Need help finding "butterworth"

I have Audacity 2.1.0 and I’m trying to find a specific filter, that I was able to find in Cool Edit Pro 2.1. In Cool Edit Pro, I went to Effects->Filters->Scientific Filters. If someone could help me find the 3 things that I have circled below, I would appreciate it:

Audacity 2.1.0 is an old version. If you download the current 2.1.2 version from us at there is a Classic Filters effect that may be what you are looking for.

Note that the effect is disabled by default, so you have to enable the effect first at Effect > Add / Remove Plugins… (not Effect > Manage… as it says on the page I link to). Also due to a bug, the effect has no dB scale in 2.1.2, although the image on the linked to page shows a dB scale.


Audacity has “High Pass” and “Low Pass” filters. These are “Butterworth” filters:

The “Rolloff” setting is equivalent to the “filter order” where:
6dB per octave ↔ 1st order
12dB per octave ↔ 2nd order ↔ 2 pole
18dB per octave ↔ 3rd order ↔ 4 pole
24dB per octave ↔ 4th order ↔ 8 pole

For higher order / steeper slop filters you can apply these filters multiple times.

If you really want a Band Stop in one step, you can try See how to install a Nyquist plug-in.