The record groove: how a scratch in plastic becomes a voice

the air moves

The shape is the sound

Nobody agreed what a wiggle means. The groove in a record is the same shape the air made when the sound happened, cut into plastic. A needle dragged along it wiggles the same way, and the air wiggles again.

That is why you can play a record with a sewing needle and a paper cone, with no electricity and nobody to teach you the code. There is no code. There is a copy of the shape.

Build one with a needle and a paper cone

Two ways to keep a sound

By resemblance

The groove, a tape, a sundial's shadow, a footprint. The record is the same shape as the thing. Anyone who works out the relationship can read it, and a scratch gives you a crackle in the middle of the music.

By agreement

A CD measures the wiggle forty-four thousand times a second and writes the heights down as numbers. Nothing resembles anything. Damage one number badly enough and you can lose the whole file rather than one crackle.

Both are on this page. The wiggle above is resemblance. The numbers below are the same wiggle, measured and written down, which is where hexadecimal and every other machine code on this site comes from.

Why it matters here

A code that resembles its subject cannot keep a secret: anyone who notices the resemblance can read it. A code by agreement can, because the agreement can be private. That is the line between this section and the ciphers.

Back to signals The ciphers

Where it comes from

1877 onward

The shape is the sound

Edison's first machine scratched a groove into tinfoil and traced it back with a needle. Nobody agreed what a wiggle meant; the groove is simply the same shape the air was. That is why a sewing needle and a paper cone will play a record with no electricity and nobody to teach you the code, because there is no code.

Try it away from the screen

Look at a record under a bright light and tilt it. The groove is not a smooth spiral: it wobbles, and the wobble is the music.