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August 13, 2008 > TechKnow Talk: How do noise canceling headphones work?

TechKnow Talk: How do noise canceling headphones work?

In 1986, pilots Dick Rutan and Jeana Yeager set out to fly a flimsy-looking airplane, dubbed Voyager, around the world. Without some form of noise suppression, they were expected to temporarily lose as much as 30% of their hearing during the trip. The Bose Company offered an experimental headset designed to reduce unwanted noise. Using this device, the pilots completed the first non-stop around-the-world flight with little or no hearing loss.

What was unique about this technology was that it didn't just muffle ambient noise, it eliminated some of it, and it did so without external equipment. The noise-canceling hardware was integrated into the headphones themselves.

There are two general categories of noise reduction technology: passive and active. Passive noise reduction has been around for a long time. Headphones that completely surround the ear, seal against the head and are filled with noise dampening material are an example of passive noise reduction. They allow the user to hear music with less distraction from external sounds or simply to enjoy relative silence in a noisy environment. This works pretty well for eliminating high-pitched sounds but is relatively ineffective in blocking low-frequency noise.

Active noise reduction, also called noise cancellation, is a more sophisticated technology. A microphone detects the external, or ambient, sounds. This information is processed by electronic circuits which then send "anti-sounds" to the speakers, effectively canceling the external sounds without affecting the sound of the music or other input.

To understand how this works, some very basic background in the nature of sound is required. Sound waves are longitudinal. This means they are comprised of air molecules moving through the atmosphere in regions of high and low pressure. Consider a vibrating guitar string. As it moves from side to side, it pushes air out then draws back, alternately forcing the air molecules together then spreading them out.

These pulses of more- and less-densely packed molecules enter the ear, where we perceive them as sound. The frequency, or number of dense regions per second entering the ear, determines the pitch of the sound - the more packets of dense molecules per second, the higher the pitch of the tone. The wavelength is the spacing, or how far apart the dense areas are - the shorter the wavelength, the more enter the ear each second and the higher the pitch. How tightly compressed the dense areas are - the pressure difference between low and high pressure regions - is the amplitude of the sound, and determines how loud it is.

As the microphone detects each of these sound waves, the noise canceling device sends a signal to generate a sound at exactly the same frequency (pitch) and amplitude (volume); the generated wave matches its dense, high pressure areas to the external wave's thin or low pressure areas. This is called a 180 degree phase shift, resulting in destructive interference. The effect of these two waves coming together is no sound at all. One cancels the other.

A useful analogy to envision this is to think of the sound wave as an ocean wave. This is a transverse wave, in which the water molecules move up and down as the wave travels along the surface. But the principle of cancellation is similar to longitudinal sound waves in which the molecules move along the direction of travel. Imagine generating an ocean wave with the same height (amplitude), the same distance between crests (wavelength), and traveling at the same speed as another wave but shifted one-half wavelength, so its crests match the troughs of the original wave. The interaction of these two waves would produce calm water.

Active noise cancellation has been in use since the 1950s and 1960s, when U.S. and Russian engineers experimented with it for military pilots. These systems were external devices mounted in the cockpits, not integrated into headsets. Dr. Amar Bose has been credited with the invention of the integrated noise canceling headset, reportedly spurred by a commercial flight in 1978 during which he was frustrated by the inability of the airline-provided headphones to block engine noise. By the late 1980s, buoyed by the success with the Voyager flight, both Bose's company and German company Sennheiser brought integrated noise canceling headphones to market.

The relatively simple concept of destructive interference has been difficult to put into practice. The noise to be cancelled is never a single tone, but a cacophony of different pitches at different volumes, and varying in both frequency and amplitude from one moment to the next.

The device must detect the external noise, separate it into its components, determine the proper canceling signals, produce these signals, and feed them to the speakers-all in the blink of an eye. And it must do so while responding to a continuously changing tapestry of sound. Further, it must ensure the canceling signal in no way detracts from or adds to the signal the listener wishes to hear, be it music, movie, or silence. These requirements present very difficult technical problems, and engineers continue to refine the electronics and improve performance.

Noise cancellation technology is most effective in erasing low frequency sounds. Since the passive techniques of physically isolating headphones work best in eliminating high frequency sounds, most consumer designs incorporate a large, enclosing ear cup in combination with active cancellation.

Current technology is also better at canceling relatively constant background noise than dealing with rapidly changing sounds. As a result, noise canceling headphones are particularly effective in airplanes and trains, but are not as useful in very dynamic acoustic environments such as sporting events or city streets.

In addition to the microphone, electronic circuitry, and associated wiring, portable noise canceling headphones require power, so must incorporate a replaceable or rechargeable battery. This adds even more bulk and weight. While this may not be a major consideration for an airline passenger, it is unacceptable for a jogger. Thus, as the electrical engineers work to improve dynamic response, much of the current acoustics research is focused on reducing size and weight to promote comfort and portability. One approach is to create an acoustic seal within the ear canal itself using very small earplug-sized devices.

Overall, the noise canceling headphone is a technology not yet fully mature. While currently available consumer products are adequate for their intended use, look for enhancements in coming years that will offer a more satisfying user experience in a broader range of applications.

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