2015-12-16

Noise cancellation headphones - "how it works?"

"Noise cancellation" is a feature in recent high-end headphones that aims to actively remove the outside noise while listening music. You can also use them without music, however the efficiency is lower. "Noise cancellation" is a relatively new technology and you must be aware of what is reasonable to expect from such headphones.

Noise cancellation - how it works

Headphones with "noise cancellation" have special microphones that captures the environment noise with the intent to create a sound that will cancel the original noise. This is a very tricky stuff, as the cancelling sound should be in exact phase with the original sound, but upside-down: when the noise sinusoidal is up, the noise cancellation should be down and reverse. Actually, there is no physical time to create this sound as the microphone receives the noise. The only way to do it is to predict the next noise pattern and create the cancelling sound for that future expected noise.

As a result, noise cancellation only works for monotonic/recurring sounds, like motors in airplanes, trains, subways, fans in air conditioning. Actually, in these areas they can do very good job. After half of hour in the office with such headphones on, when I remove the headphones I notice the air conditioning rumbling very loud - although I barely notice it normally. When I put them off in the subway, I can't believe how loud the noise can be, in contrast. The voice is not predictable and noise cancellation will have limited impact in reducing it.

There is no such thing as producing absolute silence. Even with noise cancellation, you will still notice the motor rumbling, it will just be less disturbing. Actually, from my tests, you can obtain slightly better noise reduction with industrial noise isolation headphones. The difference is that using noise isolation headphones puts a huge mechanical pressure on your head and you cannot listen music - except if you sneak two earbuds underneath. With noise cancellation headphones I can stay hours without much discomfort while using industrial noise isolation headphones I barely resist half of hour. Moreover, because of active noise reduction, I can use a lower music volume, protecting my hearing from damage.

In the office, noise cancellation can create a difference if the noise is not too loud. You can still hear people talking 1 meter away. However, you can easily ignore people talking not too loud in the next office cube. This is not exactly done by the noise cancellation, it seems more like a... side effect of it. Even when there is no noise around, "noise cancellation" headphones produce a low volume white noise, probably artifacts of the noise cancellation system. This has an interesting effect for low environment voices: it can lower the signal to noise ratio, enough so that it will not take your attention anymore. You will still hear that people are talking something, but you will not understand what they are saying and you can better concentrate on your work.

Even when the noise is low, when you put such headphones over ears you realize that it is more quiet and easy to work using them. Some people might feel a little like "under-water" without any music but this is far more lighter compared with the "ear pressure" of industrial noise isolation headphones.

You should expect a low white/static noise when using noise cancellation headphones. This is not disturbing usually, however it will not sound so clean as a pair of good passive headphones. The good news is that noise cancellation headphones can still sound better than cheap headphones, because they are usually high end headphones and have better drivers. Expect that lower price models to be very modest as "noise cancellation" and sound quality.

On some headphones, the noise cancellation cannot be disabled, so they only work when battery is charged. Others can disable noise cancellation but they still have a static noise as long as you keep the internal amplifier on. Noise cancellation will slightly change the frequency response, some frequencies will be amplified, other are lowered. Audiophile level quality just don't mix well with noise cancellation. If you only need passive headphones, you should be able to find the same audio quality (or more) at a lower price.

Bottom line
With this said, you should be prepared to limited efficiency from any model of "noise cancellation" headphones. However they can be of real help when traveling in noisy environments and when working at the office or at home.

Products
I own and used 2 types of headphones with noise cancellation: "Bose Quiet Comfort 15" and "Sennheiser MM 550-X TRAVEL", click an the previous link for a comparison between them.


P.S. Please share this article if you find it interesting. Thank you.

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