Centre Channel Speaker
"In the same way as the central speaker, the Surround speakers should be chosen from the same manufacturer as those of the main and centre channel speakers. There are three specific types of Surround speakers, corresponding to different profiles and uses. In the best case scenario, they should be made up of a second pair of main speakers, or at least a pair of hi-fi speakers from the same model range as the main ones, but in a smaller version ("bookshelf" speakers).
The first specific Surround speakers that made an appearance on the market were in fact small, white, very flat, as discreet as possible, and weren’t very expensive. They were simply placed in a high position behind the listener. They weren’t particularly coherent with the speakers you had in front of you, and with good reason, as they were of a different brand. This didn’t pose too much of a problem in as much as the system of sound encoding used at the time was Dolby Surround, which only uses a limited register of mid-tones and upper frequencies in mono in the rear section. It thus was not badly adapted to non-coherent speakers. The advantage was at least to be able to bring Home Cinema more easily into people’s homes without frightening the dog or the neighbours. This kind of speaker still exists and is well adapted to more modest installations.
The second generation of Surround speakers was marked by speakers that were coherent with the principal ones, that is to say they had the same speaker-cones, only smaller. These speakers were thus bigger than their ancestors, with results that were equally more convincing. This became more important when Dolby Digital appeared, which doesn’t work well with mediocre speakers, as it uses the full frequency range from bass to treble in stereo in the rear speakers.
The third generation of Surround speakers was developed at the initiative of the THX programme launched by George LUCAS himself, starting from a simple observation: in a cinema, the Surround speakers are placed very high above the spectators, so it is hard to localise them. In addition, the sound of the rear channels should be sufficiently well mixed so as not to distract the attention of the spectator from the screen. In the home, it’s exactly the opposite. You have just one pair of rear speakers placed quite close to you… sometimes less than one metre. It is therefore evident that this proximity could be a problem when the sound effects are too spectacular. It’s for this reason that you should slightly reduce the volume of the Surround speakers if they are placed very close to where your "audience" is sitting. Another alternative consists of placing them in such a way that they don’t radiate directly towards you, by turning them so the sound bounces firstly off the back or side walls. If your room is sufficiently large to be able to do this, it’s a plus, because nothing beats a real pair of decently sized hi-fi speakers in the back of the room. In the opposite situation, the concept speakers developed by the THX programme are extremely ingenious, allowing you to re-create the Surround sound atmosphere of a real cinema. This Surround speaker is called a Dipolar speaker, because it possesses two opposite "poles" of dispersion. More clearly, it’s a double-sided speaker, with the two sides being positioned in such a way as to create a kind of sonic "magma" over your head, becoming consequently much more difficult to localise. "