spectrogram : solve the spectrogram issue with a xor, the fft formula and his readability

A ticket was published about the compiler complaining about an XOR that was used instead of a power, this error noticed by the compiler was correct, but there were other small issues with the formula, the definition of variables, and the display of frequencies that are present in very low proportions in the audio.

Fixes #28506

Co-authored-by: Disha Baghel <bagheldisha708@gmail.com>