On winter nights, the white-noise app on my phone is tuned to Air Conditioner: a raspy, metallic whir that sounds like the mechanical noise that might echo deep inside the duct work of a huge commercial building. (Among the app's other offerings are Dishwasher Rinsing, Crowded Room and Vacuum Cleaner.) It lulls me to sleep nonetheless, because it blankets the din in my apartment (the ragged snore of a roommate; the clanking of the steam radiator; the cat's skidding pursuit of something only he can see).
It may also soothe because it replicates an early sound environment, probably that of a Manhattan childhood, though perhaps it suggests something much older. Some sleep experts note that babies, their ears accustomed to the whisper of the maternal circulatory system and the slosh of the womb, sleep better accompanied by a device that mimics those familiar whooshings.