John Cage- 4'33"
Cage's famous and controversial 4'33" is based on the idea that the only shared feature of all sound is length. Whether you like it or loathe it, the piece was significant enough to be performed in the Great Albert Hall. Note that there is actually sheet music to this piece, but that it's just three pages reading 'Tacet' which means not to play. Also note that in the above performance, this sheet music is written in comic sans.
Alvin Lucier- I am Sitting In A Room
Alvin Lucier reads a short explanation of the piece, then plays the explanation back on tape recording the results, which are in turned played and then recoded. The repeated process of playing back and rerecording means Lucier's speech is slowly converted into an ambient sound scape displaying only the resonant frequencies of the room he was in. Perhaps the most remarkable thing about the piece, is the undeniable musicality that results from a seemingly arbitrary process.
Steve Reich- Pendulum Music
Several microphones are swung over amplifiers feeding back as they pass over until they lose momentum. Steve Reich's musical sculpture is, at it's heart, a simple and elegant demonstration of feedback and interlocking tempos.