This is a work which resonated with me immediately, even though it was completely puzzling. Decades later, I still find new interactions and timbres every time I listen to a recording.

KURZWELLEN ("Shortwaves")
For tam-tam (gong) with microphonist, viola (with contact mic), electronium, piano, 4 shortwave radios and sound mixer
1968 (55')

"An undreamed intensity of listening and of intuitive playing is reached – and shared by all co-players and listeners –
through the concentration of all players on unforseeable events coming from the realm of short-waves,
in which one only very rarely knows who composed or produced them, how they came into being or from where,
and in which all possible acoustic phenomena can appear."
- Stockhausen

Link to my write up on KURZWELLEN:
http://stockhausenspace.blogspot.com/2014/10/opus-25-kurzwellen.html

I recently got a look at Winrich Hopp's book on KURZWELLEN. He did a fantastic job exploring the work, as well as KURZWELLEN MIT BEETHOVEN. Recommended.