Octave Analysis Bandwidth Setting

Updated Feb 7, 2019

Reported In

Software

  • LabVIEW
  • LabVIEW Sound and Vibration Toolkit

Issue Details

I would like to do octave analysis on my previously acquired sound or vibration data in LabVIEW using the SVT Fractional-octave Analysis VI found in the LabVIEW Sound and Vibration Toolkit, or alternatively with the Octave Analysis Express VI. I would like to set the Bandwidth parameter so, that the individual bands in the resulting octave spectrum will have constant width, for example 1 Hz. How is it possible to do that?

Solution

Neither for the SVT Fractional-octave Analysis VI, nor for the Octave Analysis Express VI it is possible to set the bandwidth to 1 Hz or to any constant value for fundamental reasons. The configurable way of dividing the frequency domain to bands is logarithmic, not linear. The available options for the Bandwidth parameter are 1 Octave; 1/3 Octave; 1/6 Octave; 1/12 Octave and 1/24 Octave. In general, if you set the bandwidth to 1/N Octave, then two consecutive bins will have a centre frequency ratio equal to the Nth root of 2, since one octave means exactly a factor of 2 ratio. That's the mathematical reason, why one cannot set the bandwidth to any constant value.
Use FFT-based functions like the Window FFT PtByPt VI to have linear frequency axis setting.

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