Room size is mainly relevant only when other variables are known. For example, a common room layout(regardless of size) is to have the seating positions 12-15ft away from the wall with the screen(video). The subwoofer will often be placed in/near the front corner closest to the key seating. Let's say 5 meters from the seats on average. At this point, the room size almost becomes a non factor. You have a couple of considerations though.
a) PVG (room gain). The smaller the effective room area the more you get. But even larger rooms will show a bit unless you get into auditorium sizes. It may seem counter intuitive to some but if the 6000 cu-ft is somewhat enclosed I'd bet you'll still see a little PVG starting around 35hz. 1-2dB by 30, 2-3dB by 25hz, 3-4dB by 20hz, etc.
b)in a smaller room the seating positions could be up against a boundary/wall. In a 6000 cu-ft room, unlikely unless the system was oriented "sideways". Seats against the wall have pro/con. The pro is you'll tend to get more overall bass volume. The con is you will also tend to have more room induced variations in the frequency response.
But otherwise, the subwoofer is going to provide fairly similar performance to the seating positions regardless if the room is 3000,6000,9000 cu-ft.
There is a very good article on this site about subwoofer room size ratings and how they formulate it. There's not too many variables to consider here(starting with the outside 2m CEA stuff). Boundary effect(is it corner loaded?), distance between sub and key seats(remember the inverse-sq won't apply in room), and PVG(expected room gain).
This isn't to say room size is completely irrelevant of course, it is one of my first questions when someone email/phones in with a "what should I get" inquiry. With a smaller(say 2000 cu-ft or less) room the subwoofer will be close to the seats(within 3-4 meters) barring some very unusual room dimensions. In a 6000 cu-ft or larger room the subwoofer-to-listener distance may still be similar to that. Or, it may be MUCH further. This is really the key. Then just find out how low/loud is required and you're finished..
Tom V.
Power Sound Audio
