[Audacity-quality] Scientific Filter graph

[<ga...@au...>]

Usual disclaimer for any any ignorance, but I am confused when I 
look at http://wiki.audacityteam.org/wiki/Low_Pass_Filter
and see that the graph for Butterworth filter in Scientific Filter
does not conform to attenuation of -6 dB at double the corner 
frequency. Or should I not expect conformance?
 
In the attached image (1000 Hz first order low pass, range set to
+6 to -61 dB but the range makes no difference AFAICT), 1000 Hz 
seems to be at -3 dB as expected, but 2000 Hz is at -7 dB instead 
of -6 dB.   

Similarly with first order low pass at 250 Hz, 500 Hz appears to 
be at -7 dB instead of - 6 dB.

The rolloff above double the corner frequency seems to be 
"correct" (-6 dB per octave).   

The Manual (most of it verbatim from Norm's -devel post) says 
for Butterworth that  "attenuation increases at approximately 
6 dB per octave times the filter order". Why "approximately", or 
is that another way of describing what I see?  

Now if I change my Butterworth first order 1000 Hz LP filter to 
"Chebyshev I", there is less attenuation than Butterworth 
(at default 1 dB "Passband Ripple") - look at 5000 Hz for example.
As greater attenuation is expected with a Chebyshev than a
Butterworth filter, is 1 dB the correct default for "Passband Ripple"? 

At what points in the stopband part of a Chebyshev filter do you
expect a first order rolloff of -6 dB per octave? For example with
Chebyshev I Passband Ripple of 1 dB (or Chebyshev II Stopband
Ripple of 1 dB),  5000 Hz has a gain of -9 dB, but 1000 Hz has a 
gain of what looks like -16.5 dB.



Gale