Is it possible to go below 100Hz with those kind of absorbers?
Like John said: That type of device is not efficient at such low frequencies. Helmholtz resonators are far more useful in the mid-range. As you can see from your graph, it appears to be working (there is a small reduction in the mode at 88 Hz, but the effect is nowhere near as big as you need. That's why bass trapping is almost always done with either panel traps or with deep absorption. John is saying the same thing I said on one of your other threads: That room needs a LOT more treatment. It is a small room, and therefore will need a huge amount of low frequency treatment.
In fact I ve put a lot of absorption on the front wall,
Walls are not much use for bass trapping with absorption: Corner are. Walls are good for panel traps (membrane traps, which are pressure-based devices). Corners are good for absorption (which is velocity based). What you need are large bass traps in the room corners.
I thought curved Helmholtz was a good way to reflect 1st reflections
Why would you want to put REFLECTION at your first reflection points? That's where you need ABSORPTION! You should be reducing all first order reflections as much as possible, not making tings more reflective. You COULD do that if you angle it steeply enough to ensure that all first order reflections are going way past the listening position, but the small angle you are using for that perforated panel does not look like it is enough for that. Did you ray-trace to figure out the angles?
without adding absorption on the side walls
Why do you not want to put absorption on the side walls? That room is small, and needs lots of absorption anyway. 317 sabins of absorption, to be exact. In other words, roughly 50% of the ENTIRE surface area of the room needs to be perfect absorption: That means half of the floor, half of the ceiling, and half of each wall needs to be perfect absorption: Since it is not practical to put absorption in the floor, that means that your entire ceiling should b absorptive, plus 50% of the area of each wall. That is how much absorption you NEED, according to the equations. It is a small room, so it needs a LOT of absorption, and most of that needs to be aimed at low frequencies.
So forget Helmholtz devices: they are not efficient and too hard to tune at low frequencies. They would take up way too much surface area, considering that half of each wall needs to be absorptive.
I think I need wood because you feel good in a room when there is some wood right?
You don't need wood to "make you feel good". You need wood where it is necessary to give the acoustic behavior that you need. That's why I suggested slot walls.
I found difficult to make slats around that frequency (hudge slats and tiny tiny slots...)
That's not the purpose of slot walls: They are not meant for low frequencies. Rather, they are meant as general broad-band devices, that absorb, reflect and diffuse all at once. They also work as absorbers at low frequencies (well below the tuned range), to a certain extent.
I closed the back-panel with a 5mm plywood because it will be against the wall I think the plywood shouldnt vibrate
5mm is way too thin, and it will vibrate. On a Helmholtz device, the cavity needs to be rigid. That should be 19mm plywood at the back and on the sides.
- Stuart -
