How to generate graph showing the EEG as Hz?




I am trying to create (a) graph(s) which show the different EEG wave (alpha, beta etc.) where the value is given in Hz.
Until now I have only been successful in gathering the absoulute band powers, but from here I can’t figure out how (If possible) these values can be transformed into hz-values.

The reasoning behind this is that I am trying to create a mobile application that can record the values, in Hz, while sleeping. (I do know some of all the cons of this experiment).