How to find frequency of wave data
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Marcella Roth
le 18 Mar 2021
Modifié(e) : Cris LaPierre
le 23 Mar 2021
Hi, I have a set of data (shown below) and I would like to get the frequency of the wave Thank you!
![](https://www.mathworks.com/matlabcentral/answers/uploaded_files/559222/image.png)
So far I have tried:
a=readmatrix(sprintf('sampledata')) %read in data
x=a(:,2)
t= a(:,1)
z = x(t>171 & t<174.5)
zz = t(t>171 & t<174.5)
% Get sine fit
F = fit(zz, z, 'sin1');
% Plot result
plot(F,zz,z);
grid on;
xlabel('Time (secs)');
ylabel('Amplitude');
title(['Estimated frequency = ', num2str(F.b1/(2*pi)), ' Hz']);
and I get a frequnecy of 0.00075267 Hz
I can see by hand counting that the frequency is about 27-28. Any help is appreciated!
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Cris LaPierre
le 18 Mar 2021
I would probably try to use findpeaks. With the right setting, you can use that to count the number of peaks. You could also do this interactively using the find local extrema live task in a live script.
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Cris LaPierre
le 22 Mar 2021
You are not using the same window you showed above. Also, you only need to call findpeaks once. Finally, cycles/sec is already Hz. Don't divide by
.
![](https://www.mathworks.com/matlabcentral/answers/uploaded_files/558572/image.png)
Assuming you are interested in the approximate frequency between 171 and 174.5 seconds, only pass the corresponding rows into findpeaks
% load the data
a=readmatrix('sample data.txt');
t= a(:,1);
y=a(:,2);
% Plot result
plot(t,y)
xlim([170 175])
grid on
xlabel('Time (secs)')
ylabel('Amplitude')
% Select rows corresponding to desired time window
ind = t>171 & t<174.5;
[pks,locs] = findpeaks(y(ind),t(ind),'MinPeakProminence',0.5);
% visualizse the selected peaks
hold on
plot(locs,pks,'rv')
hold off
% calculate the approximate frequency
meanCycle = length(pks)/(locs(end)-locs(1))
title("Estimated frequency = " + round(meanCycle,1) + " Hz");
.
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