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How to compare a rolloff figure in Hz to one in dB/oct

When you are considering the rolloff of a filter there are two characteristics that can be discussed. The bandwith that the rolloff occurs over, which is expressed in Hz, and the slope of the attenuation that occurs over this bandwidth, which is expressed in dB/oct.

The passgain, stopgain and rolloff values within Net Station can be used to calculate the slope. This is expressed by the following equation: 

dB/octave = (passgain - stopgain - 3) / (log2 (freq + rolloff / freq))

where: passgain and stopgain are in dB, freq is the nominal cutoff frequency in Hz, and roll off is the rolloff in Hz.

If we look at the default settings for a 20 Hz low pass filter, we will see the following values:

passband gain = 99% (0.1 dB)

stopband gain = 1.0% (40 dB)

rolloff = 2.0 Hz

Therefore these can be entered into the above equation:

dB/octave = (-0.1 dB - (-40 dB) - 3) / (log2 (20 Hz + 2 Hz / 20 hz)) 

dB/octave = (36.9 dB) / log2 (1.1)

= 268.4 dB/octave

This allows us to express the rolloff of Net Station's default 20Hz low pass filter as either 2.0 Hz or 268.4 dB/oct.  

The Net Station 5 User Manual discusses the optional filter controls that exist in the Net Station 5 software. 

Additional Info

  • Product Type: Net Station 5
  • Information Type: Theoretical Background
Last modified on Monday, 24 April 2017 22:44

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