The stereo system leaves a "hole" in the middle of the sound field, which is filled in virtually, in that a ghost image positioned in the middle is formed. This takes place because the same sound comes from both the right-hand side and left-hand side speakers. Hence the reason why the sound that is obtained when the two are combined is perceived as coming from the middle. This solution is however flawed, in that the ghost image tends to move depending on the listener's position. It is rather obvious therefore that in a cinema hall every audience member perceives the central sound in a different way. In particular the central sound varies for everyone apart from those sitting directly along the central axis of the cinema, perpendicular to the screen. To make up for this shortcoming, a central independent channel was introduced. With this setup film dialogues are sent to the central loudspeaker whereas the music (in stereo) and sound effects (i.e. the telephone ringing in the off-screen room next-door) are sent the lateral speakers. In order to add an extra environment sound, one more speaker was set up, this time right behind the spectators (either mono or stereo depending on the systems being used). Yet another signal is present on some systems in order to reproduce very low frequencies and is connected to a sub-woofer (LFE - Low Frequency Emitter) [Types of loudspeakers ] ; one of the most common usages of such signals can be found in apocalyptic explosions in some American movies when it feels like the whole cinema is about to crumble down.
The following table illustrates various types of cinema Surround Systems and their characteristics: