A self contained instrument is one that plays on its own, is not directly interacted with, and is not controlled directly by a player or the audience. These instruments are often Sound sculptures and very rarely play along with other instruments. Often the sounds produced by these instruments are continuous without brakes, Stochastic, and Corporeal.
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This is kind of a difficult concept to explain. The best example I can give is a Zimoun sculpture, where the sculpture is playing when you enter the room to view it and continuously plays in a cacophonous way and continues to play after you leave the room. No one interacts directly with it, the process of the sculpture being turned on and off is never seen by anyone (exept the museum/institution and the artist) and all aspects of how it gets started and ends is obfuscated to the audience, they only see and hear it playing and nothing else.
A music box can be considered self contained, but the act of winding it and allowing it to play for a short duration before stopping is not ideal to this concept. If there was a music box that plays continuously and in a loop without any direct interaction, that would be a better example of the instrument being self contained.
The John Cage piece Organ2/ASLSP (As Slow as Possible) being performed at the Saint Burchardi Church in the city of Halberstadt is another excellent example of a self contained instrument (at least for the duration of the piece), the work will last several hundred years, being played continuously and without much direct interaction to make the piece play. 1)
