AMSTERDAM — There was a palpable buzz on a recent afternoon in the Gashouder, a big, round former industrial building in a park on the west side of Amsterdam.

A buzz, that is, beyond the four helicopters whose steady drone was being relayed through banks of speakers set up to rehearse a crucial part of “Aus Licht,” a marathon performance of selections from Karlheinz Stockhausen’s eye-poppingly extravagant, never-fully-performed, seven-opera, 29-hour “Licht” cycle. (Divided into three hefty evenings, it runs from May 31 through June 10.)

Four women walked onto a stage inside the building. A moderator explained that they were the Pelargos Quartet, and they were about to perform from inside the helicopters everyone was hearing.

The women walked out a side door. Above the stage, four screens flickered to life. The audience saw worried-looking men wiping raindrops from a passing shower off the helicopters, which were idling a few hundred yards away.