I often want to isolate a particular sound from background sounds/noises of higher and/or lower frequency. It would be great to have a ‘Remove special’ tool that deleted everything above and below the selection. Even better would be to have the existing ‘Remove special>Trim audio’ tool be smart enough to change its behavior to remove above and below instead of left and right when in Spectral selection mode.
You could modify the “Spectral Delete” effect to do that.
The Nyquist code is a bit complex, so if you need help with that then I’d be happy to help. Here’s the code: https://github.com/audacity/audacity/blob/master/plug-ins/spectral-delete.ny
and an introduction to Nyquist plug-ins here: Missing features - Audacity Support