[sdiy] 8th order?

[JH.]

>What does 8th order mean? Is it simply another way of describing the
>number of poles in a filter or db/oct?

8th order means 8 poles for high pass, low pass, and all pass filters.

dB/oct means the steepness of slope; depending on the application, this can 
mean the slope "infinitely" beyond some cutoff point (as often used in LPF 
filters), or the slope near resonance (as often used in BPF filters), or the 
slope near the cutoff point (as in the infamous diode ladder circuits).
The steepness of slope is *not* linked to the number of poles: A filter can 
consist of a 2pole LPF stage and a 2pole  APF stage, and have 12dB/Oct (and 
not 24dBOct). For filters without an APF component, there is a fixed 
realtion between number of poles and zeros, and the slope at infinite 
frequency, however.

Band Pass filters are different in that there is no universal convention for 
the meaning of "order".
One common definition is: Order = number of poles, regardless of filter 
type, so this applies to BPF as well.
However, there is the very common method of BPF design that starts with a 
reference LPF and then performs a transformation, where each LPF pole is 
transformed in a pair of BPF poles. In that context, some (serious!) 
literature speaks of a 4-pole BPF as a 2nd order Filter.
So for BPFs, we must live with two different conventions.
Also telling the number of poles helps to avoid confusion here.

JH.