That element is the very space itself, the environment in which the sound occurs. Mono recording captures sounds mixed into a single audio channel, and mono playback systems render a very limited single-channel representation of sound in an acoustic space. The advent of stereo recording and playback was the first step in more accurately recreating the real audio field for listeners. Stereo playback locates sonic events in a relatively flat field between left and right speakers, and audio trajectories and locations are plotted along the x and y axes of a two-dimensional acoustic space. However, as noted earlier, the real auditory field is a 360-degree space, with sound not only in your front left and right areas, but above, below, and behind you as well.
Common stereo lacks fully realistic acoustic presence.
The use of a subwoofer helps dissociate bass frequencies from a fixed location, but the movement of the sound is still relatively static. 3DSE is a signal-processing effect designed to create the illusion of an extended stereo space. It is not true 3D audio, but it can have a striking effect on common stereo systems. These technologies would seem to resolve the issue of creating realistic sound spaces, but they have disadvantages even compared to stereo. For casual listeners, stereo works just fine, and stereo is the common audio spatialization for media as diverse as broadcast radio and television, VHS videotape, MP3 audio files, CDs and DVDs, and commercial cassette tape.