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I am currently looking at the STFT for Librosa. I was wondering how to understand the output of the STFT function, specifically what kind of frequencies the different values represent.

Say I have a n_fft of 256, that means that the shape output of the STFT will be 129 (1+n_fft/2). I therefore understand that for each frame I have 129 bins, these bins represent some sort of frequency within this frame. I am assuming that the bins start with a frequency of 0hz and go up to some value, with each bin representing an equally large range.

But how do I figure out what the maximum frequency used for STFT is? And what about the range of each of the 129 bins?

Just to be clear, I have tried looking at the source-code and at the documentation, but I have not really become much wiser.

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  • $\begingroup$ have a look on this question: FFT resolution $\endgroup$
    – Irreducible
    May 8, 2019 at 8:53
  • $\begingroup$ So if my sampling frequency is 16000, and I have 129 bins, then the resolution is ~124, meaning that each of the bins has a frequency range of 124? Is that a correct way of looking at it? $\endgroup$
    – Cralle
    May 10, 2019 at 9:47

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Having a look into the sources of librosa.display.specshow reveals how bins are converted into frequencies internally: The plotting uses librosa.core.fft_frequencies, which shows that it is basically the same as numpy.fft.fftfreq:

>>> librosa.fft_frequencies(sr=22050, n_fft=16)
array([     0.   ,   1378.125,   2756.25 ,   4134.375,
         5512.5  ,   6890.625,   8268.75 ,   9646.875,  11025.   ])
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