Spider DSA User’s Manual
233
Nominal center frequencies (mid-band frequencies)
Nominal center frequencies are “round” numbers that were historically
established for analog octave filters. The nominal mid-band frequencies for 1/1-
octave and 1/3-octave are listed in the ANSI S1.11-2004 Annex A. This standard
also describes how to determine the nominal mid-band frequencies for other
fractional octave bands.
The exact center frequency of the filter band is usually not equal to the nominal
frequency. For example, in a 1/3 octave, the exact center frequencies 794.33 Hz,
1000 Hz and 1258.9 Hz are used to correspond to the filters with nominal
frequencies 800 Hz, 1000 Hz and 1250 Hz.
Band Edge Frequencies of Fractional Filters
The low and high frequency band edges of a filter can be calculated based on the
frequency ratio, G and the fractional octave resolution N (=1, 3, 6, 12…)
The bandwidth of the filter is: BW =
When starting or resetting the filtering operation of the fractional-octave filters, a
certain time is required before the measurements are valid. This time is called the
settling time and is related to the bandwidth of any particular filter. The lowest
frequency band has the smallest bandwidth and thus defines the settling time
required before you can consider the complete fractional-octave measurement
valid. A good rule of thumb is that the prudent settling time estimate is five
resolution reciprocal time periods, that is:
Note the settling time depends on the bandwidth which changes with center
frequency. A narrower filter and a lower frequency band requires a longer settling
time.
Analysis Frequency Range
You can decide the analysis range by changing the lowest and/or highest
as the
Analysis Parameters:
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