How can i calculate the individual magnitude from my band?

I took out 50ms from a signal and then apply a fft on it. Afterwards i separate it in 3 band. How can i calculate the individual magnitude in a band? Because i want to sum all of the magnitude into a value and then square it with 2. Therefore i would get the power from my band, which i want.

%%MATLAB
%_________________________________________
[y,fs]=audioread('Undertale - Megalovania.wav');
% audioread = Read WAV-file
% y = Vector, which contains audio signal 
% fs = Sample Rate
% 'dontstopmenow' = WAV-file
%_________________________________________

%PARAMETER FOR STFT
%_________________________________________ 
t_seg=0.03; % Length of segment in ms

fftlen = 4096; %FFT-Points

%Definition of frequency bands
f_LOW= 1:200;    %lower frequencies
f_MEDIUM= 201:600;  %medium frequencies
f_HIGH= 601:1000; %higher frequencies
%__________________________________________

segl =floor(t_seg*fs); 
% Length of segment, on which we use the fft
% "floor" rounds off the result
windowshift=segl/2; 
% size of window which goes to the next segment
window=hann(segl); 
%hann function
window=window.'; 
% From a row vector to a column vector
si=1; 
%Start index
ei=segl; 
%End index

N=floor( length(y)/windowshift - 1);
% Number of segements in audio signal

f1=figure;
    % New window

      f=0:1:fftlen-1;
      f=f/fftlen*fs;
      % frequency vector

      Ya=zeros(1,fftlen);

for m= 1:1:N 

      y_a = y(si:ei);
      y_a= y_a.*window;
      Ya=fft(y_a, fftlen);
      Yb=abs(Ya(1:end/2));  

        drawnow; %Updates graphical objects

      figure(f1);
      plot(f(1:end/2), 20*log10(Yb));
      %STFT __Audio signal after stft every 50ms

      %%L,M,H - Bands

      subplot(3,1,1)
      y_low = abs(Ya(f_LOW));
      ya_low = sum(y_low).^2;
      plot(f_LOW,y_low);
      ylim([-90 50]);
      title('Spektrum (LOW)');
      xlabel('f(Hz)');
      ylabel('dB');
      grid on

      subplot(3,1,2)
      y_medium = Ya(f_MEDIUM);
      plot(f_MEDIUM,y_medium);
      ylim([-20 30]);
      title('Spektrum (MEDIUM)');
      xlabel('f(Hz)');
      ylabel('dB');
      grid on

      subplot(3,1,3)
      y_high = Ya(f_HIGH);
      plot(f_HIGH,y_high);
      ylim([-20 30]);
      title('Spektrum (HIGH)');
      xlabel('f(Hz)');
      ylabel('dB');
      grid on;

      si=si+windowshift; 
      % start index updated    
      ei=ei+windowshift; 
      % end index updated

end

[Nirja Mehta]

Do you mean to compute the following? Band Power

There are multiple functions in Signal Processing Blockset that helps you anaylze spectral content of a signal. Spectral Estimation

Also, do change plot(f_LOW,y_low) to plot(f_LOW,ya_low) on Line 52.