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How do Active Noise Canceling Headphones Work?


I've been traveling quite a bit recently and the drone of the plane engine is a major annoyance. While I have a pair of noise dampening ear buds which are much more comfortable and produce better sound than the default iPhone earbuds, I have been wavering about purchasing a pair of active noise canceling earphones for myself.

Although I understand the physics (and accompanying math--specifically trigonometry) of how the earphones work, I was uncertain if the technology was sophisticated enough to cancel ambient noise for the average flyer at a reasonable price.

Curiosity about the practicality of spending so much money for these contraptions led me to ask several frequent fliers their opinions. Overall, they expressed resounding enthusiasm for the devices, so I did some further research. When I learned Bose had created active noise canceling headphones for astronauts on the space shuttle, I was sold, in theory. I just had to wait for a pair to be created that was within my budget and a good size for traveling.

As I finally found myself a reasonably compact pair in a price worth testing, I thought I'd find SciAm readers a video explaining the science and technology of active noise canceling headphones work, and surprisingly found ONLY one. It comes from James May at HeadSqueeze. I think it does a fairly good job at explaining the technology, even if he completely forgets to use the term 'destructive interference'!

And in case you want a little more visual science, I found this brief but effective demonstration on the fundamentals of sound wave interference, both constructive and destructive.

There are several websites that explain how these headphones work, so a simple google search will lead you to them, but really, Wikipedia does a fine job if you don't mind a bit of physics terminology.

Sound is a pressure wave, which consists of a compression phase and a rarefaction phase. A noise-cancellation speaker emits a sound wave with the same amplitude but with inverted phase (also known as antiphase) to the original sound. The waves combine to form a new wave, in a process called interference, and effectively cancel each other out - an effect which is called phase cancellation.

Modern active noise control is generally achieved through the use of analog circuits or digital signal processing. Adaptive algorithms are designed to analyze the waveform of the background aural or nonaural noise, then based on the specific algorithm generate a signal that will either phase shift or invert the polarity of the original signal. This inverted signal (in antiphase) is then amplified and a transducer creates a sound wave directly proportional to the amplitude of the original waveform, creating destructive interference. This effectively reduces the volume of the perceivable noise.

A noise-cancellation speaker may be co-located with the sound source to be attenuated. In this case it must have the same audio power level as the source of the unwanted sound. Alternatively, the transducer emitting the cancellation signal may be located at the location where sound attenuation is wanted (e.g. the user's ear). This requires a much lower power level for cancellation but is effective only for a single user. Noise cancellation at other locations is more difficult as the three dimensional wavefronts of the unwanted sound and the cancellation signal could match and create alternating zones of constructive and destructive interference, reducing noise in some spots while doubling noise in others. In small enclosed spaces (e.g. the passenger compartment of a car) global noise reduction can be achieved via multiple speakers and feedback microphones, and measurement of the modal responses of the enclosure.

Ultimately, I made my decision based on CNET reviews In addition, I found a recent review/explanation over at Forbes. Are Noise-Canceling Headphones Worth It?

Ultimately, I chose this pair, at least for now.

Do you have a pair you love (or hate)?

The views expressed are those of the author and are not necessarily those of Scientific American.

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