The womb is not a quiet place. For nine months, a developing baby exists in an acoustic environment that averages around 72 decibels — roughly as loud as a vacuum cleaner in the same room. Blood flow through the placenta, maternal digestion, the muffled sounds of the outside world — it's a constant, rumbling hum.
Then birth happens. And suddenly: silence.
The Acoustic Shock of Birth
For a newborn, a quiet room is not calm — it's an absence of the only acoustic environment they have ever known. This partly explains why many newborns are easier to settle in environments with consistent background noise. Silence is unfamiliar. Sound, particularly low-frequency rumbling sound, is home.
Harvey Karp's Research
Pediatrician Harvey Karp popularised this concept in his research on the "fourth trimester" — the idea that newborns benefit from conditions that approximate the womb environment during the first three months post-birth. Sound is one of the five core elements he identifies.
Karp's clinical observations, supported by subsequent research, found that white noise between 60–80 dB consistently reduced crying and improved sleep onset in newborns, particularly when the sound had characteristics similar to the womb: low-frequency dominant, sustained, not rhythmically complex.
What Works Best
For babies, avoid music with lyrics or complex melodic patterns — these engage the auditory cortex in ways that impede sleep rather than support it. Simple, sustained sounds are better: brown noise, rain, static. The sound should be roughly as loud as a shower running in the next room.
For older infants transitioning away from sleep sounds, gradually reduce volume over several weeks rather than stopping abruptly. The nervous system adapts to the acoustic environment it has learned to associate with sleep — give it time to recalibrate.