Originally Posted by Danboy24
I was thinking about extending by just 2.00M so I end up with a room 6.68M wide by 5.58M deep. Do you think this will be OK acoustically?
Yes, there is over 1M of difference between width & length to keep it from being square.
Originally Posted by Danboy24
I know a bit about AV but clearly not enough as I have no idea what the various Modes are that you mention or what the web page is actually showing me
If you blow across an empty bottle, you can get the air in there to resonate (make that booooh sound). If you enlarge the dimensions of that small chamber to the size of your room, the air in there will still resonate (of course at different frequencies than the bottle). These resonances are called room modes or standing waves. We're mostly concerned with room modes occurring in the low frequencies (in the bass range). Modes result in peaks & nulls at fixed locations throughout the room, which means different listeners might hear wildly different bass response depending on where they're sitting.
For example, your 6.68M width will result in modes at 26Hz, 52Hz, 77Hz, 103Hz, etc. The first 4 of these are mapped on the graph below. Each problem frequency is colour coded, so you can see where its peaks & nulls fall across the width of your room.
You can cancel the first 3 of these width modes (26Hz, 52Hz, 77Hz) simply by placing a pair of subs at the ¼ and ¾ points of room width, thereby greatly minimizing their peaks & nulls. Listeners sitting across the width of the room will hear much more consistent bass response from seat to seat.
The problem with having a square room is that you have the same dimensions for width & length, which will result in width modes that are the same as length modes. So you place your subs as described above to get rid of peaks & nulls at 26Hz, 52Hz and 77Hz. When you go to measure, you find out that you still have problems at 26Hz, 52Hz and 77Hz. How can that be when you just got rid of them? Turns out the subwoofer placement got rid of the width modes but not the length modes, which are at the same frequency.
So, room designers prefer to have room dimensions that are different enough to have width modes and length modes separated (not at the exact same frequency). Enlarging your room length by 2M to 5.58M will result in modes at 31Hz, 62Hz, 92Hz, 123Hz, etc. You can see these on the graph below. Again, each problem frequency is colour coded, so you can see where its peaks & nulls fall along the length of your room.
Notice that the problem frequencies across the width of your room are separated enough from the problem frequencies along the length of your room that they won't overlap.
Also notice in the graph above that the midpoint of room length is the worst place to sit (acoustically), since the problem frequencies are either loud peaks or deep nulls (no moderation). By comparison, sitting at 2/3 back from the front wall has most of the problem frequencies at roughly the same level.
The extra room length will be helpful and won't make the acoustics worse. So do it.