Which are the best noise cancelling headphones?
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Richard Baguley
Published on April 07, 2008
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 Recently, "noise cancellation" has become a favorite buzzword feature of headphone manufacturers. Stopping external noise is obviously important, as it dilutes the quality of the sound you're hearing. We gave a brief overview of noise cancellation in our buyers' guide, but the subject really merits additional information. As mentioned in that article, there are two different types of noise cancellation: active and passive.
Passive noise cancelling is caused by causing a physical obstruction that stops sound from entering your ear drum. Cupping your hands over your ears or wearing ear plugs are examples of passive noise cancellation. In-ear headphones like the Etymotic ER6i and the Shure SE210 fit into the ear, almost completely blocking the ear canal with a tight-fitting sleeve. In many ways, the best in-ear headphones do emulate ear plugs.
Active noise cancelling headphones (such as the Bose QuietComfort 3s we reviewed) are much more involved than those with passive noise cancellation. These headphones listen to external noise with a small, external microphone. It then analyzes that sound, producing a sound of the opposite phase. Like two waves on a pond cancelling each other out, the noise is effectively cancelled. If you want to learn more about this process, How Stuff Works has a good article here.
So which approach works best? The answer is that it depends on what type of noise you are dealing with.
Our testing system allows us to do an in-depth examination of how effective headphones are at blocking noise. We do this by putting a noise source (a speaker) near our head and torso simulator (HATS), which is wearing the test headphones. We then measure how much noise reaches the ear-simulating microphone inside HATS. Our testing system is sensitive enough to ascertain how much sound the ear picks up at a wide range of frequencies. We do this test three times: with no headphones on the HATS, with the headphones on (but with any active noise cancellation turned off), and with the noise cancellation enabled. For more details on how our tests work, see here. We can then compare the results to figure out how good a job the headphones do of blocking the sound at different frequencies, which is shown on the graph below.
On this graph, the higher the line, the more noise is cancelled at that frequency. The frequencies go from 100Hz (a low-frequency rumble) on the left, to 10Khz (a high-pitched squeak) on the right. The red line is for the Etymotic ER6is, which are in-ear headphones. The green and blue lines are for the Bose QuietComfort 3 over-ear headphones: the green is with noise cancelling turned off, the blue is with it on.
This graph shows that the efficiency of noise cancellation really depends on the frequency of the noise. Low frequency noises were best cancelled by the QuietComfort 3 headphones' active cancellation. The Etymotic ER6i headphones, however, could cancel higher-frequency noises better than the QuietComfort3s, even with active cancellation.
It's also interesting to see the difference active noise cancellation has on the QuietComfort 3s' ability to repel external noise. When turned off (again, the green line), the headphones don't cancel much noise at all at low frequencies; turning it on (blue line) made a 20 decibel difference. At higher frequencies, active noise cancellation didn't make any difference, as seen by the green and blue lines overlapping.
So which set of headphones did a better job? Again, it depends on what you're looking for. If you'll be riding airplanes or railways, or generally around a lot of low-frequency sound, the QuietComfort 3s and their active noise cancellation will definitely help you out. The ER6is tended to block out more mid-range and higher-frequency sounds, so they should better block out speech or the high-pitched whine of electronics.
  In the end, the choice may come down to the type of headphones that you prefer. While in-ear headphones like the Etymotic ER6i (on the right) and Shure SE210s do a better job of blocking out noise overall, many people find them uncomfortable. Further, they can be extremely uncomfortable on airplanes, since they don't allow the ears to equalize air pressure; you'll constantly feel the need to pop your ears. On-ear ones like the Bose QuietComfort 3 (on the left, or their over-ear cousins the QuietComfort 2s) don't have this problem. Coincidentally, they're also most effective with the low frequency sounds that are the bane of frequent travellers.
Do you have any questions on our tests or noise cancelling headphones? Post them below and we'll do our best to answer them.
[Photo by Flickr user DC_Meatloaf]
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