Ocean

Slow waves, each 6 to 12 seconds, swelling and draining away.

Each wave is a different length and height: a deep body that swells, then the hiss of foam that trails it and fades. The loop is about 87 seconds long and built wave by wave to fill it exactly, so it repeats without a seam.

The slow rise and fall suits winding down. For sleep through the night, a steady noise masks sudden sounds better, since the quiet moments between waves let them through.

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