FFT shows different frequency

Question

I have a question regarding my FFT result. I have 3 audio files from a welding process with 120A, 130A and 140A with a audio length of about 0,085s.

You can see the audio file in the first row from the plot.

After that i've created a FFT from every audio file which are located in the second row. The maximum peak from every audio file is at about 200Hz. 120A-140A_085sec.jpg

When i calculate the period between the welding pulse i have a period of about 0,00871s at a welding current of 140A. This is exakt the first peak in the FFT plot. 140A_v20.jpg

My question is, why is the maximum peak exactly at twice the frequency at about 200Hz and not like i calculated at about 100 Hz?

I hope someone could explain this to me

Many thanks in advance! BR

    clear all
    close all

    % 120A
    [data,Fs]=audioread('Mono_120A_v20_08sec.wav');
    [nSamples,nChannels]=size(data);
    waveFileLength=nSamples/Fs;

    t=[0:length(data)-1] / Fs;

    subplot(2,1,1);
    plot(t,data)
    grid minor
    xlim([0 0.09])
    title('120A 0.085sec')
    xlabel('Zeit')
    ylabel('X(t)')

    y_fft = abs(fft(data));               %Retain Magnitude
    y_fft = y_fft(1:nSamples/2);       %Discard Half of Points
    f = Fs*(0:nSamples/2-1)/nSamples;  %Prepare freq data for plot

    subplot(2,1,2);
    plot(f, y_fft)
    grid on
    xlim([0 400])
     ylim([0 10])
    xlabel('Frequency (Hz)')
    ylabel('Amplitude')
    title('Freqenzbereich 120A')`

paisanco paisanco · Accepted Answer · 2019-07-05T15:08:18

I don't think there is anything wrong with your code. You've taken the absolute value of the FFT, which will give you the spectral envelope of your signal. Determining the pitch of the signal is a different problem and cannot be done by just taking the absolute value of the FFT. If that is your goal, you need to do a bit of research on pitch detection techniques.

You have a situation where the amplitude of the first harmonic of your acoustic signature is greater than that of the fundamental. This is not unusual and can happen as a consequence of the physics of the acoustic source (in this case a welding torch).

There is a post over at Physics.SE that discusses this in a bit more detail for musical instruments: https://physics.stackexchange.com/questions/246020/is-it-possible-for-a-harmonic-to-be-louder-than-the-fundamental-frequency

FFT shows different frequency

1 Answers