A side note. Even if you do use that program, you’re not going to get a sharp shelf filter. You’re actually going to get a much more gentle curve such as the green line, even with the filter Length turned all the way up.
At 90Hz, it’s 36dB down. It’s not gone. If you’re making critical decisions based on this filter, you should know that.
If the filter Length is not turned up, the green curve is much more gentle. Not only is the attenuation much more graceful (90Hz is only 24dB down), but it’s attenuating some of the work at 70Hz and 75Hz.
Does this affect your job?
There’s a bug listed in the Audacity release notes:
In the Equalization effect if you create a “curve” with a vertical segment, then that vertical segment will not be recalled properly by Audacity when you next use the effect. Also note that you cannot use the “Manage” or the “Save/Manage Curves” - as these will exhibit the same corruption of the curve.
what I’m doing overall is making songs sound better
That’s one reason we sometimes ask—firmly—that you give us the actual job. That filter and curve won’t do that. That’s the filter if all you want to hear is trucks rumbling by, thunder and earthquakes. It’s probably the exact opposite of the curve you want. The one that removes low pitch distortion, trucks rumbling by and thunder.
There’s another thing to note when you try tricks like that. Low Rolloff (there’s one built into the Audiobook mastering suite and most Hollywood sound shoots.) is normally done much more gently and not drop like a rock at a certain frequency. Natural sounds aren’t just one tone. They’re usually a tone and then overtones and harmonics. That’s the difference between a good and crappy violin even if they’re both playing A above middle C. The drop-like-a-rock filter takes out the tone, but leaves everything else.
So instead of a large, thumpy truck rumbling by, it gives you crisp, tighter, well defined truck rumbling by. Probably not the goal.
I have a field sound mixer with that curve built in, but it’s much more gentle and not a vertical line “fall off a cliff” at 80Hz. I do have a picture of it. That’s it going out on a job. There’s a picture of what the knobs and switches used to say stuck to the bottom if I forget.
By the way, in that first curve you posted? It failed because you told it to do two different things at the exact same frequency or pitch tone. “Reduce the music volume by 6dB and reduce the music volume by 60dB. Call me when you’re done.” It probably just followed the last instruction it could deal with which is where the straight line came from.