# How to plot FFT?

3 views (last 30 days)
Sara Boznik on 9 Sep 2023
Commented: Sara Boznik on 9 Sep 2023
Hi,
I am trying to plot FFT of tuning fork. I know that peak must be at 440 Hz. I am trying with my Matlab code but nothing works. Please for help.
plot(t,p)
title 'Časovni potek zvočnega signala'
grid on
xlabel '{\itt} [s]'
ylabel '{\itp}'
m = length(p);
n = pow2(nextpow2(m));
y = fft(p,n);
figure(2)
f = (0:n-1)*(1/n)/10;
amplituda = abs(y).^2/n;
plot(f(1:floor(n/2)),amplituda(1:floor(n/2)))
xlabel('Frekvenca(Hz)')
ylabel('Amplituda')

Paul on 9 Sep 2023
Hi Sara,
The frequency vector needs to be corrected. And for this data it's helpful to subtract out the mean before taking the fft.
% load in the data and run the original code
vilice
t = 1001×1
1.0e+00 * 0 0.0001 0.0002 0.0003 0.0004 0.0005 0.0006 0.0007 0.0008 0.0009
p = 1001×1
2.6691 2.6593 2.6508 2.6496 2.6447 2.6386 2.6435 2.6508 2.6557 2.6508
jCorrect expression for frequency
f = (0:n-1)*(1/n)*1e4;
Subtract mean
y = fft(p-mean(p),n);
amplituda = abs(y).^2/n;
figure
plot(f(1:floor(n/2)),amplituda(1:floor(n/2)))
xlim([200 600])
Sara Boznik on 9 Sep 2023
Thank you so much, I am satisfied with that result.