Hi Charlie,
The first thing I would do in that room, is pull out the carpet! There's two major issues with that: First, carpet does the exact opposite of what the room needs, acoustically, and secondly it is on the floor, which prevents your ears and brain from correctly perceiving the real acoustics of the room (psycho-acoustic response).
In more detail: Carpet absorbs high frequencies very well, mids to a certain extent but randomly and rising with respect to frequency, then does absolutely nothing at all to low frequencies. That's the opposite of what a small room needs. All small rooms need huge amounts of low frequency absorption, some in the mid-range but falling with respect to frequency, and little to none in the high end. Here's what carpet actually does, according to a test that was done by Riverbank laboratories, one of the most respected acoustic test labs in the world:
carpet-absorption-spectrum-RVBK-S01.jpg
That's for typical 1/2" thick pile carpet. Thinner carpet is even worse.
Carpet makes your room sound dull, boomy, thuddy, muddy, lifeless, etc.
Secondly, it is on the floor, which means it destroys the reflections from the floor that your brain relies on to build an "acoustic picture" of the room. All your life, wherever you go, your ears are exactly the same height above the floor, and your brain is very, very accustomed to figuring out the acoustic signature of the room based on the reflections it hears from the floor. If you sit down, your brain recognizes that, and adjusts it's "image" of the room accordingly. It does not use the ceiling or the walls for that, because the distance from your ears to the walls changes all the time, many times per second as you walk around, so the "signature" is not constant or consistent. Ditto for the ceiling, to a lesser extent: ceilings are different heights, and when you walk outdoors, there is no ceiling at all! But there is still a floor, and it is still the same distance from your ears as every other floor.
If you have carpet on the floor, your brain no longer has any reflections to use for this.
So firstly, get rid of the carpet. You'll find it really hard to have a good acoustic setup in a room with thick carpet on the floor. Take a look at some photos of high-end pro studios: how many of those have thick carpet on the floor?
There's a reason for that...
Now for the actual room layout and geometry:
Orient your room so it is facing towards the wall at the bottom of your sketch, marked "2.75m". That's the only orientation that makes any sense for that room.
Set up your speakers on stands such that the acoustic axis of each speakers is 1.2m above the floor, or maybe a little higher. Up to 1.25 is fine, 1.3 is borderline. Note that I'm talking about the acoustic axis here! Not the top or bottom of the speaker, not the center of the cabinet, not the center of the woofer cone: the actual acoustic center of the speaker. The manual should tell you where that is, or if not then ask the manufacturer.
The stands must be massive! As in hugely heavy. Some people make them from hollow steel and fill them with sand, others stack up concrete blocks or bricks. That's the type of mass you need. Use Sorbothane pads under the speakers, to decouple them fully from the stands.
For your room, set up the speakers 77cm from the side walls. (Once again, that's the measurement to the acoustic axis, not the side of the cabinet.) That will put the speakers 121 cm apart (acoustic axis to acoustic axis). The speakers must be tight up against the front wall! leave a 10cm gap between the rear corner of the speaker and the front wall. You will put acoustic treatment in that gap shortly.
So the acoustic axes of your speakers are 77cm from the side wall, 10cm from the front wall, 121 cm apart, and 1.2m above the floor.
Now set up your chair on the left/right center-line of the room, such that your ears are 167 cm from the front wall. Set up a vertical pole of some type 197cm from the front wall, on the room center-line (in other words, about 30cm directly behind your head). I often use a mic stand for this, set up vertically. The pole is your aim point...
Angle each of your speakers inwards until the acoustic axis is pointing exactly at your vertical pole. I use a simple laser pointer to do this. Set it on top of the speaker, exactly above the acoustic axis, and exactly perpendicular to the baffle (front surface of the speaker box).
That's it! Now your geometry is correct for that room.
That's the theoretical optimum, but once you get the treatment in, you can fiddle around a bit to see if you can get an even better spot. The best spot will be right there, or very close. Only adjust the speakers slightly closer together of further apart, but do not move them away form the front wall. Also try moving your chair slightly forward/backward and adjusting the speaker angles as needed to maintain the focus 30 cm behind your head. You might find a better spot like that, but it won't be far away. And only do this AFTER the room is treated, not before...
Now for treatment:
- 10cm thick OC-703 in that gap between the speakers and the front wall, wide and tall.
- 60cm superchunks in all room corners
- 10cm OC-703 on all first reflection points (both walls and the ceiling)
- 20cm thick OC-703 across the rest of the back wall, between the superchunks.
Test the room using REW after you have the geometry set up correctly but before you put any treatment in, so you have base-line data, then test again with REW after you put in each treatment panel, to check that they are working as expected, and to see what still needs to be done.
... first impressions on where you would put things if this were your room.
That's the way I would do it, if I were designing that room for one of my customers. Or for me!
- Stuart -