Brown noise

Deep, steady rumble, like a waterfall heard from a distance or the inside of an airplane cabin.

Brown noise (also called red or Brownian noise, after the random motion of particles, not the color) loses 6 dB of power per octave: every step up in pitch is half as strong as the one below it. Most of what you hear is low rumble, with a soft hiss on top.

It is a common choice for sleep and for people who find white noise harsh. Many also use it to focus: it covers traffic, fans and distant voices without the brightness of white noise.

Using it for a baby? Keep the phone across the room, not in the crib, at a low volume (why).