How to find corresponding frequencies after viewing amplitude in descending order?

yun-hun-jmai - 2021-03-01T10:34:31+00:00

Question: How to find corresponding frequencies after viewing amplitude in descending order?

Good day. Below I have a piece of  code written.     %% Peak detection Part % In this part, we have defined a threshold value. So, all peaks above % threshold value will be detected. temp_fft = data_filt_ft; % Normalize the FFT temp_fft = temp_fft/max(temp_fft); % threshold th = 0.02; % i.e. 1% of maximum amplitude temp_fft(temp_fft < th) = 0; %you can skip this step by commenting this. % Plot FFT after thresholding figure; plot(fr, temp_fft); xlabel('Frequency(Hz)'); ylabel('Amplitude'); grid on; % find peaks of the remaining data [peak_ft, peak_loc] = findpeaks(temp_fft); % frequency corresponding to peak [peak_fr] = fr(peak_loc); %Sort amplitude values into descending order DescendAmp = sort(peak_ft,'descend'); % this fuction returns the fourier transform of the signal function [ft, f] = fr_t(x, Fs) L = length(x); % NFFT = 2^nextpow2(L); % Next power of 2 from length of y NFFT = L; X = fft(x,NFFT)/NFFT; f = Fs/2*linspace(0,1,floor(NFFT/2+1)); ft = abs(X(1:floor(NFFT/2+1))); % ft = 20*log10(ft); % plot(f,ft); end I am able to sort the amplitudes into descending order but I am unable to view the corresponding frequency values in that order. How can I view the corresponding frequency values in that order?    

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Expert Answer

Prashant Kumar answered . 2025-11-20

% Sort amplitude values into descending order and get the corresponding frequency
[DescendAmp, sortIdx] = sort(peak_ft, 'descend');  % Sort amplitudes and get sorting indices

% Reorder the frequencies using the sorted indices
DescendFreq = peak_fr(sortIdx);

% Now DescendAmp contains the sorted amplitudes, and DescendFreq contains the corresponding frequencies
% You can plot the results or display them

% Plot the corresponding frequency vs. amplitude in descending order
figure; 
plot(DescendFreq, DescendAmp, 'o-');
xlabel('Frequency (Hz)');
ylabel('Amplitude');
title('Sorted Frequency vs. Amplitude');
grid on;

Explanation of the Changes:

1. Sorting Amplitude and Getting Indices:

   • [DescendAmp, sortIdx] = sort(peak_ft, 'descend');
   • This line sorts the amplitude values in descending order and returns the sorting indices sortIdx, which tell you how to reorder the original data.

2. Reordering the Frequencies:

   • DescendFreq = peak_fr(sortIdx);
   • Using sortIdx, we reorder the frequencies (peak_fr) to match the sorted amplitudes.

3. Plotting the Results:

   • plot(DescendFreq, DescendAmp, 'o-');
   • This line creates a plot showing the sorted frequencies against their corresponding amplitudes.

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Output:

• DescendAmp: Amplitudes sorted in descending order.
• DescendFreq: Frequencies corresponding to the sorted amplitudes.

Now, you can directly view the frequencies corresponding to the peaks in descending order of their amplitudes.

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