Separating audio file in 3 different frequency areas - matlab

Question

I apologize in advance, if the title is confusing. Basically, i have a Audio-file, on which I perform a STFT every 50ms. My File is about 11 seconds long (10.8526s), which i have cut off from a soundtrack. Btw, i am not allowed to use the built-in function in Matlab for the STFT. I am aware it is much more easier. Anyway, after i run my code, every 50 ms a STFT is performed and the picture is being plotted.
Now i want to seperate it in 3 different plots. In the first plot i have the lower frequencies (0-300Hz), in the second plot medium frequencies(300-5kHz) and in the last plot i have high frequencies(5Khz-fs/2). fs=44100 --> Further explanations below in the code. How can i define now the areas?
%AUDIO-FILE
%______________________________________________________
[y,fs]=audioread('UnchainMyHeart.wav');
% audioread = Reads Audio file
% y = A vector, which contains the audio signal
% fs = sample rate
% 'UnchainMyHeart' = Audio file
%______________________________________________________
% Paramter for the real-time spectral-analysis
%______________________________________________________
NFA=2;
% Every second picture is being plotted
% Don't need every picture
t_seg=0.05;
%Length of the audio signal on which is a STFT performed
fftlen = 4096;
% Length of the FFT, frequency resolution
TPF= 300;
BPF= 5000;
HPF= 22050;
% Trying to define the frequencies areas
% Isn't working right now
LOW=((TPF*fftlen)/fs);
MEDIUM=((BPF*fftlen)/fs);
HIGH=((HPF*fftlen)/fs);
% Contains the number of FFT points in the frequency
%_______________________________________________________
segl =floor(t_seg*fs);
windowshift=segl/2;
window=hann(segl);
window=window.';
si=1;
% Start Index
ei=segl;
% End Index
AOS= length(y)/windowshift - 1;
f1=figure;
f=0:1:fftlen-1;
f=f/(fftlen-1)*fs;
Ya=zeros(1,fftlen);
n=0;
for m= 1:1:AOS
y_a = y(si:ei);
y_a= y_a.*window;
Ya=fft(y_a, fftlen);
n=n+1;
if n==1
Yres=abs(Ya);
else
Yres=Yres+abs(Ya);
end
if n==NFA
Yres=Yres/NFA;
n=0;
drawnow;
%Updates the graphical objects which are being plotted every 50ms
figure(f1);
plot(f(1:end/2), 20*log10(abs(Yres(1:end/2))));
ylim([-90 50]);
title('Spektrum of audio signal');
xlabel('f(Hz)');
ylabel('dB');
grid on;
end
si=si+windowshift;
% Updating Start Index
ei=ei+windowshift;
% Updating End index
end

Answer 1

I am not able to run you code as I do not have your audio file but I will try to explain conceptually, and use pseudo code.
Frequency brick-wall
If you just want to separate the frequencies for visual purposes you can just use brick-wall filters.
Perform an fft of the full signal. Define a frequency vector.
SigFD = fft(signal);
n = length(signal); % number of samples
fs = 44100; % sampling rate
deltaF = fs/n; % frequency resolution
F = [0:floor(n/2)-1, -(floor(n/2)):-1]*deltaF; % frequency vector
Slice the signal based on the frequency range that you want.
lowF = 0;
highF = 500;
part1Range = abs(F)>lowF&abs(F)<highF;
Fpart1 = F(part1Range);
Sig1FD = SigFD(part1Range);
Note that I am unable to test the code on your waveform so this should be considered more of pseudo code!