Vinyl Heads: Good Vibrations

Vinyl LPs are made by lacquering a substance called polyvinyl chloride (PVC) between two wafer-thin groove-bearing discs called "stampers." PVC is a polymer — a dense molecular compound composed of multiple smaller molecules — and is also used in credit cards and construction materials, especially plumbing.

Sound recordings are inscribed from magnetic tape (usually called a "master tape") to the stampers on a lathe, where a cutting stylus — a hard, sharp needle sometimes made of diamond — inscribes the disc with one continuous groove by vibrating according to the signals it receives from the tape. Stereo recordings have different signals inscribed into the left and right sides of the groove; the groove's width depends on the volume of the recording. Louder passages yield wider grooves.

The disc is then electroplated with nickel and processed to yield two stampers, one for each side of the recording. Tiny PVC particles are simultaneously heated and pressed between the stampers to make an LP, a process that takes about 25 seconds.

When the resulting record is played on a turntable, the vibrations made by the needle brushing against the walls of the groove are picked up by a nearby device called a transducer. Here, the stylus's motion turns a magnet inside a wire coil, which produces electric current. The current is fed through an amplifier and then the loudspeakers, which produces sound waves when the current hits an electromagnet, causing vibrations in an adjacent, thin cone-shaped disk called a diaphragm. Most speakers today contain several such cones to better reproduce different ­frequencies.

(Source: How in the World: A Fascinating Journey Through the World of Human Ingenuity by The Reader's Digest Association, Pleasantville, New York, 1990.)