 Stockhausen had dreams of flying throughout his life , and these dreams are reflected in the Helikopter @-@ Streichquartett ( the third scene of Mittwoch aus Licht ) , completed in 1993 . In it , the four members of a string quartet perform in four helicopters flying independent flight paths over the countryside near the concert hall . The sounds they play are mixed together with the sounds of the helicopters and played through speakers to the audience in the hall . Videos of the performers are also transmitted back to the concert hall . The performers are synchronized with the aid of a click track , transmitted to them and heard over headphones ( Stockhausen 1996c , 215 ) . 
