Dolby Laboratories Pro Logic IIz
Nine speakers may seem like overkill, but Dolby's Pro Logic IIz fulfills the promise of surround sound: audio that envelops you from all over, not just the sides. The new technology for AV receivers pipes sound to the usual center, side and back channels, plus two extra ones positioned three feet above the main left and right speakers, adding a vertical dimension to the surround sound. In existing soundtracks, the system recognizes nondirectional sounds like falling rain and sends them to the new speakers. But audio engineers and videogame designers can also create sound channels specifically for the speakers, more accurately placing sounds like a helicopter coming in for a landing or a sniper on a roof. And it's not unaffordable: The first receiver to incorporate the tech is the $650 Onkyo TX-SR607.
I've been listening to six-channel ambience synthesis for more than 20 years. I never noticed that it didn't surround me.
"Envelopment" depends more on how directionality is encoded and presented. With the "right" system (such as Ambisonics) you don't need a huge number of speakers.