George Prochnik, author of In Pursuit of Silence: Listening for Meaning in a World of Noise in The New York Times: 

Mammalian hearing developed primarily as an animal-detector system — and it was crucial to hear every rustle from afar. The evolved ear is an extraordinary amplifier. By the time the brain registers a sound, our auditory mechanism has jacked the volume several hundredfold from the level at which the sound wave first started washing around the loopy whirls of our ears. This is why, in a reasonably quiet room, we actually can hear a pin drop. Think what a tiny quantity of sound energy is released by a needle striking a floor! Our ancestors needed such hypersensitivity, because every standout noise signified a potential threat .

There has been a transformation in our relationship to the environment over the millions of years since the prototype for human hearing evolved, but part of our brain hasn’t registered the makeover .

Every time a siren shrieks on the street, our conscious minds might ignore it, but other brain regions behave as if that siren were a predator barreling straight for us. Given how many sirens city dwellers are subject to over the course of an average day, and the attention-fracturing tension induced by loud sounds of every sort, it’s easy to see how sensitivity to noise, once an early warning system for approaching threats, has become a threat in itself.