Background noise is annoying under the best of conditions. But when you're wearing headphones, it's all too easy to find yourself boosting volume to hazardous levels to drown out air conditioners, lawn mowers, cars, and jet engines. Noise-canceling headphones, which generate a compensating signal to reduce the din, can help. Here, we review four popular models (the Bose QuietComfort 2 is reviewed on the Gear page).