Magic Transistor

That's sound wave you see in the picture above. Here demonstrating how an acoustic lens focuses sound from a a horn, the wave was made visible with the device at left-an alluminium 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 fed the lamp. where ever 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. Neon lamp traces sound waves, 1950.