Do untreated rooms look like this?

[producer12999]

One of these is 0.36 m2, so 1.4m2 for 4. They look nice and membrane absorber is the way to go, but you might calculate the size of your back wall (or front) and then compare. It will increase (average) wall absorption by about 0.2 if your wall is 3 x 2.5 m.
So be prepared that you might want to use more of these. But your pictures show that you are already planning that way ;-)

thanks!

 

[olieb]

I have another question, we are only talking about my 80 Hz dip all the time. In my measurement you can see a big 70 Hz peak.

The tuning of the absorbers would be to the peak. They work by absorbing energy.
The dip comes from the fact that the speakers at their position do not couple to those room modes that have an amplitude at the listening position for 80 Hz. Or these do not exist.
To help with the dips the best idea is to modify the positions.
Absorbers help too, because damping of modes results in changing the width of the modes. Wider modes give a (somewhat) flatter FR.

 

[producer12999]

The tuning of the absorbers would be to the peak. They work by absorbing energy.
The dip comes from the fact that the speakers at their position do not couple to those room modes that have an amplitude at the listening position for 80 Hz. Or these do not exist.
To help with the dips the best idea is to modify the positions.
Absorbers help too, because damping of modes results in changing the width of the modes. Wider modes give a (somewhat) flatter FR.

All right, thanks! It would probably be best to order the acoustic modules from the pictures first. (as basic equipment)
Maybe not all of them to leave some budget. Then take measurements again and move the setup. Now in my empty room, these dips at 80 Hz and 120 Hz just don't go away, no matter where I place myself or the speakers at the short wall. The long wall is no opinion for me. With the remaining budget, I can then fine-tune the setup. Is this a good plan?

 

[olieb]

All right, thanks! It would probably be best to order the acoustic modules from the pictures first. (as basic equipment)
Maybe not all of them to leave some budget. Then take measurements again and move the setup. Now in my empty room, these dips at 80 Hz and 120 Hz just don't go away, no matter where I place myself or the speakers at the short wall. The long wall is no opinion for me. With the remaining budget, I can then fine-tune the setup. Is this a good plan?

I do not know the exact dimensions in your room, but in this arrangement there are no real nulls, just dips and peaks ;-) well up to about 145 Hz (there is a null).
If these could be dampened the result might be not so bad. The peaks are the modes along the length of the room. They can be EQed to a certain degree, too.
But every room has modes and in the region with single (isolated) modes the FR is not flat.

I know only one way around that: Dual bass array (DBA). You might look here.

Simulation Overview of the 'Double Bass Array' Configuration.

www.audiosciencereview.com

 

[-Matt-]

The tuning of the absorbers would be to the peak. They work by absorbing energy.
The dip comes from the fact that the speakers at their position do not couple to those room modes that have an amplitude at the listening position for 80 Hz. Or these do not exist.
To help with the dips the best idea is to modify the positions.
Absorbers help too, because damping of modes results in changing the width of the modes. Wider modes give a (somewhat) flatter FR.

In the case that the dips are caused by SBIR or LBIR wouldn't you try to attenuate the wave reflected from the boundary. I.e At the frequency that the dip occurs (80Hz), with panels either behind the speakers for SBIR, or behind the listener for LBIR. If the interfering reflected wave is attenuated relative to the direct sound then the cancellation, and hence the dip wouldn't be as deep.

FWIW I'd favour the sub... multisub... room correction route over a large investment in absorption panels.

 

[olieb]

In the case that the dips are caused by SBIR or LBIR wouldn't you try to attenuate the wave reflected from the boundary. I.e At the frequency that the dip occurs (80Hz), with panels either behind the speakers for SBIR, or behind the listener for LBIR. If the interfering reflected wave is attenuated relative to the direct sound then the cancellation, and hence the dip wouldn't be as deep.

Yes that is the theory and true if there is only one wall. In a room it is more complicated as SBIR and LBIR are just another way of looking at room modes. And a SBIR null at a frequency with a length mode reacts different than in case it is a width mode. A SBIR null at a frequency that is rather far from all room modes is worst of course. And on top of that all this depends on listening position too.

FWIW I'd favour the sub... multisub... room correction route over a large investment in absorption panels.

Exactly my point. Clever arrangement of multisub (source and sink).

 

[-Matt-]

Likely a single sub (probably near a corner and preferably with EQ) with the crossover set quite high (maybe 150Hz) could improve things a lot. Then the main speakers won't be exciting those problematic frequencies as much, and you will have more placement freedom with the sub.

Would the OP mention which speakers and electronics are planned to be used? Will there be the possibility of EQ? If a computer is the source then maybe look into EqualizerAPO if hardware EQ is out of the question.

Also not yet mentioned - do you have an absorbing floor covering like thick carpet?

By the way, if you do end up installing a lot of absorption panels it would be excellent if you would share your before and after measurements. Something like this thread...
https://www.audiosciencereview.com/...in-a-small-room-subs-ma1-absorbers-avaa.27288