How to get signal level values ​​at different audio frequencies?

Question

Sound processing

I really want to make my own audio visualizer, these are the same steps in the equalizer that dance depending on the frequency.
But I don't know how to get signal level values ​​from audio files for further processing.
Maybe someone came across a ready-made solution (open source) on this topic?
Any C-like language will do.

Answer 1

Google "Fast Fourier Transform". Well, ready to calculate it.