When you can hear it, the illusion is like the difference between night and day. Or an IMAX theatre and an everyday picture house. It is like boosting an anemic garden hose and suddenly experiencing a wall-to-wall water spray.
In my opinion, it takes several factors to fool the ear/brain system into believing that sounds, actually coming from a couple of speakers, really represent a walk-around sound-space.
The ear/brain system includes a complex neural network that effectively operates in the digital domain. When individual hairs inside your cochlear resonate to an incoming frequency, they transmit electrical pulses which indicate the initial timing and frequency. The volume is indicated by the rapidity of repeating pulses. These digital signals feed into a real neural network which 'learns' the meanings of the signals overs a few days and weeks, as individual neurons can disconnect and reconnect in a phenomenon known as brain plasticity. (I sometimes wonder if the break-in period quoted for components and cables is actually the time it takes to reconfigure the listener's neurons!)
The ear/brain system has a much better chance of making sense of the audible world if it is fed clean signals on which to build its processing network. In my opinion, by far the biggest muddlers of the waters are loudspeakers, especially if they have drivers which are widely separated. That's over 99.9% of them, right?
Why is this so? Well, you can correct for time alignment only in one dimension, for example the vertical with D'Appolito driver arrangements. In every other dimension, reflections from walls, floor and ceiling are not time aligned - they arrive in a confusing mess which is why audiophiles with such speakers think the room, and its reflections, are so important.
But if the speaker behaves like a coherent, single point-source of sound, the ear/brain system stands a good chance of making out an illusion of something sensible, like a walk-around soundscape. This is especially true if the listener has attended many live-music events, preferably un-amplified.
In my personal experience, very few speakers have been designed to mimic a point source of sound. (They cannot actually be a point source, because the energy density would rise to infinity at the point!) The speakers I am most familiar with are Quad electrostatics from the ESL-63 and later. Although they have a big flat panel, they are driven to emulate a point source a foot behind the panel. These have been my main speakers for about four decades, plenty of time to tune my personal neural network. They don't have a sweet spot - you can stand up and walk around them and the image stays much the same. When I audition speakers, I usually walk around to see what happens.
The other speakers I am familiar with are KEF speakers also designed to emulate a point-source. Normally when I hear box speakers, I hear a box, but one day I walked past a pair of KEF LS50 speakers and the sound-field felt solid. Later I auditioned their big brother Reference 1 speakers against conventional speakers from Sonus faber, B & W and so on. By comparison, the KEFs just throw a huge, wall-to-wall soundstage. I do not know if other people hear this!
(It is possible that speakers which emulate a single line source are coherent enough but I have not had a chance to listen to them)
I'd better describe my set up. It looks like a decent enough two-channel arrangement with either Quad electrostatic or KEF Reference 1 speakers driven by a class A Krell amplifier, plus an 18" servo-controlled subwoofer. I get the walk-though effect with a sound stage that extends way past the speakers, from any decent two-channel recording on CD or record.
The room is just my living room with no special sound treatment, though the floor is carpeted and there are pretty heavy curtains. A 65" OLED TV sits between and just behind the speakers, but I do not consider this to be a home theatre, although the TV plays though the main speakers. Actually, it can play though additional rear speakers and four height speakers if supported by the program. So I have a 4.4.1 set-up capable of playing 5-channel SACD (but deliberately without a centre speaker) or Pure Audio Blu-ray including Dolby Atmos.
With good source material, this is truly immersive. Classical music has been widely available on SACD for 25 years and I do not think I have a poorly recorded SACD. Norwegian label 2L (2L - the Nordic Sound) have stunning immersive sound delivered on SACD and Pure Audio Blu-ray (DTS and Atmos).
Last week I went to an organ recital at the Sydney Opera House, and in the interval picked up a ten CD set of all Olivier Latry's organ recordings from Deutsche Grammophon. The set had a bonus Blu-ray including multi-channel DTS and Dolby Atmos recorded in Notre Dame, Paris. I have never been to Notre Dame so I don't know what it sounds like in real life, but in Dolby Atmos my entire room became the sound-stage.
