|I believe 6.1 require two rear speakers (in addition to the two side speakers of 5.1) to create an enveloping sound field. The difference with 7.1 is that the two rears become separate channels.|
Correct: 6.1-channel and 7.1-channel systems are both
run on 7.1-speaker set-ups. (Extra points for not confusing number of channels
with number of speakers
|how noticeable is the difference between 5.1 and 6.1/7.1? Is it a substantial one or is it fairly minor?|
Depends on several factors: the room, the speaker layout, your hearing, listening material, etc.
But even with less-than-ideal circumstances, there are three thing you should notice when going from 5.1 to 7.1 playback: better localization
(4 speakers allow more distinct placement of surround content than just 2 speakers); greater envelopment
(4 speakers can 'surround' you better than 2 speakers can); and increased stability
of imaging in the surround field (no matter where you sit in the listening area, sounds at your sides and sounds behind you stay locked to those directions because there are speakers physically placed at those locations).
It's difficult, if not impossible, to get all that from only 2 surround speakers. Even if you have the 2 best surround speakers in the world, they can't be in two places at once.
|for a 6.1/7.1 system to be most effective, the rear speakers should be a few feet behind the listeners, with the surround speakers on either side?|
That would be ideal. In a 5.1-speaker set-up, it helps to have the surrounds at your sides but rearward of the listening area. This helps strike a decent compromise between side and rear imaging in the surround field. This compromise isn't necessary with a 7.1 set-up because you have speakers at your sides and behind you.
Our ear/brain mechanism creates phantom imaging by comparing the imput from both ears. This works best up front, where our human hearing is most acute. But it is weakest directly to our sides, where we're basically using a single ear (the ear on the other side of your head isn't helping much). So it really does help to have a pair of speakers at that location: directly to our sides.
Of course, this leaves a big hole in the surround field behind us. The easiest way to solve that problem is to simply place speakers behind us. These sounds should actually sound like they're coming from behind, not above or some other direction, so it helps to have some distance between the seating area and the rear wall; even if it's (as you said) a few feet.