Constant Percentage Bandwidth Filter
A band-pass filter whose bandwidth is a constant percentage of centre frequency.
1/3 octave filters are constant percentage bandwidth, this is 23.1% of the 1/3 octave centre frequency.
Note that it makes sense to plot constant percentage bandwidth data on a logarithmic frequency axis.
1/3 octave data plotted on a linear frequency scale.
1/3 octave data plotted on a logarithmic frequency scale.