How it works: Noise-cancelling headphones

Panasonic's RP-HC500 noise-cancelling headphones. (Panasonic Canada)

Technology is at its best when it appears to work like magic, and that's exactly what noise-cancellation headphones do. Place a set on your ears, and the sounds of the world outside simply disappear.

At first you might be inclined to think that the headphones' snug ear cups simply block outer sounds. To an extent, you'd be right — much like quality traditional headphones, noise-cancelling models (both headphones and fitted ear buds) tend to have good physical sound barriers that help to block outside noise. But this blockage is passive and has nothing to do with the dynamic eradication of unwanted sound.

Noise-cancelling headphones employ a technology referred to as active noise control, which essentially erases or destroys unwanted sounds before they can reach your ear.

The effect is both wondrous and disconcerting. Don a set of noise-cancelling headphones while in flight on an airplane, and the drone of the engine will disappear, replaced first by silence, then by the thumping of your beating heart (heard not through the air, but via conductivity through your body — it becomes that quiet). When the music starts and you close your eyes, you might find that you forget the rest of the world even exists.

A decades-old idea

The concept of cancelling noise rather than simply blocking it out goes back to the 1930s, when a U.S. patent was issued to German inventor Paul Lueg for "the process of silencing sound oscillations." His patent stated that noise, composed of tiny and rapid variations or waves in air pressure, can be eliminated by introducing an opposing wave that is out of phase with (or a mirror image of) the original wave. When the two waves join, they essentially cancel each other out.

Over the ensuing decades, Lueg's noise-cancelling techniques were applied here and there in the aviation and military industries. The technology turned out to be successful in limiting the whine of engines in airplane cockpits and sound vibrations in air ducts.

With the advent of affordable microprocessors, noise-cancelling became more feasible in consumer products, and companies like Bose began working on noise-cancelling headsets. The first of these, the Bose Aviation Headset Series I, went into production in 1989.

The technology has since slowly scaled up to where entry-level headsets are affordable to the consumer masses, though high-quality noise-cancelling headphones are still priced in the hundreds of dollars.

A cutaway view of the electronics inside an earcup of the Bose QC2 noise-cancelling headphones. (Bose U.S.A.)

How it works

Whereas traditional headphones typically comprise little more than a pair of small speakers, there is substantially more technology at work inside a pair of noise-cancelling headphones.

In order to effectively eliminate incoming ambient noise, the headset has to identify and evaluate sound waves approaching the listener. This is done with a microphone, which can be located either inside or outside the ear cups. Headphones with a microphone located inside the cups generally perform better, because the microphone is able to sense waves that more closely approximate what the listener is hearing.

This is where the technology comes in. The collected sound is sent to a circuit board in the headphones that analyzes it and quickly formulates an opposing wave. This cancelling sound wave is usually emitted from a secondary driver - that is, a speaker separate from the one used to create the sound to which you are listening - to intercept and cancel out incoming ambient noise.

This process goes on continuously while you are using the headphones. The changing flow of incoming sounds is monitored, and the appropriate cancelling waves are generated.

"A question I'm often asked is whether we're hiding or actually decreasing the sound," said Dan Gauger, research group manager in Bose's noise-reduction technology division. "The answer is that we're decreasing it. The microphone constantly senses changes in air density — sound — and the electronics determine whether you want to hear those changes. We destroy the sound you don't want to hear."

Down with the low

But as magically effective as noise-cancelling technology might be, it's not perfect.

Noise-cancelling headphones are extremely efficient at eliminating low-frequency sounds. Slip on a pair while in a car, plane or boat and the rumble of the engines all but disappears. On the home front, they do a good job of reducing the sound of a neighbour's lawnmower or the traffic on busy streets nearby. But they have a harder time with higher frequencies.

"They won't cancel the sound of music, telephones or the human voice," said Vien Bantog of Panasonic Canada, whose company makes a wide variety of noise-cancelling headphones. The most powerful model in Panasonic's line cancels 92 per cent of ambient noise with a frequency of 200 hertz or less. (In terms of sound, hertz is a measure of wave oscillations per second. One hertz equals one oscillation per second. To imagine what a 200 Hz. noise sounds like, think about the low tones made by the native Australian wind instrument the didgeridoo).

Bose's Gauger explains that lower-frequency sounds — that is, those at a lower hertz — are more easily cancelled because the sound waves change less rapidly. "Their wavelengths are longer and the sound is more consistent over a larger space," he said. "It's easier for a noise-cancelling system to track those changes."

The fact that higher-frequency noises aren't as easily cancelled might seem like a limitation, but it can be beneficial in some situations. The sound of a car horn or fire alarm, in many instances, is something a listener would likely prefer to hear rather than have cancelled.

An illustration explaining noise-cancelling technology. (Panasonic Canada)

Noise cancelling really works

To research this story, I tried a set of Bose QC2 noise-cancelling headphones. Lightweight and of average size, they looked just like traditional headphones. But the moment I slipped them on, I could tell there was more to these phones than met the eye — or ear, for that matter.

The first time I flicked the switch to turn them on, the experience was otherworldly. My nearly noiseless environment seemed unreal. I imagined the way I felt — slightly alarmed and a little spooked by the lack of sound — to be vaguely akin to the way someone who has just lost his hearing must feel.

However, the uneasiness subsided once the music, almost completely unpolluted by external sounds, began to flow.

I wore them at home while working on a computer, and the sound of my fingers tapping the keyboard vanished. I could still hear my family and television in the background, but both were reduced in volume so that I could only hear them in the silence between songs.

I also had the chance to wear the headphones on a plane during two business trips, and, sure enough, the rumble of the jet engines was completely silenced. In fact, the attendant had to nudge me to get my attention during one flight.

Conclusion:

There's a lot of science at work behind noise-cancelling headphones, and the result is impressive. They act like a toggle to turn off the sounds of the world around you.

And when facing a Saturday afternoon with construction on the road that runs by my apartment, the sound of silence was magical indeed.