to cut frequencies below 30 Hz I usually use the “High/Low Pass Filter” plugin, or an equalizer. But in both cases (at least I think) the frequencies are lowered gradually, following a curve.
Is there a way to completely cut frequencies below 30 Hz (a “stepped” cut) and leave those immediately to the right of the cut unchanged?
There are tradeoffs to doing so, the sharper the curve the more resonance in the filter, so you end up with significant boosts near the filter frequency. There are plugins that allow for a ‘brick wall’ filter, but you always have tradeoffs. And no Ardour does not come with one.
You can do that with plugins:
ChowMultiTool (Linux, Windows, Mac)
LSP Filter (Linux)