Whenever I need to focus on mental-intensive work but still need to have some background music to help enhance my concentration (as opposed to distracting me with distinct melodies, rhythms, and lyrics), I will put on my ambient/soundscape playlist. I thought I'd share some of what I listen to, so those who have similar needs could try these, and others could add to this thread with more suggestions.
The most important criteria for this type of music/soundscape are:
-No prominent sense of rhythm or overtly strong drum/percussion
-No overtly distinct melodies
-No distinct lyrics or vocals (unless used like a synth pad or soft choral wash of sound)
-No harsh and jarring sounds or random distinct noises to jolt you out of the "trance"
Here are some examples of what I currently have on my "Writing Soundtrack" playlist, which is what I listen to when I'm writing my novels:
And since I only listen to this type of music/soundscape when writing, I also categorize them into "Dark and moody," "Light and soothing," and "Scary and disturbing," so I can match the mood to the scene I'm writing.