I've been reading about the importance of room treatment. I'm sure that it would improve the sound quality but by how much? They're so expensive and I'm not much of DIY guy. I did the "hand clap" test, I didn't hear any echo but I did hear a slight ringing sound almost immediately after I clapped my hand. Not sure how much the sound would improve if I install absorber, diffuser panels and bass trap. It sounded good as it is right now. I can hear dialog and voices clearly (not muddy). I have big drapes on one of the side wall (it's actually a big window ), big bookshelf on one of the side wall behind the listening area, poster frames around the walls.

How much improvement would I get if I put those expensive room treatment? Is it really noticable? My room size is 16x13 btw

The benefit you get depends on how good or bad your room is to begin with. Some people are fortunate to have really good rooms from day 1, ie they made lucky or appropriate choices in dimensions, floor and wall finishings, drapes, bookshelves, wall hangings, furniture and choosing flabby, sound-absorbent friends

If your room is pretty good already, and it sounds like it might be, you won't see the same obvious improvements from randomly adding room treatments that you would get in one of those "rooms from hell" that some people end up starting with. You need to be much more selective with the treatments you choose in order to get value from the investment.

I wouldn't rush into room treatments if I were in your situation. First thing to look at would be clarity of imaging -- you probably have much more absorbency on one side of the room than the other courtesy of the drapes -- but if your imaging seems good (play some recordings using just the mains and see how clearly the different instruments can be located between the speakers) then I would just kick back and enjoy.

You can also make things sound worse. You might want to check the Axiom Technical advice articles from Alan on the home page, at one point he addressed treatments and the pros and cons.

Try these:

http://www.musicplayer.com/ubb/ultimatebb.php?ubb=forum;f=26

This one is IB sub specific, but has some cross refs and links to acoustics and eq procedures:

http://ibsubwoofers.proboards51.com/index.cgi