How does a needle lightly rotating over grooves create a sound frequency of a specific song?
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All sounds are vibrations in the air. Those vibrations cause very sensitive muscles in your ear to vibrate, and your brain processes those vibrations as a sound.
When you sing into a microphone, the microphone has a thin membrane inside of it called a diaphragm, and the diaphragm vibrates in response to the changing vibrations in the air coming from your mouth.
So, imagine you have a microphone connected by a wire to a really sensitive pen, like a seismograph. You sing into the microphone, that makes the diaphragm vibrate, those vibrations are passed down the wire to the pen, and that makes the pen vibrate. A piece of paper rolls past the pen, and the pen draws the waveform that corresponds to those vibrations on the paper. Boom, you’ve recorded sound. But that sound isn’t very convenient for playback.
Instead of a pen on paper, what if it was a sharp needle on a soft piece of rubber? And instead of rolling it, let’s make it spin instead. Now as you sing, the needle is scratching the wave form into the spinning disc of rubber. Someone else can now take that piece of rubber, and play it back, using a really sensitive stylus. The stylus vibrates the same way that the needle did while you were singing, and those vibrations get passed along a wire to a speaker, causing the speaker cone to vibrate, and now the speaker plays the same sound that you recorded.
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