How to calculate the energy levels of frequency intervals of an audio signal?
You may research for spectrogram. The above comments from pichennets and B Z are correct, but you have to pay attention to the effect of just take some subset of coefficients and calculate the power. When you take this way you're applying a retangular window in frequency, and some peaks in the frequency domain power. You must apply a window in time domain before take the FFT and calculate the power.


I want to add this as a comment, since it's not a real answer but I'm not abble to do this.