Since some people like to use ambience tracks in their hypnosis files, here's a 17 minute ambience piece: http://www.mediafire.com/download/okh35k22uuc7lbk/deepambience.mp3
It starts out playing only deep tones, but after a minute or so higher tones are added.
The instrument I used to create it: http://www.mediafire.com/download/7zg3r3efma377o2/ambience.nmsv
Do whatever you like with them.
The instrument requires the Massive VST.
It is designed to make a simple, varying ambience track when a note is held.
A good technique for using it is to play several notes at once for a very long time, waiting several seconds between each note start unless you want the instruments seeming to echo each other or be in sync. It's practical across the entire range of human hearing, although you might want to quieten it at midtones.