A while back I was thinking about how a membrane absorber is similar to a sealed speaker but the difference being that with a speaker we just look at the volume of the cavity and a membrane absorber we look at the depth. Turns out they're working on the same principles but with membrane absorbers we simplify the calculations by assuming the membrane covers the entire front instead of only partially covering the front like a woofer (unless you had a cylindrical cabinet the same size as the woofer...).

This got me thinking about if there were any practical uses for a membrane absorber with the membrane not covering the entire surface area. For the most part the answer is no but one situation could be to save space.

Say you have thick membrane absorber A beside thin membrane absorber B in a studio. What if you made A with an "L" shaped cavity which B then sit in? Effectively reducing the depth of A by having some of it sitting behind B (which could then end up being flush with A).

Within a more complex constrained assembly you're going to lose some performance from the new placement of B since there's no free lunch but it could work.

Not all that useful especially if your goal is full range absorption but it could be a useful tool in some cases.

My question is, does anyone here have any experience building membrane absorbers like this where the membrane doesn't cover the entire front area of its cavity?

A speaker radiates sound into a room.
That sound spreads for low frequencies from the speaker omni directional in the room.
Now if you want to catch/absorb that sound, you need either another, similar speaker that is radiating the same sound but delayed and out of phase (active absorption), or absorption on every surface the sound from the speaker is bouncing of (passive absorption).
If you don't cover every piece of surface you won't get 100 % absorption.
Tuned absorbers are hard to work with (show me a thorough investigation where they work really good), and tinkering with the effective surface makes things worse, I guess.

I'm not asking about active treatment or using speakers for absorption... all I mentioned were the similarities in how a sealed speaker is designed vs a membrane absorber. The physics are the same. Yes, in theory you can use a sealed speaker as an absorber but it would be much more complex since you need to consider the compliance, Mms, etc but that's not what I'm asking here.

I also mentioned that this approach has its limitations and will not give you a full range absorber but that also doesn't mean it doesn't have applications where you can only afford to have selective absorption (when your wall can't be 1m thick...).

I tried to explain how absorption works.

It would be very comprehensive if we had all these parameters for membrane absorbers. It could even be that they would work then.