Music to Your Ears

Noise Cancellation 101

Few things annoy people more than the din inside an aircraft--except, perhaps, being stuck on a noisy plane between a snoring executive and an unhappy child. That's precisely why the makers of headphones tend to advertise in in-flight magazines; they know that, after a couple of hours, you'll be desperate for a little peace and quiet.

Headphones that promise to block outside noise do so in one of two ways: passively or actively. Passive headphones rely on the fact that sticking things in your ears makes it harder for noises from the outside to penetrate. In-the-ear headphones such as the Shure E4c and the Etymotic ER-6isolator fit like earplugs, sealing the ear canal tight so sounds can't get in.

Active noise-cancelling headphones go a step further: They "listen" to the environment and create an inverse sound that (in theory) cancels background noise. The air pressure waves that make up sound, like waves in water, have peaks and troughs; an inverse sound has peaks where the ambient sound has troughs, and vice versa.

We received mixed results with both of the active noise-cancelling headphones in our test group, the Bose QuietComfort 2 and the Sennheiser PXC250. Although each set blocked at least some ambient noise, neither blocked all of it. Low-frequency noises (such as those of jet and bus engines) decreased significantly, but high-frequency noises persisted. And neither of these models was as effective as the ear-canal headphones, which, when properly fitted, blocked all outside noise equally well.

Richard Baguley is a freelance technology writer based near San Francisco. Say what you will to him: When he's wearing his headphones, he can't hear you.