Independently from the underlying hardware, I know about two approaches.
1.) Natural sounds (forest, raining above a lake, river, creek, waterfall), all 1-1.5 hours long and could be repeated more times or one time when going to sleep. That kind of sounds are very easy to process and result in a more relaxed state.
2.) other possibility, it is less natural, but proved to be effective, i.e. binaural beats, which are resulting in specific states (alpha, delta waves), helping to fall asleep and to reach deeper sleep.
The first possibility needs only a clear sound recording, which can be played everywhere, while the second can be generated by computer program (there are a few commercially available).
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