A spring reverb used a spring in a metal case to run an audio signal through to make echos of the music, voice, or whatever. Sounds like you are in an hard walled empty room with echos only much more so if you turned it up. Most of my experience is with guitar amplifiers, where reverb is still very common, but usually electronically generated today. We called the old ones "reverb tanks." The ones in the cars would make crashing noises if you went over some rough bumps or tracks and the spring(s) crashed into the box or each other. Same with music amplifiers. Back in the day we would kick or rock the amplifier about to make the reverb crash. "Heavy metal." Actually, I was (and am) more into blues and jump.