White noise vs brown noise, side by side.

Why it works

White noise has the same energy at every frequency, so the ear, which is most sensitive up high, hears it as hiss. Brown noise falls away by six decibels per octave and is heard as a low rumble. White masks more, including speech. Brown is easier to have in the room for a long time.

People often move from white to brown because white started to grate. Sometimes the trouble was the recording coming round again, and the mind noticing. Neither sound here repeats. If it still is not enough, the missing thing may not be a colour. Try the voice.

What you are hearing

• Soft white noise. True white noise, equal energy at every frequency, with the sharpest part above 4,000 Hz taken off so it is easier to live with.
• 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.