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The Creative Aurvana X-Fi headphones have the rare option to turn their active noise cancellation on or off; typically noise-cancellers just have a power switch, and when it's off (or when the batteries die) the headphones won't play back music. Interestingly enough, the X-Fi headphones have substantially different audio quality depending on whether it's on or off. Generally, we'd recommend you keep cancellation on for the best overall audio quality.
Even at it's best, however, the X-Fi headphones only achieve average audio quality. They had good (read: low) distortion levels, and great isolation for active cancellers. On the bad side, they had weak frequency response, and leaked a lot more sound than we would've expected.
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About our testing:
For our performance tests, we use the same hardware and software employed by headphone manufacturers for their own testing. On the hardware side, we use a head and torso simulator (HATS). For software, we use SoundCheck, an electroacoustics analysis program developed by Listen, Inc. If you'd like to learn more about our audio quality tests, you can find more information here. To learn more about a specific test, you can click the orange information icon after each section's title.
Frequency Response (3.32) 
How the test works:
We first fit the headphones on HATS, then have SoundCheck play a series of frequencies through the headphones, which HATS listens to. These frequencies run from 100 Hz to 20 kHz. HATS listens to output, measuring the decibel level of each frequency. HATS then relays the data back to SoundCheck, which very thoughtfully plots it all on the graph below. As you can see in the legend, the green line represents the left channel, and the red line represents the right channel. The dotted black lines represent the limits the channels' frequency responses should fall between. If you'd like to know more, click the orange information button above.
What we found:
The first thing we noticed: the headphones perform very differently with noise cancellation off than they did when it was on. After we ran the test a bunch of times with it on and when it was off, we noticed that, in terms of the way we score, there wasn't too much of a difference between them. The results with noise cancelling on were slightly better, so we used those figures and graphs for scoring. In this section and the distortion score section, we've also provided a graph of the headphones' performance with noise cancellation turned off. The graph on the left has noise cancellation turned on, the graph on the right has it turned off.
As you can see, with noise cancellation on, the overall frequency response is significantly quieter than when it's off. We're assuming this is partially because, with cancellation off, your playback has to compete with far more external noise, and Creative didn't want their headphones to be ill-prepared. Whatever the reason, switching noise cancellation off will give you a far more dynamic -- in fact, a little too dynamic in the bass -- frequency response. The bass starts out overly emphasized, then dips back down to a good level of emphasis, then tends to get a bit over-zealous towards the high end. With noise cancellation on, the line is far more flat, slightly underemphasizing anything from the middle frequencies on up. If you're looking for a more dynamic sound, keep the cancellation off if you can. Of course, if you think keeping the cancellation off is a sure-fire way to get better audio quality, you'd better read our distortion section below (or click here if you are a lazybones).
How the Creative Aurvana X-Fi compares:
Most of the headphones below, noise-cancelling or not, performed poorly. Relative to other noise-cancellers, the X-Fis' score isn't bad. Compared with the HD 555s, which lack cancellation but have good audio quality overall, there isn't much of a difference. The Pioneer SE-A1000s trounce the X-Fis firmly, however, with a really controlled frequency response while still adding emphasis to the bass and some high frequencies. Also, the SE-A100s' left and right channel both have a very similar response; the Aurvana X-Fis and, to a much greater extent, the Bose QC2s both have some issues with the channels offering the same frequency response.
One important factor to keep in mind is that, although the X-Fis' frequency response was a bit low, it wasn't erratic. First of all, slightly quiet playback shouldn't be a huge issue with noise cancelling turned on, since there won't be as much interference from external noise. Seconly, since the line is more-or-less flat, all you need to do is bump the volume up a bit for an even frequency response.
Distortion (4.81)
How the test works:
In this test, we look for the differences between a sound coming out of the headphones and the original sound wave. What we do is strap HATS with the headphones, then play a known sound wave through the headphones that covers frequencies between 100 and 10,000 Hz. HATS records the playback, then ships the data to SoundCheck, which compares what HATS heard with the original sound. It then outputs some handy graphs, measuring the percentage of total harmonic distortion. If you'd like more information on this test, click the little orange button to the right of this section's title.
What we found:
Again, we've run this test both with noise cancellation on and off. Noise cancellation might have screwed up frequency response, but it certainly fixes a lot of distortion. We would've expected the noise cancellation to cause more distortion, since it adds extra sound waves to the mix and messes around with which noises actually make it to your ear.
Without noise cancellation on, there's a minimum of about 0.5% distortion throughout the spectrum, with a few peaks that approach a noticeable 3%. If you were to listen to a track with a pair of low-distortion headphones, then switch to the X-Fis sans cancellation, you'd notice the difference.
These results, coupled with the results of the frequency response test above, show the Aurvana X-Fi headphones are going to fall short in some way, regardless of the power switch's position. This isn't really much of an issue, however, as long as you know which setting to use when. If you're on the go, switch noise cancellation on and bump up the volume a bit. You'll get low distortion, good isolation, and a slightly flat response (whether that's a good or bad thing is up to the listener). If you're at home watching a movie, keep cancellation off. You'll get deep explosions, sibilance and other high-pitched sounds won't get lost in the action, and the distortion won't matter as much.
How the Creative Aurvana X-Fi compares:
The Aurvana X-Fis received an average score on this test when noise cancellation was on. If the feature were switched off, they would've gotten a downright terrible score. We have seen better performance from noise-cancelling headphones, but unlike these comparison headphones, the Aurvana X-Fis have the ability to turn their cancellation on and off.
Tracking (5.15)
How the test works:
Again we play a frequency sweep through the headphones, this time from 20 Hz to 20 kHz. What we're looking for are points where one cup is louder than the other. We score this section based on the most pronounced shift from left to right; gradual changes of a few decibels won't ruffle anyone's feathers. Again, we don't score the extreme low or high end, since the results aren't 100% accurate, but we show them anyway as a means of showing the general trend.
What we found:
The Aurvana X-Fis have average tracking. In the bass it looks like there's a weird jump towards the left ear cup, which means either the right ear cup was a lot quieter or the left ear cup was crazy loud. Either way, after that blip the graph evens out a bit, with the volume slowly meandering towards the right channel. Towards 1 kHz the volume quickly shifts towards the left, but only by a handful of decibels. Afterwards the tracking gets a bit erratic. Fortunately, none of the swings are particularly noticeable: the sharp ascents and descents only jump a few decibels at a time.
How the Creative Aurvana X-Fi compares:
This is another test with average performance. While the tracking certainly wasn't perfect, compare the X-Fis' result to the QuietComfort 3s'. The biggest shift the X-Fis showed was a 6-decibel shift before 1 kHz; the QC3s had a 20-decibel jump, plummet and jump within a very short spectral range. All the other headphones featured slightly better tracking than the X-Fi since they had smaller, less noticeable jumps from the left channel to the right.
Maximum Usable Volume (8.70)
How the test works:
To test the maximum usable volume, we basically perform our distortion test at varying volume levels. As volume increases, so does distortion. We increase the volume until the headphones output 3% distortion, which is a noticeable amount.
What we found:
The Creative Aurvana X-Fi headphones were capable of outputting 114.55 dBSPL of music with acceptable distortion levels. This isn't bad: we award maximum points for 120 decibels, which isn't too far off from what the X-Fi was capable of.
Isolation (7.97)
How the test works:
Isolation refers to how well the headphones can stop external sound from getting to your ears. To test how well headphones isolate their user, we blast HATS with some pink noise and see how much of it get through the headphones and into HATS's ears. The graph below shows the total amount of of sound the headphones were able to block out at each decibel level. The green line represents the passive cancellation, or, basically, how well the headphones act as ear plugs: no fancy electronics, just a solid object covering your ears. The blue line is active cancellation, or when the headphones shoot incoming soundwaves with inverse soundwavese, essentially cancelling out the incoming noise.
What we found:
Like most noise-cancelling headphones, the fancy electronics really only give you a boost in blocking out bass frequencies, but actually create more noise in the middle frequencies. In terms of total sound cancelled out, the X-Fi headphones did well. They would definitely serve you well on an airplane ride.
How the Creative Aurvana X-Fi compares:
The X-Fi headphones beat all comers on the battlegrounds of active noise cancellation. The two non-cancelling headphones below also have semi-open backs and cloth padding, which is notorious for not providing a good seal with the ear. Although the amount of bass blocked out is impressive, the X-Fi, even as the new site active-cancelling champ, can't outperform a good set of in-ears. Etymotic Research's 6isolators weren't as good at blocking out bass, but they blocked out quite a bit more high-end noise.
In terms of headphones for a commute by bus or train, however, we think the X-Fis are the best choice.
Leakage (3.71)
How the test works:
For leakage, we set a microphone 6 inches from HATS's ear, then put the headphones on HATS. We pipe some pink noise through the the headphones go on HATS, and a microphone is placed 6 inches away from HATS's ear. We then play back pink noise through the headphones. If any of the pink noise leaks out, the microphone picks it up.
What we found:
First of all, contrary to what seems logical, noise cancellation doesn't make a difference on the amount of sound the headphones leak out. This is especially important for those who are planning on using their X-Fis in a library, study lounge, or the quiet car on Amtrak. The X-Fi headphones may stop sound from getting in, but they let more sound out than you'd think. On a bus or other environment with a lot of external racket, the person next to you would probably be able to make out the song you're listening to. The music probably wouldn't be so loud as to be obnoxious, but it would be noticeable. If the room is quiet, even moderate volume levels would carry pretty far. Also, the active cancellation would block out others' attempts to shush you, making you seem like a callous jerk.
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