Stft - Alternative way of visualising In theory I have 3 axis: time, frequency and amplitude (which is scaled in by $20\log$). I have an audio signal. I noticed that I have very high amplitudes in the range of $1$-$1000$ $\mathrm{Hz}$. So I made 3 frequency bands: $y_{low}$, $y_{medium}$ and $y_{high}$, where $y$ refers to the signal and the subindex to the frequency band. 

Now I defined a frequency vector for my bands:

LOW = 1:200
MEDIUM = 201:600
HIGH = 601:1000


Afterwards I wanted to know the values of my bin, depending on the frequency band. Basically y_LOW/4096   --> (fft points). Now I did an arithmetic mean with the values of my bins, which is saved in a single variable. 
Before that happens, I am doing a STFT. So, a spectrum is shown every $50 \ \mathrm{ms}$. I want to plot my 3 values (since I have 3 frequency bands) every $50 \ \mathrm{ms}$. The values should be shown as a point or something similar.  

If the code is required, I can post it.
Instead of calculating the STFT, you could just implement a 3 band filter bank as shown in the image below.

 

Your input signal x[n] could then be broken into 3 subbands: x0[n], x1[n], x2[n] which contain the energy associated with each frequency range. You could then feed each individual signal into an accumulator block in which the output would be the sum of the last x number of points. Or in you case, the x number of points that occurred in 50 ms. This would give you a single number in which you could map to some visualization output. You could implement normalization factors however you would like.

All you would have to do is define the filters H0, H1, and H2 as bandpass filters corresponding to your desired cutoff frequencies.