How a vinyl record
makes sound.

A mastering engineer plays the finished album into a cutting lathe. Its cutter head works like a speaker run backwards: the music drives two coils, the coils swing a heated sapphire stylus, and the stylus carves one continuous spiral into an aluminium disc coated in soft nitrocellulose lacquer. The cut happens in real time, one side at a time, at the same 33⅓ turns a minute the record will play at.

Before it reaches the cutter, the signal goes through RIAA equalisation. The bass is turned down by up to about 20 dB, so the deep notes don't swing the groove into its neighbours, and the treble is turned up by about the same, so it sits well above the hiss of the vinyl. The engineer also lets the lathe space the turns: loud passages get more room and quiet ones less, which is how a side holds twenty minutes or more.

Lacquer is far too soft to press records from, so it is turned into metal. It is sprayed with a thin coat of silver and lowered into a nickel plating bath, where the nickel slowly grows over the grooves. Peeled away, that nickel is the father: an exact negative, with ridges where the grooves were. Pulling it off ruins the lacquer, so the father is precious.

The father is plated again to make a mother, a positive you could actually play, and each mother makes the stampers: negatives again, one for each side. A stamper wears out after a thousand or so pressings, and a fresh one is grown from the mother. The numbers etched in the run-out groove record which lacquer and stamper made a copy, which is how collectors tell a first pressing from a later one.

The two stampers are clamped to the faces of a hydraulic press, one above and one below. PVC pellets, black or coloured, are melted into a soft puck about the size of a hockey puck, set between the two paper labels, and laid on the lower stamper. Steam heats the dies to around 150 to 180 °C, the press closes with roughly 100 tons of force, and the vinyl flows outward into every groove on both sides at once.

Then cold water runs through the same channels the steam came through. The vinyl sets, the press opens, and a blade trims the ragged rim of excess vinyl from the edge. The whole cycle takes about 25 to 35 seconds. The labels go in already baked dry, or the moisture in the paper would blister in the heat.

On the turntable the record turns at a steady 33⅓. The stylus rests in the groove under about two grams of tracking force, and from the moment it drops in at the edge, the groove does the steering: it pulls the arm in toward the label, one turn of the spiral every 1.8 seconds, for about twenty minutes.

The groove passes the stylus at about half a metre a second at the edge and less than half that near the label. That is why the inner tracks of a side are the hardest to get right: the same note has to be cut into a shorter stretch of groove.

The two walls of the groove are cut at 45° to the surface, one channel on each: left on the inner wall, right on the outer. As they wiggle, they push the diamond tip from side to side and up and down. The tip sits on a tiny hollow cantilever, and in a moving-magnet cartridge the far end of the cantilever carries a magnet sitting between two pairs of coils.

A moving magnet makes a current in a coil next to it, so every wiggle of the groove becomes a matching wiggle of voltage: about 5 thousandths of a volt for each channel. That is the whole trick. Everything after this point is making that signal bigger.

The phono preamp does two jobs. It reverses the RIAA curve from the cutting room, turning the bass back up and the treble back down, which also turns down the surface hiss along with it. And it raises a few thousandths of a volt to the line level of an ordinary audio input, roughly a hundred times bigger.

The power amplifier then gives that signal real current: enough volts and amps to push a speaker cone back and forth hundreds of times a second.

Inside the speaker, the signal runs through a voice coil that sits in the gap of a strong magnet. The current makes the coil push and pull against the magnet, and the coil is glued to the cone, so the cone moves in and out in exactly the shape of the groove that was cut in the lacquer.

Each push squeezes the air in front of the cone and each pull thins it out. Those pressure waves travel at about 343 metres a second to your ears. The wiggle that a lathe carved into lacquer months earlier has become sound in the room.