How well your front L/R speakers are able to create a convincing "phantom" center image is dependant upon a number of things - including the dispersion pattern of the speakers, the room reflections off of side walls, ceiling and floor, and your seating position and its proximity to the wall behind you and to the side walls.
In general, if a speaker maintains flat frequency response, even at off-angles, it will be better able to produce a "phantom" center image over a larger seating area. Typically, the goal is to maintain flat frequency response over a 30 degree "window" - that is, 15 degrees to either side of dead center.
Therefore, the typical goal is to have all listeners' ears within that 30 degree "window". What you will normally find if you try to achieve that goal is that you have to toe in the speakers so that the person sitting on the right couch cushion has his/her right ear just within the 15 degrees-to-the-right portion of the Front R speaker's dispersion, while the person on the left couch cushion has his/her left ear just within the 15-degree-to-the-left portion of the Front R speaker's dispersion. The person in the middle couch cushion is going to thus have the Front R speaker pointed almost right at him/her.
Now, the thing is, a lot of speakers do NOT have flat frequency response at any angle other than dead on. Many speakers immediately begin to roll-off as soon as you go off angle. And many speakers also have diffraction problems, where the soundwaves hit the front side edge of the speaker cabinet and bounce off at weird angles. This is why you'll see speakers where the sides of the front baffle are rounded or chamfered or bevelled.
Certain tweeter designs tend to only have flat frequency response when dead on. Ribbons, electrostats, ring-radiators, some horn designs. With those designs, you tend to have to have the speakers pointed straight at you, creating a tiny "sweet spot" that only a single listener can enjoy.
On the opposite end of the spectrum, some speakers have extremely wide and even dispersion. With such speakers, you don't have to toe them in very much at all because they maintain a flat and even frequency response over a very wide "window". Sometimes as much as 45 degrees to either side of dead on!
It sounds as though whatever speaker you own have very limited dispersion. If you're finding that they only create a convincing center image when pointed straight at you and they are unable to maintain that image when you move even a hair out of that one "sweet spot", then your speakers likely have uneven frequency response at off-angles and/or diffraction problems.
That said, it could also easily be your room. If your side walls are flat and hard, and if you ceiling and floor are flat and hard too, then you are getting some very strong reflections. If you are also sitting fairly close to the wall behind you and that back wall is flat and hard, then you are getting strong reflections off of the back wall as well.
Those reflections interfere with the direct sound that is coming from your speakers. That interference can destroy a speaker's imaging and can make dialogue in particular a lot harder to clearly understand. What you would need in that case is some acoustic treatments.
The basics are as follows:
- place bass traps in all four vertical corners of your room. Focus on the wall-wall-ceiling corners. Don't worry too much about treating areas that are below your seated ear height.
- place bass trap panels directly behind your front speakers and directly on the side wall beside your front speakers
- *most importantly - place absorptive panels at the first reflection points on your side walls and ceiling. If you have a fairly long seating distance, also place absorptive panels at the second reflection points.
- place an absorptive panel directly behind your seating position on the back wall.
- treat the back half of your room (side walls and ceiling) with diffusion panels.
The idea is for the sound coming from the front of the room to be direct from your speakers and free from most of the reflections off of your room's surfaces. You want that sound from the front to "wash over" you and then "disappear" behind you - hence the absorption placed directly behind you.
You do not want a totally "dead" sounding room though, so that is where the diffusion comes in. In the rear half of the room, you want to "scatter" the sound so that you maintain the sound energy and some of the sonic signature of the room, but you do not get strong, direct reflections that will "muddy" your soundstage and interfere with the direct sound coming from your speakers.
You likely have a combination of both room reflections and less than even dispersion from your speakers. But I do not know what speakers you are using, so it's tough to say for sure.
If you use something like electrostats, they can basically sound like headphones - with incredible detail and resolution, but only with them pointing straight at you and only for an extremely small "sweet spot". If your speakers merely roll-off at off-angles, you have a choice: you can point them straight at you to glean maximum detail and resolution, but a small sweet spot. Or you can barely toe them in at all, or even face them completely straight forward. You'll gain a much larger sweet spot, but have less resolution and detail.
Hope that helps!
