Why treat modal resonances below the speaker cutoff? (eg @40Hz)?

bass traps and low frequency control

Treating the bass in your studio is hard.

Especially the lowest octave.

To fully absorb 100Hz you theoretically already need 86cm (2’10”) of porous material depth (¼ wavelength rule).

Of course this is deep in voodoo territory, because what is technically true isn't even remotely necessary in practice.

I can assure you: If you were to cover your entire room in 86cm deep absorptive material, it would absorb FAR further down than 100Hz..

Still, how far down in frequency does it actually make sense to go in practice?

Is there even a point in absorbing lower down than the natural cutoff of your speakers, for example (@40Hz typically)?

What's a good target to set for your low end absorption?

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