mmm, in the past the old spectrum analyser like Klark Teknik was using really selective bandpass filter for each frequency in the audio spectrum + regular LM3914 or something like that bargraph driver for front plate LEDs. But for sure PC software(like spectra-lab or Smaart) and Neutrik ML1 or else have to use those kind of calculation. If you find the great formulas post them here. We can probably do something.