From cylinders to CDs

The thought of having up to 60,000 songs on a small electronic box the size of a wallet would have seemed like magic to Edison in 1877. However the essential ingredients for recording and playing back sound at today’s standards were developed in Edison’s lab well over a century ago.

In the groove

Sound is vibrations and those vibrations have to reach your ear drum for you to be able to hear a sound. Whether it’s the air, water or a solid, there has to be something vibrating so that those vibrations can travel to your ear and cause your ear drum to vibrate for you to be able to hear a sound.

Edison realised that if you could somehow map vibrations on a material, then you could reverse the process and get the mapped vibrations to once again vibrate the air and reproduce the sound. Edison achieved this by speaking into a horn to focus the sound energy, and make a metal needle vibrate.

The vibrating needle created grooves in a thin sheet of tin wrapped around a rotating cylinder. The louder the sound, the deeper the groove. To play this back, Edison put the needle at the start of the etched grooves and turned the cylinder. The needle faithfully retraced the grooves causing a diaphragm to vibrate, producing a faithful yet scratchy rendition of the original sound. 

Music in bits

This isn’t hugely dissimilar to how recording and playback works today. Sound vibrations cause the diaphragm in a microphone to vibrate and these vibrations are then converted to electrical currents.

The value of the electrical currents changes depending on the pitch, loudness and tone of the incoming sound, and this is stored digitally as ‘bits’ which form strings of binary code; essentially a series of 0s and 1s.

To play the sound back, the 0s and 1s are converted back to electrical currents which in turn vibrate a speaker cone. The speaker cone vibrates the air in a similar way to the sound that went into the microphone originally.


See our top links on the physics of music

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