Hello tkingen and welcome to the board. You have a few problems that are difficult to diagnose from our vantage point. But I'll give it a shot. But right off the bat, with only this information to go on, it is purely a guess. OK?
It is the room size in combination with carpet and comfortors, adjacent to low frequency(rumble)producing vehicles. The only freqency range left in the room is the sound of traffic rumble. You don't hear it as much as feel it, because it is low frequency, as your "treatments" absorb the mid and high frequency. This is exactly why small room acoustics are so difficult to treat. Let alone with rumble producing traffic nearby. The room also has dimensions which create "modes" that cancel a range that you are also sucking up, which leaves very little of in that band of frequencys, and then accentuates other frequencys which contributes to the "muddiness" of the sound that does exist. To treat properly, you are looking at significant soundproofing, and then correct acoustical treatment, of which translates into loss of floor space. That is a very simplistic answer, and the solutions are not.
There is only one way to ELIMINATE this type of sound transmission. That is to "float" a room WITHIN the existing room. There are ways to REDUCE the sound transmission into the room, but you will NOT eliminate it with these other methods. However, if you do not eliminate this type of sound intrusion, your engineering efforts will constantly be compromised. And since this room is only 10' x 12' your situation is difficult in the least, as floating a room, requires just what it implies. Building a complete room within the existing room. Floor, walls, and ceiling structures, built as a unit, floated on neoprene pads, with doors and jambs of equal STC(sound transmission class). And THEN, how do you breath. That is the next problem as soundproof equals airproof. I hate to rain on anyones parade, however, it is best to know the truth from the outset. At least from my perspective, cause I HATE wasting my money and time.
Let me add this though. This intirely depends on your prioritys when recording and the TYPE of recording. With microphones, and the existing "treatment" you will end up with "muddy" recordings. You will NOT be able to EQ it out of the recordings though. Because when you play them back, the room does it AGAIN. So how do you know if it is the recording or the room. Get my drift? And thats just the beginning. When you take the recording into ANOTHER room, and system, it doesn't TRANSLATE. In other words, the new room adds its own "character" to the existing "color" which now sounds nothing like what you heard in the first place. And NO amount of "treatment" will eliminate this sort of sound intrusion, that is IF I am interpreting your description of the culprit in question correctly.
Now, you CAN do things like frame in the sliding glass door and sheetrock it flush with the existing sheetrock, and then ADD another layer to the whole room as MASS is what HELPS sound transmission attenuation. But then, structural transmission of vibration by way of the existing structure, may cause this additional mass to be rendered useless. Hence floating a room.
I know my reply is disheartning, but these are the sort of things everyone here deals with one way or another, when building a "studio" under these circumstances. Small rooms, adjacent to noise producing agents are the WORST. I would seriously consider defining EXACTLY what it is you want to accomplish in your "studio", and then ask how you can reach these goals. If your looking to record perfect acoustical and vocal performance recordings, this will take the most serious approch to construction and treatment, and may leave a space slightly bigger than a vocal booth and very little room for equipment. If on the other hand, you only want to do demo level midi, computer generated or direct to media type recording whereby no mic's are used, then the existing room as is will work, except for monitoring through speakers. Then you need acoustical treatment of the space, which I will not go into at this point as it will only confuse the situation. Also, monitoring a performance in the same space as it is being recorded, now creates another set of problems. Mainly, you have to do it in headphones. This has one advantage in that you only treat for reflection into the mics, not from the speakers themself. But then how do you correctly percieve how the recordings sound AS YOU RECORD. Headphones do NOT truthfully replicate what is being recorded. Yes, they may sound good, but upon playback, through a set of speakers, in the room as it exists, will not tell you the truth.
So, now that I have illustrated the problems, maybe you can tell us more of what you want to do in your studio. MUCH more information is needed to properly give the help you need. Also, I'm not the only one here with opinions and solutions, so don't take my word as gospel, as my disclaimer reads "NOT AN EXPERT". But even the people who are need more information. That being said, come on back and tell us eveything you can about the conditions that exist, your expectations, type of recording, etc, and I am sure that you will find that people are willing to share their experienc and knowledge in regards to solutions, but they MUST have info to do so. Well, thats my .02. Hope it helps enlighten you to the problems you face without distorting them. Studio construction is not "easy" nor cheap if you are after perfection. However, Homerecording seldom aspires to perfection. Usually only fun. It is to what extent your aspirations dictate your goals that is in question.
fitZ
