Noise Cancellation Headphones

Posted by Paul Hochman, special to MSN Tech & Gadgets

In space, no one can hear you scream. But on a recent flight back from Los Angeles, I wanted to: my seatmate would not shut up; the jet’s engines roared at about 80 decibels; three rows back, two siblings were fighting over candy (or somebody had smuggled a flock of howler monkeys in their carry-on). I pulled out my noise-cancelling headphones. Sweet gods of silence: they worked.

No, noise-cancelling headphones are not a gimmick. They can dramatically clarify the music you’re listening to; reduce the volume you need to hear your music or movie soundtrack; and even reduce the fatigue caused when your ears are exposed to hours of low-frequency sound.

But be careful: when you’re looking for a pair, know that there are 2 kinds of noise cancelling headphones – passive and active. Passive versions are only about half as effective as the active “noise-cancellation” versions. Here’s how they work:

Any sound your ear detects comes to your ear in a wave. Think of a guitar string – when it’s plucked, it vibrates back and forth, first pushing up against the air molecules around it, and then pulling back away, then pushing again. The string’s movement compresses the air rhythmically (the faster the rhythm, the higher the sound). Like a pebble thrown in a pond, the compressed air travels outward in all directions in a repeating ‘wave.’ Your ear senses that wave and interprets it as sound.

Noise cancelling headphones simply detect that outside sound wave using a tiny microphone on the exterior that ‘hears’ it and then knock down that wave before it gets to your ear. Using a small speaker inside the earpieces, noise-cancelling headphones produce an equal and opposite ‘inside’ wave of sound. Some call it ‘anti-noise’: every time the outside sound wave rises, the headphone speakers produce a wave inside the headphone that ‘falls,’ and vice-versa. Add the ‘up’ waves from your environment to the ‘down’ waves in your headphones, and you get zero sound. Or at least, less sound.

How much less? Between the foam padding of a standard headphone, which reduces that 80 db by about 20 db and then noise-cancelling device that cuts it down another 20 db, you cut the ambient noise by about 70%. 

One note: active noise-cancelling headphone use batteries to power the little inside speakers.  The “passive” noise cancellation or “noise reduction” version headphones have no electronics. Instead, they depend on the insulative or noise ‘baffling’ foam and plastic material in the headphones’ construction to intercept sound waves before they get to your ears.

So remember: if you work or relax in a nice, quiet place, all you need is some passive noise reduction. But if you’re habitually hanging with the howler monkeys, it’s time to get active. Cancel that noise.

(below) GearDaddy Video: Music players and noise cancellation

Paul Hochman is the TODAY Show’s Gear and Technology Editor, and is the host of GearDaddy and Consumer Electronics Show coverage on MSN Tech & Gadgets. He's also a contributing writer for Fast Company Magazine, and is a former teacher and dedicated consumer advocate.