Pink noise

Balanced noise with equal energy in every octave: softer than white, brighter than brown.

Pink noise loses 3 dB of power per octave, so each octave (say 100–200 Hz and 1,000–2,000 Hz) carries the same energy. Our hearing groups pitch in octaves, which is why pink noise sounds even rather than hissy. It resembles steady rain or wind through trees.

It is a good middle ground for focus and for masking speech in an open office: enough treble to cover voices, without white noise’s sharpness. Audio engineers use it to test and tune speakers for the same reason.

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