Stft - Alternative way of visualising
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.