The figure in a clearing is a rich musical topic or texture. Something happens occurs before, or against, or within, a background. That background may be somewhat atmospheric or suggestive (musically or extra-musically), but it must also be somewhat neutral, audible but not listened-to. How is such a background made in a work of music?
(1) silence (Cage's rhythmic (metrical), and — later — time bracket (ametrical) structures.
(2) a scattering of small events (Berlioz: "intermittent sounds"), distributed irregularly and unpredictably.
(3) erdodic form (Tenney): completely and randomly filling the available time with events, so that no orientation, temporal or otherwise, on the basis of the background is possible.
(4) events occuring in absolutely regular and preditable intervals of time, providing a strict temporal reference, possibly references for pitch, timbre.
(5) a drone, a continuous sound providing a pitch, but no temporal reference.
(6) a contrapuntally independent stream of music, speech, or environmental sounds.
I love the coincidence here of completely full and completely empty backgrounds. [Is a blank canvas white or black? The complete physical spectrum of light tells us one thing (white is a broad spectrum of light, black is an absence of light) while the real experience of visible light, the colors with which a painter works (with a spectrum that our mind folds back up on itself, connecting reds to blues) suggests the opposite: white is the absence and black is completely full.] This figure usefully focuses on the issue of audibility/inaudibility. While the figure and ground idea is, at root, contrapuntal, the immediate compositional issue is whether or not the listener should be aware of the counterpoint, and this audible/inaudible quality (now you hear it, now you don't) is determined by the material relationship, the similarity or dissimilarity, between the figure and ground.