I'm trying to plot my FFT of File. This i my result. I made my FFT application:

I want to obtain something very similar to plot of WaveSpectra, please tell me my mistakes..


The result from WaveSpectra The wave Spectra version Other result from WaveSpectra Other with WaveSpectra

Here my code:

double[] Samples = null;
try {

  RandomAccessFile rafr = new RandomAccessFile(TheFile, "r");  //TheFile String with path and WAV filename
  //double[] getSamplesFile(rafr,"left",0,8192)  //returns the first 8192 samples of left channel
  Samples = getSamplesFile(rafr,Channel,iPosition,iSizeFFT);
catch (IOException e) { System.out.println(e.toString()); }

Applying Window Function

Now I'm trying to apply Window function to Samples, check the code below

//double[] getWindow(8192,"hamming"); //was verified against matlab
Samples = applyFilter(Samples,getWindow(iSizeFFT,sWindow));

Trying to Normalize

Get the Log10 of of absolute value of each complex item of array of FFT, later multiply by 20

//fft:  Returns a complex array with FFT (the result was tested and it is equal to Matlab)
//getArrayComplexMod: Returns a double array with absolute value of each complex number
//getRealArrayLog10: Returns a double array with Math.log of each double element
//getRealArrayScalarMult: Returns a double array multiplying each double element with 20.0

Samples = getRealArrayScalarMult(getRealArrayLog10(getArrayComplexMod(fft(Samples))),20.0);

Obtain the Frequency in Hertz

//Now the freq or bins
double[] bins = new double[iSizeFFT];  //Size 8192
Integer SmplRt = 8000; 
for (int i = 0; i < iSizeFFT; i++) {
  bins[i] = (double)i*SmplRt/iSizeFFT;

The code to apply filter (or Hamming window) to Samples.


  static double[] applyFilter(double[] dSamples, double[] dFilter) {
    return applyFilter(dSamples, dFilter, 0);

  static double[] applyFilter(double[] dSamples, double[] dFilter, int PosSample) {
    double[] filteredSamples = new double[dSamples.length - PosSample];
    for (int i = PosSample; i < dSamples.length; i++) {
      filteredSamples[i - PosSample] = 0;
      for (int j = 0; j < dFilter.length; j++) {
        if(!((i - j) < 0)) {
          filteredSamples[i - PosSample] += dSamples[i-j]*dFilter[j];
    return filteredSamples;

EDIT 1: Following the answer of @hotpow2, solved!

enter image description here


2 Answers 2


You appear to be convolving the data with your window instead of just multiplying the data by your window. e.g.

windowedSamples[i] = window[i] * data[i]

without any nested convolution loop.


This is generally a programming question that fits to Stack Overflow, but you missing one part. Frequency scale in case of Wave Spectra is logarithmic, on your plot it is linear. Usually functions like semilogx are doing job. Then it's simply setting the x-axis ticks.

  • $\begingroup$ maybe Stackoverflow, My question is represented in Java Code, but the algorithm belong to DSP technics... I want to know the formula or procedure to deal with FFT to represent it in X logarithmic mode... no matter if not java... $\endgroup$
    – joseluisbz
    Commented Jan 22, 2015 at 23:57

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