Pendulum music by Steve Reich is the first piece of sound art I saw. I was shown a video of this piece by my music teacher in my early teens and it made me realise that there was a place for experimental music. It also opened me up to different styles of composition, the score being text based instructions for performers to recreate the piece.
In the version I saw 4 speakers were laid out on the floor facing upwards, with four microphones each suspended in the air at the same distance above the speakers, four performers each go and hold the microphones up to themselves and let them go, the microphones swing over the speakers like a pendulum and the speakers are turned up until there is feedback when the microphones swing over.
I really enjoy the element of performance captured within the score, having the performers go and sit within the audience was particularly interesting to me. The sound of the microphones slowing down and becoming one sound over the period of the piece was impressive.