This reviewer has a very loud air-conditioning system in his office that we use for testing noise-canceling headphones (it isn't quite as loud as the inside of an airline cabin but it's not that far off). The QuietComfort 15s were able to almost completely silence the sound of the rumbling fan. Oddly, the impact of engaging the circuitry may seem a bit weird--it almost feels as if you're at the bottom of a pool, almost completely shut off from the sounds above.
As with all of Bose's noise-canceling headphones, you have to engage the noise-canceling to listen to music, and when the battery dies, so does the music. Luckily, battery life is good. Bose rates it at 35 hours, and the single AAA battery, which resides in the right earcup, was still going strong after we left our tunes in a loop overnight. Those looking for a rechargeable option can either opt for the QuietComfort 3s, or invest in their own third-party battery and charger. On the plus side, sticking with standard alkalines means there's no wall charger or AC cord to worry about when traveling.
While there may not be a huge difference between this model's noise-canceling prowess and the QuietComfort 2's, a distinction is definitely noticeable. According to Bose, these headphones are designed to defeat a wider range of frequencies, going up to over 90 decibels; the QuietComfort 2s peaked more in the 84-85db range. We also tried the QuietComfort 15s in the New York City subway system, and they did an impressive job of muffling noise.
Of course, the only problem is that because these are over-the-ear headphones that offer a tight seal, it can get a bit steamy inside the cups, especially on hot days, though they "breathe" fairly well for over-the-ear headphones. On long plane rides, your skin will also get a bit moist underneath the cushions, so expect to take them off for short periods to give your ears a little air. Also note that the 3.5 millimeter cord is detachable, so if you just want to block out the outside world and catch a nap, you can do so.