Depth is an interesting subject when it comes to audio systems. Properly setup your loudspeakers should be able to disappear and provide good front to back depth, meaning the sound should go from the front plane of the speakers to the back of the room behind the speakers.
The amount of depth depends on the program material. The closer the performer is to the microphone the closer to the front the sound will appear and when it's really closely mic'd it'll sound like she's right in the speaker.
Because depth is relative, the loudspeakers usually act as the reference for how far the actual sound goes back. Of course this is all an illusion, but none the less, I have noticed that some recordings use what I would refer to as fixed markers to establish and extend the image depth. Those markers are performers and when placed properly, the illusion of depth can be uncanny.
A good example of this can be found by dusting off a copy of an old Reference Recording: Red Norvo the Forward Look. This is a great three track recording made in 1957 by the (then) unknown Keith Johnson on all vacuum tube electronics. On most systems this sounds like a fairly distant recording, where the image is pushed way to the back of the stage, but on several tracks (track 2 is a good example) the depth seems to actually increase when a foreground instrument starts playing. This appears to allow you to hear deeper into the stage, which, of course, isn't correct since nothing in the recording is actually changing but your perspective.
I think this is similar to a technique used in photography to help dramatize depth in a two-dimensional image: a
foreground anchor. Most people have a really tough time with their cameras showing depth. You've probably run into this problem yourself with a camera. Before lies a great scene that to your eye stretches off into the distance and you try and capture that magnificent depth. When you view the photograph later you realize the depth you experienced live is gone forever.
The technique of capturing depth is actually rather easy: the photograph simply needs a point of reference. Take a look at this photograph from
Gary Luhm,an excellent nature photographer, and note the terrific depth it achieves.
Note how the rock with the leaf on it is the foreground anchor. This gives the eye a reference to gauge the depth of the surrounding scene - which is what you actually use in real life to gauge depth - but most people don't get in their photographs. It's an easy technique if you can remember it.
Here's a simple cropped version of the same picture without the foreground anchor to show you the difference.
Note how little depth this has relative to the first.
This is a good example of how we benefit by being aware of the required elements needed to make a convincing three dimensional illusion from a two dimensional medium.
Photography, as well as stereo, are both two dimensional mediums trying to mimic three dimensionalevents.
Getting the depth to sound real is an art in both disciplines.
