How to plot FFT?

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')

Answer 1

[Paul]

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]

Thank you so much, I am satisfied with that result.