So where does the sense of vertical come from?

Paul McGowan writes:

We discussed the effects of depth of soundstage, characterized by some as “layered depth” to describe the spacing in the horizontal plane that we hear in our high end audio systems.  I mentioned that there is yet another dimension to the depth aspect of our systems, that of height.

The height of the soundstage is quite evident in many recordings, in particular live indoor recordings give a sense of height that is many times equal to that of width and depth.  But how is this accomplished?  We can easily understand how a stereoscopic image is created in our systems because of the two sound sources: left and right.  Most of us have seen the amazing depth afforded by a 3D stereoscopic viewer – and we understand this happens because of the slight shift in the image perspective between the left eye and the right eye.  Our brains assemble the two differing images into what we perceive as rounded three dimensional objects, based on two slightly different 2D views of a scene.  It isn’t really 3-dimensional it is, as one of my readers suggested yesterday, a pleasing illusion of reality.

However, two discrete left and right sources of sound don’t account for height information, now do they?  They handle width and depth, but since we don’t also have a top and bottom loudspeaker, how in the world can we hear differences in height?  On my system I can easily hear the stepped risers of a choral group and it’s rather obvious that some singers are higher than others – yet there is apparently no information that gives me this sense.

Our unofficial technical editor, Soundminded (aka Mark Fisher), gives us the clues we’re looking for in one of his comments and I find this both illuminating and obvious – although obvious only after you hear the answer:

“So where does the sense of vertical come from? From reflections. We perceive the height of the ceiling just as we perceive the width and depth of a room from reflections.  Without reflections, as sounds come from farther away they don’t sound distant or remote, they just sound fainter. Somewhere in our long ago evolutionary past earlier life forms we came from must have been cave dwellers.”