Brown noise vs a fan. One has a motor.

Why it works

A fan is close to brown noise already: moving air is broad, low and steady. What a fan adds is its motor, a faint hum at the mains frequency and its double. Some people find the hum is the comforting part, because it sounds like a machine quietly doing a job. Switch between the two and see.

The fan in your room also moves air, and no browser will do that for you. What a browser can add is a voice. If you have slept beside a fan for years and still lie awake narrating, the fan was never the missing piece.

What you are hearing

• 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.
• Warm fan hum. Brown noise low-passed at 240 Hz for the moving air, with two faint tones at 60 and 120 Hz underneath for the motor.
• 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.