Stereo adds two dimensions to that... Spread and phase. In addition to the depth cues and room character, you get a left to right spread, so the violins are on the left and the cellos on the right. When combined with depth cues, this can create a three dimensional space in front of you. For a vivid demonstration of this, check out John Culshaw's recording of Wagner's Ring with Solti. He set up mikes and laid a grid on the floor and gave the singers blocking. You can hear the actors move around the stage in front of you. The other aspect of stereo recording is phase. Anyone who has heard a Pink Floyd album or a later Beatles record has heard phase effects, where the sound opens up and appears to be coming from weird directions. This is an exaggerated effect, but even in regular recordings, there is out of phase material that seems separate from the rest of the sound. The direction of out of phase sound can't really be controlled. It just sounds separate.
Surround sound takes what Stereo adds and squares it. Instead of having a soundstage that sits in front of you, it's possible to place a specific sound anywhere in the room. Singers can whisper in your ear or walk behind you. Phase also becomes a tremendously powerful tool. The meshing of the speakers around you can create any spacial envelope you want. This means that you aren't limited to just the sonic sound of your living room. By altering the phase, the sound can be opened up to the size of a concert hall. The front channels are still presenting a clear soundstage in front of you, but the sound of the room around you is altered by the rear channels. When a recording combines both the synthetic room ambience with the ability to move the position of the sound all around you, the effect is quite uncanny and can make the hairs on the back of your neck stand up. It's a type of realism you aren't used to hearing in a recording.
Mono sound can be great. So can stereo. But 5:1 can do everything those two do and add a whole new dimension that they can't.
I never thought I would hear a surround rechanneler that I would like, but my new Yamaha amp changed my mind. Yamaha has done a lot of research into creating 5:1 ambiences, and there is a stereo to surround setting that opens up both stereo and mono recordings and makes them fill the room. The soundstage is preserved in front, the violins are stll on the left and cellos on the right, but it adds a sonic envelope that opens up the room and throws out of phase material to the appropriate rear channel to create a dimensional front/back spread. It's very, very good. I use it for all of my music listening now.
The amp also has a variety of ambiences from small club to stadium, but I rarely use those because most recordings have their own hall ambiences. Doubling up the acoustics doesn't work well. However there is one called "Concert Hall in Vienna" that is fantastic for making dry, boxy recordings like late 40s Toscanini sound very good.
I tend to be a sceptical luddite, but when something works, I jump on it. 5:1 is a major breakthrough, and I think that if equipment manufacturers can find a way to make it easier to set up surround systems in people's homes, it will become the standard. With the shift from stereos to home theaters, that may already be happening.
Edited by bigshot - 7/7/12 at 11:08am