"Shhhhhh....” It's the sound we make to hush each other in libraries and museums. It's also the sound of wind through trees, traffic noise, the ocean, rivers, and television static and several eras of relaxation records, tapes & cds. I've read several speculative statements in a variety of parenting publications that the playback of a uniform spectrum of a wide range of frequencies (white noise) is in ways similar to the sound of blood rushing through the veins and arteries around the womb: consequently babies often find the sound calming, and sleep-inducing. Maybe it's just noise, but many new parents swear by it.