Dark noise vs brown noise. One of them drifts.

Why it works

The two are close in pitch. The difference is movement. Brown noise here is perfectly steady. Dark noise has its level raised and lowered very slowly by two cycles that never line up, so it breathes a little and never settles into a rhythm. Some ears prefer the stillness. Others find stillness easier to stop hearing.

A steady sound disappears, which is what you want from a mask. A moving one stays faintly interesting, which is what you want from company. If company is what tonight needs, there is a better kind on the second fader, and it has a beginning and a middle.

What you are hearing

• Dark noise. Pink noise low-passed at 320 Hz, with a very slow rise and fall in level that never repeats. Soft, close and a little uneven, like air moving at night.
• 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.