Spider DSA User’s Manual
232
Two descriptive systems are commonly used to describe proportional bandwidth
filters: Base-two (fractional octave) or Base-ten (fractional decade). A full octave
filter has a G of 2, numerically equal to 10
3/10
. Hence this same filter is termed a
3/10-decade filter. Crystal Instruments CoCo analyzers describe proportional
filters by their decade resolution.
Proportional bandwidth analysis divides frequency information uniformly over a
log scale which is very useful for analyzing a variety of natural systems such as the
human response to noise and vibration. Many mechanical systems also display
behavior that is best characterized by proportional bandwidth analysis.
Fractional Octave Filters
To gain finer frequency resolution, the frequency range can be divided into
proportional bandwidths that are a fraction of an octave. For example, with 1/3
octave analysis, there are 3 band-pass filters per octave where each center
frequency is 2
1/3
(10
1/10
) the previous center frequency
.
In general, for 1/N
th
octave analysis, there are N band pass filters per octave such
that:
where 1/N is called the fractional bandwidth resolution.
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