A performance requires: four helicopters, each equiped with a pilot and sound technician, television transmitter and 3-channel sound transmitter, and an auditorium with four columns of televisions and loudspeakers, a sound technician with mixing desk, and a moderator (optional), as well as the members of the string quartet. The piece focuses on the simple idea of a string quartet, with the rotor blades acting as a second instrument, with microphones placed so the helicopters may blend with the instruments themselves, whilst the instruments remain louder than the blades. The piece is played as follows. A moderator, who may be the sound technician, introduces the quartet, and then explains the technical aspects of the piece. The players must then walk, or be driven if necessary, to the helicopters, always being visible to the auditorium audience by camera. The embarcation is also shown, the musicians and instruments remaining constantly in the view of the cameras, with no camera changes. Behind each player the ground can be seen, as well as the glass. Then the piece begins.
The original version lasted approximately 18½ minutes, but the 1994 revision was extended to 21½ minutes. The helicopters circle at a radius of 6 km from the auditorium, changing altitude constantly to create the 'bounce' of the piece. All 12 incoming signals are controlled by the sound technician. The descent lasts five minutes, with the decreasing sound of the rotor blades acting as a background as the quartet re-enter the hall. The moderator then takes questions and leads applause. (from Wiki)

http://en.wikipedia.org/wiki/Helikopter-Streichquartett