How does a needle lightly rotating over grooves create a sound frequency of a specific song?
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Because it makes the needle vibrate, and what sound frequency *is*, is vibrations.
Also, when they talk about ‘grooves’ in a record. There’s only One groove. It’s wound around the record from the outside to the inside and the needle stays in it the whole time.
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.
All your eardrums are doing are moving back and forth in a particular pattern to create the sound. This movement is created by air pressure changes.
Speakers move in that same pattern to send these pressure waves out into the air.
Think of a simpler record like an early 78RPM, played using a phonograph.
The stylus sits in the groove which is sliding past underneath with the rotation of the record and the movement of the groove moves the stylus side to side. That side to side movement is the same movement the speaker will do, and the same movement your eardrums will move with.
The only difference is in early phonographs this movement was transmitted mechanically up to a horn via a diaphragm [like this](https://www.clpgs.org.uk/uploads/4/9/3/8/49389291/_9550023.jpg)
Later record players right up to the modern day built on this basic principle.
All sounds are vibrating waves in the air, or another medium if you are for example underwater. When you add up all these different waves together (the waves from the singer plus the waves from the guitar plus the waves of the bass etc.) you get another wavey function that looks quite messy, but is still fundamentally just a bunch of waves. The grooves in the vinyl match this wavey function and make the needle vibrate in the same way that the recording has all those waves added up. That way you effectively hear all the frequencies from all the instruments at the same time.
Tiny ridges and grooves inside the vinyl disk make the needle vibrate and produce the correct sounds. Fun fact, if there’s bass that’s panned too far left or right, it creates big grooves on one side and causes the needle to jump out.
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