Low Cut EQ - Am I understanding this correctly?

I’m trying to do a low cut EQ on my entire project and cut out all the frequencies below 20 Hz.

I have the following parameter settings:

with_fx :eq, low_shelf: -1, low_shelf_note: 20 do

Am I understanding this correctly? The low_shelf_note: 20 means 20 Hz and I’m cutting everything below it by -15dB?

On the oscilloscope the waveform doesn’t seem to reflect the lowest-end being cut but maybe it’s not super accurate.

I notice if I change the low-shelf to -4 or lower it seems to reduce the sound in the low to mid frequencies significantly but I didn’t expect this as I thought the shelf would be only applied below 20 Hz similar to how a Graphic EQ when you adjust the 20 Hz slider.

EDIT: After bringing some exported sound into a seperate DAW it appears that everything below 100 Hz was cut given the code I shared above…

" 20 " means midi-note 20 rather than 20 Hz …

Oh I see. Even so, do you know why it would be cutting around 100 Hz? After looking at this MIDI Note to Hz Conversion Table (MIDI note numbers and center frequencies | Inspired Acoustics), it would seem that MIDI Note 20 = 25.96 Hz… I’m now thinking that maybe the “low_shelf” parameter has something to do with it. Maybe when I set the low-shelf to -4 it’s actually still affecting the frequencies above the targetted “low_shelf_note” due to bezier curve easing or something along those lines…

EDIT: I just did some further testing. When I change low_shelf to an extreme of -10, it completely cuts out most of my high-end so I think that proves my suspicion that there is some sort of easing curve that’s eating into to the frequencies above the target note. I might just have to do a bunch of test cases and export them to a DAW so I can do a visual analysis. Maybe Low-Shelf is not what I need. I’m now thinking a workaround would be to do a low cut instead and crank up the low_q to make those target frequencies drop out without effecting anything higher.

If you want to cut out everything below 20 Hz ,
a high pass filter would be more apropos …
( pass everything above 20 Hz )

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Thank you, that works much better. I’m so accustomed to using an EQ to mix in the past using a DAW and totally overlooked the HPF.