How to get the power spectral density from a Spectrogram in a given frequency range?

In the figure I have uploaded for example, is there a function to get the Power spectral density of the signal between 1 - 2 Hz? I think that I need the matrix of numbers used by Matlab to generate the Spectrogram. Is it stored in the variable S considering I used the line: [S,F,T,P] = spectrogram(x1,w,2400,2800,Fs); to generate the spectrogram?

Answer 1

[Youssef Khmou]

This problem is simple in terms of matrix manipulation, all what you need is the index corresponding to the desired range, let us take an example :

 F=rand(100,40);

suppose the frequency is represented by the is the x (40), if i want choose the range 22:25 :

 G=F(:,22:25);

[Win]

Ok, thanks for your reply. I've never calculated the energy of a signal before. Do I need to use the function periodogram for it?

[Youssef Khmou]

ok, here is an example using modulated sinusoidal signal :

t = 0:0.001:2;
x = chirp(t,150,1,300);

The number of points for frequency is :

 f=0:0.1:150; % example

Code for computing the PSD :

 [y,f,t,P]=spectrogram(x,10,6,f,1E3);

 figure; surf(t,f,10*log10(abs(P)),'EdgeColor','none');
 view(0,90);
 xlabel('times s');
 ylabel(' frequency Hz');

to choose per example the range 50,100Hz, you need the information of theirs indexes :

 x1=500;
 x2=1000;
 F=P(x1:x2,:);
 figure; surf(20*log10(F));