound wave you see in the picture above. Here demonstrating how an acoustic lens focuses sound from a horn, the wave was made visible with the device at left – an aluminum rod with a microphone and a neon lamp at the end. A small motor swings the rod in a wide arc, scanning the area. The microphone picks up the sound and turns it into electric current to feed the lamp. Wherever the sound is strongest, the light is brightest, and the wave is traced out. A complete sound photo, such as this from Bell Labs, takes 10 minutes exposure. ~ Bell Labs, Link