Shush

A rhythmic “shhh”, about once a second, like shushing a baby by hand.

Shushing close to a baby’s ear is one of the oldest ways to settle one. This sound does it for you: a band of hiss around 1–6 kHz that swells about once a second, each one slightly different, never quite silent in between, with the peaks kept even.

Parents often shush loudly at first and then quieter as the baby settles. Do the same with the volume, and keep the phone at a distance rather than next to the baby’s head (why).