4Hz modulation energy from MFCC coefficients

Hi,
with respect to your community, I'm just starting my journey with signal
processing topic. I'm currently working on small voice activity detection
(VAD) C++ code snippet. After reading .wav file, passing it trough FFT and
feeding with obtained data MFCC algorithm I got my MFCC coefficients.

Now I'm little confused. How should I calculate  modulation energy in 4Hz
frequency (and in bandwith from, for example, 200Hz to 600Hz) from this
coefficients? I assume, that I need only two or three first only for that.
Do I have to apply bandpass filter centered on 4Hz frequency and then
compute the magnitude from filtered amplitudes (as, as far I know, MFCC
coefficients stores information about it)?



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