 Roughly, because you notice that it's not, there's no measurements or anything like that. It's just a rough idea. Imagine a cylinder with basically a sheet of tinfoil wound around it that you turn by a hand crank. As someone sang into the device, a needle reacting to the vibrations would make markings on the tinfoil recording the sound. This is the earliest phonograph. You could record music on a cylinder and then you could play them back.