1/12-Octave Band Spectrum (FFT)

This analyser derives the 1/12-octave band power spectrum from a succession of fast Fourier transforms. Frequency bands are the IEC frequencies (with 1/12-oct bands based on round(40*log10(frequency)). This is different to musical semitones in two ways: (i) the tuning frequency is not equivalent to A 440 Hz; and (ii) the semitones are slightly smaller for IEC frequencies.

The advantages of deriving 1/12-octave spectra from a simple redistribution of FFT output include:

  • potential for short computation time
  • small spectral leakage

The disadvantages of deriving spectra in this way include:

  • low temporal resolution
  • the underlying filtering process is not constant Q
  • errors can occur from window steps that are too large
  • the shorter the window length, the greater the error due to redistribution from linear to logarithmic frequency, especially in low frequency bands
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