White noise is one of the most effective sleep tools for young babies. It is also one of the easiest to use unsafely, because the failure mode is silent — there is no immediate sign that a level is too high.
The number to remember is around 50 dB at the baby's ear, measured where they actually sleep. For reference: a quiet library is about 40 dB, a quiet shower or normal conversation around 50 to 60 dB. If you have to raise your voice to talk over it, it is too loud.
Distance does most of the work. Sound level falls sharply with distance — roughly 6 dB for each doubling. A machine at the far side of the room delivers a fraction of what the same machine delivers clipped to the cot. Keep the source well away from your baby's head, ideally across the room, and never inside or attached to the cot.
How to check. A free sound-level meter app on your phone is accurate enough for this purpose. Hold it where your baby's head rests, not where you are standing, and read the average rather than the peaks. If it sits meaningfully above 50 dB, turn it down.
Continuous beats intermittent. Sound that starts and stops re-triggers alertness each time it returns. A steady, unchanging sound is what allows the nervous system to stop monitoring it. This is also why lower-frequency sound — womb recordings, heartbeat rhythms, deeper hums — tends to work better than hissy high-frequency static, which is both more alerting and more fatiguing.
Don't run it all night at volume. Many parents use it to get through the settling phase and the first sleep cycle, then lower or stop it. That reduces total exposure while keeping the benefit at the point it matters most.
On weaning off: there is no fixed deadline. Many families phase it out somewhere in the second year by reducing volume gradually over a couple of weeks. Do it slowly rather than abruptly.
If your baby has any diagnosed hearing condition, or was in neonatal intensive care, ask your pediatrician before using white noise at all.