Noise Isolation vs. Noise Cancelling

I've decided to start a "mini-series" of posts for my blog regarding audio and headphones/earphones and other audio gear (amplifiers, speakers, etc.). I think these posts will be beneficial to anyone looking to get the best sound quality out of their music! Personally, I have been upgrading quite a lot over the past few years, and reading a lot of audiophile information on forums like head-fi.org. I will try my best to help others with the information I've obtained over the years.

I'm sure many of us have heard of Noise Cancelling headphones by now. But do you truly know the difference between Noise Cancelling headphones and Noise Isolating headphones/earphones?

Noise Cancelling headphones work by "fighting sound with sound" by emitting opposing frequencies that cancel out external noise. Above, Sony MDR-NC60 headphones.
Source: sony.jp

Noise Cancelling headphones, quite simply, fight sound with sound. Since there is usually ambient noise external to the headphones that leaks into the music, Noise Cancelling headphones actively "remove" this unwanted sound by emitting frequencies that cancel out the opposing sound. The end result is that you hear your music without all that external noise from outside leaking in.

Noise Isolating headphones/earphones work a little bit differently. Noise Isolating headphones/earphones (more properly, IEMs or in-ear monitors) passively remove external noise without producing opposing frequencies. How they do this is quite simple: they create a seal in your ear or outer ear. For example, many "canal phones" that you insert deeper into your ear than you would iPod earbuds, are creating a seal in your ear that reduces external noise from anywhere between -20 dB and -47 dB. This prevents external noise from coming into your ears at all, thus eliminating the need to "fight sound with sound" as Noise Cancellation does.

Noise Isolating in-ear monitors (IEMs) work by passively reducing external noise by creating a seal in the ear canal. This eliminates the need to "fight sound with sound". Above, Shure SE535 in-ear monitors.
Source: fittechnica.com

So which form of technology is better? Frankly, Noise Isolating would get the vote from me. One of the main reasons is that Noise Isolating headphones/earphones do not introduce artifacts into the music as Noise Cancelling headphones/earphones do. Since Noise Cancelling headphones/earphones fight external sounds with opposing frequencies, they have a tendency to introduce artifacts into the music that were not originally there. Noise Isolating headphones/earphones do not fight sound with sound, so they never introduce any artifacts into the music.

Secondly, Noise Isolating headphones/earphones do not require any external electricity or power to work. Many Noise Cancelling headphones/earphones require a battery of some sort in order to run the Noise Cancelling feature. This requires charging or replacing batteries, something that Noise Isolating headphones/earphones do not require.

And finally, Noise Isolating headphones simply work better in my opinion. Since sound is never introduced to the ear in the first place (because of the seal), you don't have to fight sound with sound. It's akin to solving a problem before it happens. With Noise Cancelling headphones/earphones, the external noise already gets into your ear and music while it is simultaneously alleviated with opposing frequencies. There's no saying that one technology is better or worse than the other--it's just that Noise Isolating gets the vote from me in preserving good audio quality.