BandPass/BandStop filter settings

This group contains the settings if the type BandPass or BandStop has been selected as shown below.

If BandPass is the filter type and  is checked, several bands may be filtered and thus several curves are plotted depending on the start and stop frequency and the octave fraction bandwidth. How to determine the bands is explained below.

Note that if Filter several bands is checked, you should note that if Plot->Overlay is checked, many curves will be generated.

 Determines the centre frequency. Any frequency may be typed or it may be selected from the list of standard 1/3-octave frequencies. If  is checked and the filter type is BandPass, two frequency values are to be selected, the start and end centre frequencies as shown below.

The number of curves to be plotted within this frequency range is given by the bandwidth described below. If the bandwidth is set to 1 octave or 1/3 octave, the standard frequencies within the specified frequency range is used. For other bandwidth settings the centre frequencies are calculated beginning at the start centre frequency.

 Determines the filter octave bandwidth. Any bandwidth may be typed or it may be selected from the list. A bandwidth of 1 octave or 1/3 octave is typically used.

 Specifies the filter order. The order is selected from the list and has the range 2-14, only even numbers. Filter order 6 is according to the IEC standard. If the order is greater than 6 it does not give a stable result for low frequencies, e.g. filter order 8 and bandwidth 1/3-octave will not give a stable filter below a centre frequency of 350 Hz.

 Specifies the filter type. You can select from the two types shown below

Select the type Butter Forward if you want a traditional filter function. If you select Butter Forward/Reverse the filtering will be performed once forward and once backwards. Thus there will be zero phase distortion and the filter order will be doubled.