Layers of depth

One of my readers, Alan, recently asked a really good question. "Reviewers of high resolution sound reproduction equipment frequently speak of one element of the reproduction, depth. But they speak of layers of depth; I've never really understood that concept, because in real life I always hear a continuum of depth." I am sure we've all read these terms and frankly I never gave them a second notice. But, Alan's correct, what does the term layers mean? Are recordings produced in specific layers, perhaps divided up into discrete zones by the recording engineer? I think in all our experiences live sound isn't in layers and certainly continuous in its depth. But our systems? The quick answer is that I haven't any clue what the reviewers are saying when they refer to depth. When I listen to the system all the music is presented in rather seamless fashion from the front of the loudspeakers all the way to the room's rear wall - and all the depth between those two planes sounds continuous to me. The exceptions to this might be with a very closely mic'd recording where the depth of soundstage is non-existent and everything seems to sound like it is in a "layer" up front and firmly attached to the speakers. But as a description of actual soundstage depth, the only thing I can think of is it must be a term that got coined somehow and remains in use as a literary crutch to try and use words to explain depth. If you think about it, from a writer's viewpoint, it is a difficult task to use words to explain the idea of depth. Imagine for a moment you've never heard soundstage or depth on a pair of loudspeakers - and let's face it - most people haven't ever heard that. How would you, as a writer, describe this phenomena? When someone new to audio comes to visit at PS we routinely take them into the big sound room to play something for them. Part of my spiel is to show them where the sound comes from and then walk behind the speakers to explain to them that what they will hear seems not to come from the loudspeakers at all - but rather behind the speakers. I prep people for what they are going to hear for a couple of reasons: first, because it's a totally new and sometimes overwhelming experience and because it avoids the inevitable questions afterwards of "where are the speakers and what are those big things I see in the room?" Because the sound is completely disembodied from the speakers, and appears in an imaginary space between the speakers and wall, it's somewhat disconcerting to people new to listening on a high-end setup. Unless a reviewer who uses that term adds a comment explaining something I am missing, I am going to mark the question as that of a simple tried and true term that probably should be revised someday. Thanks for asking.