Why babies fall asleep to noise
A steady noise covers household sounds: voices in the next room, the TV, the street. The baby startles less often at sudden sounds.
Many parents find that a hissing sound soothes. It's close to the familiar 'shhh' used to settle babies.
Volume and distance: the key part
In 2014 the journal Pediatrics published a study of 14 infant sleep machines. At full volume, all of them were louder than 50 dB (the recommended level for hospital nurseries) at 1 metre, and three exceeded 85 dB close to the cot.
The authors advise placing the sound source as far from the cot as possible, keeping the volume low and running it for shorter periods.
In practice: about as loud as a quiet shower behind a closed door, with the phone or speaker across the room, at least 2 metres away. Don't put it in the cot or near the baby's head. Set the timer for 30 to 60 minutes so the noise fades once the baby is asleep.
If in doubt, ask your paediatrician.
White, pink or brown
White is the usual choice for babies. If it sounds harsh to you, try pink: it's softer and still covers household sounds well. Rain works too, it's calm and even.