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Ok for the back wall. I'm going to build an helmholtz bass trap. I suppose i can hang it horizonally ?
Why Helmholtz? Are you aware of how hard they are to tune? A room mode is a very tight, high-Q resonance, just a few Hz wide. A Helmholtz resonator is a very tightly tuned, high-Q device, with a peak just a few Hz wide. Getting those two peaks to align is a lot harder in reality than the text boos make it sound: slight imperfections in your building materials, or workmanship, even changes in temperature and humidity (that make the materials expand or contract fractionally), can throw your tuning off. If you have built Helmholtz resonators before, and understand the process of tuning them, and are prepared to go through that, and have the equipment to do it, then by all means, that's a good approach. But you will need a very thick front panel (probably 40 mm at least), a very deep cavity behind it (probably 30cm or more), very tiny slots or holes, and very wide slats or hole spacing. Such a device takes up a lot of space in your room, and isn't very efficient (unless it is very large).
On the other hand, membrane traps are easier to build and to tune, much thinner, and don't need high precision to make, nor do you need an extremely thick front panel. You could build a limp membrane in less than one third of the thickness you'd need for a Helmholtz device tuned to the same frequency, and you could even build several of them, tuned to different frequencies, in the same total space that one single Helmholtz device would take up.
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i'm also worried about the hollow point at 230. I don't know what to do about it. Time will tell...
Your images only show up to 200 Hz, so I'm not seeing what you are referring to there. But a dip at 230 Hz is probably either SBIR or modal. My money would be on SBIR.