I wonder whether I should go for some bass traps like you. would you have some short recordings for comparison between before and after room treatment with the same microphones and same placement ?
BASS TRAPS - ACOUSTIC PANELS FOR THE HOME STUDIO
Hi, Didier. Any small to medium size room will benefit from bass traps/Acoustic Panels. The best thing to do is to walk around the room clap your hands and speak around the area of the piano, mics, and listening position. If you can hear flutter echo, ringing, room modes then it's worth treating with bass traps. If you have nasty resonances on certain notes, loss of definition or clarity in sound, then it's also worth treating with bass traps. It also depends on fixtures in the room, wall construction, geometry, dimensions, glass, piano/mic location, and nearby reflections.
For all practical purposes to record in the home environment, bass traps function to mainly absorb sound reflected by walls, room corners, and ceiling closest to the piano and mics. It also helps to reduce unwanted room modes, ringing, comb filtering, and flutter echo, which plague all small spaces in a typical home with 8-10ft ceilings. It also alters the frequency response of the room so that the peaks and nulls are flatter across the frequency spectrum.
The best way to treat any room is with bass traps that use fiberglass panels. I am not sure what you have in France, but I highly recommend Owings-Corning 703 (OC-703) or equivalent fiberglass panels that measure 2ftx4ft. You can also use Roxul Mineral Wool, Roxul Rockboard, or Knauf Acoustical Board which are all cheaper. However, their absorption coefficients are lower for bass frequencies, so you may have to double the thickness to get the same absorption of OC-703. They all measure 2in thick. Don't bother with foam panels that you see at music outlets - they much less effective than fiberglass because they have much lower absorption coefficients.
I made my panels 4in thick, by doubling two 2in thick 2ftx4ft OC-703 fiberglass panels. I glued and screwed together a 2ftx4ft frame out of 4.5in wide pine wood and placed the fiberglass panels inside (4in total thickness). To camouflage the panels in the room, I matched the color of the acoustic panels to the color of the walls by stapling an ivory colored burlap fabric over the entire front and sides of the panels. It looks great. To cover your construction, you must use very porous fabrics; I find that burlap is very cheap and effective. You can hang them on the walls like a picture frame with wire and hooks, or make an H-frame pedestal to place on the floors. The great thing about them is they're portable, I can take them down if guests are coming over. The entire project took me a weekend to make. Your local hardware store can easily cut the wood for you, so that all you have to do is glue,screw, and staple. If you can make a picture frame, you can make your own bass traps. Each bass trap cost me $60 including everything (glue, screws, staples, burlap, wood, OC-703, pedestal, shipping, and taxes). The retail cost for each bass trap would be $325 - $375 in the U.S. The entire room cost me around $300; Retail would have cost me at least $1750.
Amount of bass traps depend on room dimension, shape, and the location of piano in the room. Triangulate the nearby reflective surfaces from the piano source to the mics, and place the traps in the path of direct nearby reflections. Don't go crazy since you only need to treat the nearby reflections from the walls, corners, and ceiling that will interact with your mics. Don't worry about walls that are away from the mics. As you know, there are phase cancellations/summing, delays, etc. that can only hinder clarity and evenness of frequency response. By negating the majority of the nearby reflections going to the mics, you're picking almost an entirely direct sound from piano source to mic. You may even find yourself using true pressure omnis, like your Avenson STO-2, at a farther distance to 3-4ft and allowing the sound the "breath" and coelesce by the time it reaches the mics. Once you place the panels, you'll hear that the piano sounds more damped and not as loud as before, because you're limiting the reflections coming to your ear, and what you are hearing is the direct sound of the piano. Don't worry about the loss of "small" room ambiance, because you can always reverb on your DAW, without amplifying the anomalies that were there before treatment. You'll get a much clearer and more natural sounding reverb. Add only the panels that are necessary, you can add more if you need to, but address only the nearby reflections first - the walls and ceiling around you and the mics. If you find that the sound is too tight and damped, either move the panel(s) around or remove a panel and use the extra panel for your monitor speakers... Just like mic positions, you may find you have to move the bass traps around to for the best sound. For my room, I am using only 5 panels - 2 in the corners 4inx2ftx6ft, 2 rear wall 4inx2ftx4ft, 1 on ceiling above mics/piano 2inx2ftx4ft.
Below 50-60Hz it's very difficult to treat any room, but most of the music is above that anyway so it can only improve one's situation. You can build better bass traps for absorption in the 60Hz and 120Hz range by placing 1/4in and 1/8in plywood panels, respectively, behind the fiberglass panels, but I think it's over kill, unless you know for sure that there is a peak in the room mode for that frequency range. Before treatment in my room, I was getting a nasty peak @ 1.9kHz (very high B-flat and B-natural), loss of bass definition, and lack of clarity in the midrange. After treating with several bass traps, I get more bass definition, better clarity in the midrange, and don't hear that nasty peak @1.9kHz anymore. The sound going into the mics is more damped, tighter, and controlled across the range. Any loss in room ambiance, I can add reverb more predictably, without amplifying the nasty anomalies.
However, the best way to objectively judge is to use a simple room analysis software that's capable of generating a "waterfall" plot of frequency/time/decibels. Or, You can also use your Avensons as a measurement mic, and plot the frequency response from 100-15000Hz using a $30 SPL meter on your DAW software. That way you can see which frequencies are being affected and by how much for your room.
The next 3 weeks are very busy for me, so I unfortunately won't be practicing, playing, or recording much at all. I'll try to make some comparative recordings under controlled conditions, with and without traps soon after that.
If you want I can post pictures of the bass traps in the meantime?...
Let me know if you decide to make your own, I can give more details?...