Pink noise vs green noise. They are cousins.

Why it works

Green noise here is made from pink noise. A wide filter centred on 500 Hz takes away the low rumble and the high hiss and leaves the middle. So the comparison is between the whole spectrum, evenly weighted, and its centre. Pink is fuller. Green is plainer and a little further off.

The difference is real and small, the sort of thing worth five minutes one evening and no more. Past that point, choosing sounds becomes its own way of staying awake. Pick either, bring the voice up a little, and let somebody else do the deciding for the rest of the night.

What you are hearing

• Pink noise. Equal energy in every octave, made with Paul Kellet's pinking filter and rolled off above 2,500 Hz. Brighter than brown, softer than white.
• Green noise. Pink noise passed through a wide band centred on 500 Hz. The lows and the hiss are both turned down, leaving the middle, where most outdoor sound lives.
• The voice. The first four minutes of Aristotle, the Greatest Philosopher, a Lullable story, in a synthesised voice. It only plays if you bring it in.
• The ending. The preview lasts four minutes. Its last ten seconds are a straight fade to silence, so it never stops abruptly.

For a real recording that runs forty minutes, the Lullable iPhone app has rain, waves, a campfire and plain noise, with no voice and no music.