Green noise for sleep, with nothing repeating.

Why it works

Green noise is not a formal term. People use it for noise concentrated in the middle frequencies, around 500 Hz, which is where a river or a stand of trees puts most of its sound. This version is pink noise through a wide band filter, generated live, so it has no rumble, no hiss and no loop.

It is the most natural-sounding of the colours and still, in the end, a texture. A texture occupies the ears and leaves the rest of you free to rehearse tomorrow. Add the voice and the rehearsal has competition: a calm account of something old and true, in no hurry to arrive.

What you are hearing

• 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.