how to convert pressure versus time to pressure versus frequency using fft function

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I have tried to convert pressure versus time data to pressure versus frequency using FFT function
This is the pressure versus time signal
after using fft
deltaT=2.5e-6;
fs=1/deltaT;
pxx0 = fft(p);
n = length(p); % number of samples
f = (0:n-1)*(fs/n); % frequency range
mag=abs(pxx0);
figure(1)
semilogx(f,mag)
ylabel ('magnitude');
xlabel ('frequency');
This is the output of FFT function
How to convert the magnitude of FFT to corresponding pressure value like this graph ? in form of pressure versus frequency
  2 Comments
zein
zein on 23 Apr 2020
I do not understand your answer?in my case if i got the results as magnitude domain, how can i convert it to pressure?

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Accepted Answer

Mehmed Saad
Mehmed Saad on 23 Apr 2020
Edited: Mehmed Saad on 23 Apr 2020
  • set xlim between 30 and 1e6
  • set you xtick as [30 300 3e3 3e4 3e5]
  • turn off xMinorTick
  • turn off xMinorGrid
  • turn on grid
  • change linewidth to 1.5
Although it looks like window is also applied on the pressure data and fft size is also greater than the size of signal (next2pow is used)
  8 Comments
Mehmed Saad
Mehmed Saad on 23 Apr 2020
Edited: Mehmed Saad on 23 Apr 2020
fs = 1000;
t = 0:1/fs:1-1/fs;
s = exp(2j*pi*10*t);
L = length(s);
n = 2^nextpow2(L);
Y = fft(s,n);
Pp = abs(Y)/L;
figure,plot(Pp)
s = real(s);
Y = fft(s,n);
Pp = abs(Y)/L;
figure,plot(Pp)
Pp = abs(Y)/L*2; % because we take fft of real componnet only
figure,plot(Pp)

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