Envelopment requires a center channel instead of a symmetric front-back array.

Evidence

One of the prime objectives of multichannel audio is to deliver envelopment, the impression of being in a different acoustical space. As was shown in Section 7.2, adding two channels in a symmetrical front/back array, as was done here, has very little positive effect. A different arrangement is needed, with a center channel for movies. — Toole, Floyd E, Sound reproduction- the acoustics and…, 759

Categorization

• DEFINE: Defines ‘envelopment’ as the impression of being in a different acoustical space, the core objective of multichannel audio.