Pink noise vs brown noise. Hear the difference.

Why it works

Both begin as white noise. Pink loses three decibels per octave as the pitch rises and brown loses six, so brown ends up with far more of its energy at the bottom. In practice pink sounds like steady rain and brown like something large and far away. Switch between them here and the description becomes unnecessary.

Which is better for sleep is mostly a matter of what you are covering and what your speakers can reproduce. Neither one gives you anything to think about, and that is the real difference between a noise and a story. The second fader lets you compare that as well.

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.
• Brown noise. White noise run through a leaky integrator, so its energy falls six decibels per octave, then low-passed at 400 Hz. A low, even wash.
• 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.