How to find frequency of wave data

Hi, I have a set of data (shown below) and I would like to get the frequency of the wave Thank you!

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!

Answer 1

[Cris LaPierre]

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.

[Marcella Roth]

Thank you for your help. I tried findpeaks, but I'm still having trouble arriving at my desired result. I did the following:

findpeaks(y,x)

[pks,locs] = findpeaks(yy,xx,'MinPeakHeight',5); %I only want peaks of a certain amplitude

meanCycle = mean(diff(locs))

I am getting a mean cycle of 0.0128. I want to get the average number of peaks per 1 sec which should be around 27. Any additional help is very much appreciated.

[Cris LaPierre]

Consider sharing your data.

[Marcella Roth]

I shared my data above. I currently have the peaks picked out that I would like to find the frequency of (shown below). I'm not sure what the command is to find the mean frequnecy per second .

[Cris LaPierre]

You've shared several pictures of your data. However, it's easier for me to test code on the actual data. Consider attaching 'CSSR-16Dec20-1.txt' to your post using the paperclip icon.

Still, if your pks and locs are correct, then it's just a matter of understanding what you have. locs is the x location of the peak, in seconds, and pks is the y value of the peak. Average number of peaks per second is the number of peaks devided by the time interval, in seconds.

meanCycle = length(pks)/(locs(end)-locs(1))

Your current approach is calculating the average time between each peak.

[Marcella Roth]

Thank you for your help, I'm starting to get a better understanding of this process, but still am not getting the correct output. I attached my data. I read in the data as a matrix and x is time in seconds. I did the following and got a mean cycle of 0.8366. By visually counting the peaks this is not the correct output.

y=a(:,2);

x= a(:,1);

findpeaks(y,x,'MinPeakProminence',0.5);

[pks,locs] = findpeaks(y,x,'MinPeakProminence',0.5);

meanCycle = length(pks)/(locs(end)-locs(1))

[Cris LaPierre]

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 .

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))

meanCycle = 27.6002

title("Estimated frequency = " + round(meanCycle,1) + " Hz");

.

[Marcella Roth]

Thank you so much for your help, I really appreciate it!