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About our testing:
Our testing rig consists of the very same hardware and software that manufacturers use for testing audio quality. The hardware is, literally, a hearing machine. HATS, our head and torso simulator, has a humanoid shape to it and precise microphones installed in its ears. For more information on our tests, read this article.
Frequency Response (4.02) 
How the test works:
This test measures the emphasis the headphones place on any given frequency. Once we outfit HATS with the headphones, we play back a frequency sweep that spans everything from 100Hz to 20kHz. Each frequency is played at the same decibel level. HATS listens to the playback and reports a detailed map of what it heard. SoundCheck then interprets that data and gives us a nifty graph of how the headphones emphasize each frequency. The green line is the left channel, red is right, the dotted lines represent the limits of the area we score, the left side of the graph is decibel level, and the bottom of the graph runs across the frequencies we tested. For more info, click the orange "i" button above.
What we found:
The Shure SE420s had a pretty good frequency response result. As you can see, these headphones put some emphasis on the bass frequencies but trail off towards the higher end. The lines dip a bit below the lower limit here, but overall the line progressed evenly. If there were any sharp peaks or long furloughs far outside the limits, then it would have been an issue. As it is, the Shure SE420 wasn't perfect, but still performed pretty well.
How the Shure SE420 compares:
The SE420s' managed to score ever so slightly above average here compared to the comparison headphones. As you can see, the SE420s managed to perform better than the SE210s, although the general shape of the curve was about the same. The MM 50 iPs scored a bit better, because they barely strayed outside the scoring limits at all (the bass was a bit loud, but we don't score on the extreme low or high end). The other set of headphones that did better than the SE420s was the 6isolators, which had a very even kiel to their frequency response. This is great for music purists who want a faithful representation of their music. Again, overall a good showing from the SE420s, but not particularly outstanding.
Distortion (3.82)
How the test works:
Our distortion test checks for any differences between the original sound wave and whatever's coming out of the headphones. We again run a frequency sweep through the headphones, HATS listens, and SoundCheck spits out a graph. The green and red lines again represent the left and right channels respectively, and the bottom represents the frequencies we tested. The left side of the graph, however, represents the distortion levels. The big number on this test is 3%: this is the distortion level at which you'll really notice something is off. Ideally the both channels would just sit along the zero line.
What we found:
While it wasn't terrible, the Shure SE420s did show a bit of distortion. Most of it was in very low amounts across the whole spectrum, but it did have a small bump around the 1kHz mark. Overall, it stayed below 1% the entire time, which means it'll be less noticeable than a brief spike in distortion.
How the Shure SE420 compares:
The only set of headphones the SE420 had less distortion than were the Audio-Technica ATH-ANC3s. If you check out the graph you can see why. The other headphones might be less obvious due to the small increments involved and the minute size of the thumbnails. You can click the links to check a larger graph and our full review, or you can trust us when we say that all the other graphs have lines that run closer to the origin. Most have slightly bumpier lines, but the bumps are brief and, most of the time, involve a lower distortion level than the SE420 at its max.
Tracking (10.10)
How the test works:
Our tracking test looks at the relative volume of the cups. If they're playing some sounds louder than others, that can be an issue; if one cup is playing some sounds louder than the other cup, that's a separate issue entirely. We again send our frequency sweep through the headphones, and SoundCheck measures each instance where one channel was being played a bit louder than the other. One thing to keep in mind: the extreme left and right of this graph aren't totally accurate and aren't calculated towards this section's score. We leave in those bits so you can get a good picture of what happens in these areas, but don't treat the actual decibel measurement as gospel.
What we found:
The SE420s had great overall tracking. Again, what we look for here are any sharp hikes or plummets. We don't score the far high or low end, which renders those spikes around and after 10kHz moot. The response is very, very flat across virtually the entire stretch, meaning the audio will be very balanced. In the results pictured at here, the right channel was about 2dB louder virtually the entire way. This imbalance is so small it's almost insignificant, and could likely just be remedied by adjusting the ear buds.
How the Shure SE420 compares:
The Shure SE420s had a very flat tracking score. It managed to do better than just about every other set of the comparison headphones. Some of the headphones pictured below have seemingly flatter graphs, but keep in mind that these graphs are normalized: there was probably some brief spike that got flattened out somewhere along the line. Really, though, anything above 8 is really well balanced, to the point where the typical users probably wouldn't notice. The Shure SE420s simply manage to be better than they need to be.
Maximum Usable Volume (10.00)
How the test works:
This test is essentially a series of distortion tests. In it, however, the volume is increased after each pass. As volume increases, so does distortion. We keep bumping the volume until we find the point at which any more juice will result in 3% distortion. Although you technically can use volumes that exceed this number, the music will sound a bit blown out and the audio quality will be noticeably diminished.
What we found:
The headphones were able to squeeze out 122.54dB before the distortion got bad. This garners the SE420s maximum points for this score. Any more than 120dB is dangerous to anyone without pre-existing hearing problems. It can also have adverse affects on your headphones. In the SE420s case, however, anyone who likes it loud will be very pleased.
Isolation (10.12)
How the test works:
Our isolation test is a bit different than our other tests. Instead of bothering with frequency sweeps, we simply bombard HATS (and the headphones it's wearing) with pink noise. Pink noise is what you get when you play each frequency at a decibel level that's proportional to its frequency. This means bass sounds will be played louder than high-frequency sounds and, in the end, will result is each frequency being played at equal power. It sounds a lot like airplane ambience. The graph below depicts how much of this noise at each frequency was blocked out.
What we found:
The Shure SE420s have an amazing capability to drown out external noise, partially due to their wide selection of sleeves (the bits that stick onto the ear bud to protect it from the nastiness that hides in your ears). We tested out the three types of sleeves and found that, contrary to our initial beliefs, the foam sleeves were able to block out more sound than our previous pick for best isolators, the triple-flange sleeves. This makes sense, because actual ear plugs nowadays are made of foam, not little plastic Christmas trees. To the triple-flanged sleeves' defense, however, they came in a very close second place.
In any case, these foam ear buds were able to block out an impressive amount of noise. Like most in-ears, the majority of this blockage happened towards the higher frequencies, but it did block out an impressive chunk towards the lower end too.
How the Shure SE420 compares:
The Shure SE420 is, at the time of this writing, the best set of headphones for isolation. The Etymotic Research 6isolators, one of our first reviews, has finally been dethroned, and active-cacellers out there lie prostrate before its attenuating might. The SE420s are excellent for blocking out sound.
Leakage (10.00)
How the test works:
We test leakage by playing pink noise through the headphones at a set decibel level. We have a microphone set up six inches from HATS that will record anything that manages to leak out.
What we found:
These headphones barely let out a whisper. You should have no fear of anyone ever hearing your playback. These headphones would be great for hipsters that don't want to continually say they're listening to Chiptunes or La Bouche ironically.
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