As in music that you listen to while studying for days on end, not the study of music.
I can't listen to anything with words (even in another language), but voices are ok, as long as I'm unfamiliar with the piece, that way I can't sing along. I prefer mixed instrumental or orchestral as opposed to a single instrument, otherwise I focus too much on the one instrument.
I need to listen to things that I'm not used to hearing, otherwise I just focus on the music. It needs to be somewhat unpredictable without being a cacophony. I'm getting too familiar with my usual rotation. Need some new stuff.
Stuff that works: the brandenburg, handel's water music, eric satie's gymnopedies and gnossiennes and others, some of schubert's string quartets... can't remember the others.
Doesn't work: vivaldi's 4 seasons (well, winter is ok, but the other 3 don't work)
Any recommendations would be much appreciated.