How small of a budget are we talking? You might want to check out some of the DIY stuff on the Studio Building / Acoustics forum.
As for what the minimum you can do is, it really depends on what exactly you're trying to achieve. You're probably going to have to be a little more specific... Otherwise, you're just going to get the standard answers:
More Bass Traps
More Bass Traps
DIY Bass Traps
Don't listen to offki, what you need to do is knock a couple of walls down. Then rebuild them at an angle. Ditto ceiling. :-) What you need is spectrum analysis.
Small rooms are deprecated; besides which, they won't support an extended bass response. (Saves on a sub-woofer?). In a small room the fundamental resonant modes are more likely to fall awkwardly closer together in frequency than in a large room. This could make trapping them simpler (e.g. one trap may serve two modes) - or not - but you need an accurate acoustic picture before you start working on them. Also with a small room, expect at least one pug ugly flutter echo somewhere in the midband.
Buy a dozen 4x2" semi-rigid, acoustic mineral wool boards. Owens Corning 703 in the US. Rockwool RW3 or Knauf Ecosse acoustic board in the UK.
Try to break up any straight lines and large flat spaces wherever you see them. Put several boards across corners between walls, and/or the angles between ceiling and wall. Hang the rest on the walls. Maybe also on the ceiling if you can.
I've heard placing some foam behind them would be great, placing some foam up in the corners of the walls.
Say "NO" to foam!! Seriously.
Proper bass traps and broadband absorbers are made from mineral wool or rigid fiberglass insulation. Foam will knock down some of your high end and flutter echo, but leave you with the worst problem in a small room - the low end. Typical acoustic treatment panels are non-invasive to the structure (just like hanging a picture), and are easy to take with you when you move.
PLEASE, spend some time in the studio acoustics section and educate yourself on what's involved with PROPER acoustic treatment.
Money spent on decent acoustic treatment will be WAY more productive to your music-making than any other piece of gear you can buy. It is seriously the best investment you can make.
A couple of bass traps, and acoustic panels...probably about $250 worth from an inexpensive company like ATS. Their mounting system is fantastic and doesn't make or leave any permanent damage other than what you'd normally have to deal with when hanging a few pictures.
And a few panels of plyboard you can throw out on the floor when you track acoustic instruments to create some lively reflections. IMO on a carpeted floor in a small room a few panels of ply go a long way.
When you mix in a small room it's nice if you can leave the door open to help bass freq's escape and leave the closet door cracked.
Working within a converted den myself, I suggest a bulldozer. It is very frustrating to try to get a coherent sound in a small room. If you bother to pay attention to the frequency response charts of the various sound-control products it is not hard to see what works and what doesn't. You are going to have to give up some space to sound control if you are serious. If you need to DIY because you have no budget then you need to learn what it takes to DIY, or just put up anything that you think looks cool and doesn't cost much, and call it done. You cannot change the laws of physics no matter what some sales guy told you.
On a related note, someone announced yesterday that they have developed a 'diode' for sound control, which allows sound to flow one direction but not the other. It will probably take 15 or 20 years to get a viable and affordable product to market, but it is something to watch out for.
What I have done is to put a Real Traps Room Kit in place. These can be hung like paintings, so "damage" is minimal to non-existent. I filled the picture hook holes with spackle when I moved from my apartment to this place. I've also built a frame that hangs from hooks in the ceiling (toggle bolts... unscrew them and spackle over the holes.) behind my head to hold a pair of 1 inch RealTraps to prevent flutter echoes from coming at me from behind. I only hang these up when I'm mixing. I'm still not happy, this is NOTHING like my studio, but it is definitely more than a 15% improvement over the untreated room. I'm still working on it though.
Now what do I need to buy and where do I put it for STAGE 1 of acoustic treatment that doesn't leave permanent damage and I could bring with me to the next place I live?
Build a wooden skeleton frame inside the room and mount acoustic boards (703 or RW3) on that.
Placement as I mentioned previously. A few panels placed across corners is quite important. The variable depth behind the board means they absorb a range of bass frequencies. You can probably just lean some against the wall across some of the corners of the room.
This shouldn't cost a lot if you do it yourself. In the UK I could make ten 4'x2' panels for about £5 each, maybe less.
I have been tracking in that room for a long time.
Haven't done any 'real' treatment. I hang blankets on strings at various places....at various times....for various effect. I have stuff in the corners...guitar cases, rolls of carpet.....the studio couch helps a lot.
The most important thing you can do if you can't change your room is to get to know and understand it. Move around with your vox mic and listen carefully to what the room does....there is bound to be a sweet spot of some sort.
EDIT: After re-reading all of the excellent comments...for monitoring purpose, I gave up on my tiny room long ago and went to a very good set of cans for mixing. Took me a while and a lot of referencing, but I have great confidence in them now. The only time I use my monitors is to crank the mix and step outside the room
Those GIK units look pretty decent for the price. Anybody use these with good results?
There's LOTS of people on GS who use the GIK products with great results. GIK makes great stuff at reasonable prices.
As an alternative, panel traps similar to the GIK products can be built relatively easily (if you're a DIY type of person) and inexpensively. Please consult the studio acoustics section of this forum for more info.
Sennheiser HD800s with the Bottlehead Crack/Speedball amp, and put a Corda Crossfeed circuit in the Crack. That'll hold you through the construction of traps, gobos, and holes in the wall... and you may decide it's all you need.
Nope. Altogether there are three modes of standing wave: those involving reflections between two walls, those involving reflections off four walls, and those involving all six walls. Considering the first type, realigning one wall will treat only one of the three standing waves and its harmonics. Obviously it will also affect the other modes too, but given that the first mode already gives you three fundamentals and their harmonics, the full situation rapidly gets beyond intuition.
So, in addition to spectrum analysis, a software room simulator is a good thing too.
As I play with these, I'm beginning to think that the 800s with the Corda circuit dialed-in right can come close to what I hear with good nearfields. It's not the same of course, but it's translatable - certainly worth playing with more. The 800s have less of the "inside your head" thing going on by themselves, and with the Corda it's almost like listening to a good image in front of you.
Rystro, will you be recording in this room as well? Because if you are, I strongly suggest you make acoustic panels with rock wool on the one side (absorbing) and wood on the other (reflecting), so you can flip them over if need be.
Recording acoustic guitar or vocals in a room with no top end is not a good idea.
It took me a couple of days to build them, but I'm very glad I have the luxury of flippable acoustic panels instead of being stuck with a dead sounding room.
Other than that, you can make some cheap ceiling clouds by filling old pillowcases with rock wool and nailing/stapling them to the ceiling. Nothing a new coat of paint won't fix afterwards...
The KRK ERGO system works quite nicely. We spent about CAN$590 on it. You haven't answered the all important question posed by JinMaikeul (2nd post). that is.... "How small of a budget are we talking? " What is "small" for one may be out of reach for others.
OK, here is a distillation of what I think would be helpful to the OP:
1. Rectangular (almost square) rooms are tough; they breed standing waves, which screw with the accuracy of the playback, particularly in specific bass and low-mid frequencies, which makes it very, very tough to mix in a reliable way. The corners are the biggest culprits, so treat all 4 corners with bass traps (block the door with a trap while you are in the room, move it out of the way when you need to use the door). You can build them out of Owens Corning 703, some wood, some fabric, and a staple gun. Read the section of GS that deals with the various ways of building these things. I like triangular shaped chunks that sit in the corners, others like regular panels that straddle the corners. Either one will give you an improvement.
2. In any room, the first reflection spots on the walls and ceiling are very important places to treat. Again, read the section of GS that deals with how to find the first reflection spots using a hand mirror. Build some absorption panels and put them in the first reflection spots. In a small room like this, you might as well plan on hanging absorbers on every wall.
3. After that, treat as many "intersections" (ceiling to wall, wall to floor) as are still exposed and that your budget will allow. Smaller absorption panels can be built to either fit into or straddle those intersections.
4. Build and hang all of the panels described above in such a way that they are easy to pick up and move around, and to take with you when you leave this space someday.
5. After that, if you want to try to use the IK Multimedia ARC software to fix any remaining problems, some folks (like me) find that helpful, not as a substitute for room treatment, but as a supplement once you have "topped out" on what you can reasonably accomplish with room treatment.
6. Commercially sold foam (like Auralex) is not ideal for your needs, so don't buy it. You can get better results with less expensive DIY panels made out of OC703, as described above.
7. You can always just say "screw it" and mix with headphones. While real monitors in a good room are better than headphones, headphones are not a crazy choice for you, given the limitations of the room you are stuck in. If you go this route, make darn sure to buy great (not just good) headphones, choose an open back design, use at least a very good headphone amp, and consider using either crossfeed software or hardware that has a crossfeed feature (like the SPL Phonitor or 2Control). If you go "top drawer" on everything, headphones may actually give you a better, more reliable mixing environment than your little room will ever allow with real monitors. It also yields much more neighbor goodwill than loud monitors would. Something to ponder.