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About our testing:
We spared no expense when constructing our testing procedures in order to bring you the best science money can buy. Our opulent laboratory consists of two prized pieces of equipment: HATS and SoundCheck. HATS is shaped like a humanoid robot only with far better microphones in its ears than the average humanoid robot. HATS allows us to more accurately test how headphones will sound to a user. SoundCheck is an electroacoutsic analysis program that takes the data HATS gathers and spits out sharp-looking graphs. If you want to know more about our testing procedures, read this article.
Frequency Response (8.33) 
How the test works:
This test determines how much emphasis the headphones put on any given frequency. We put the headphones on HATS, then play back a frequency sweep from 100Hz to 20kHz. In this test, we play a range of frequencies through the headphones, into HATS's ears. HATS records the output, then sends it to SoundCheck, which compares the sound wave to the original. The graph below depicts how loudly each channel emphasized each frequency. The green line is the left ear cup; the red line is the right ear cup. The two dotted lines are the limits we score based on. If you'd like to read more information on this test, click the orange information icon to the right of this section's title, or underneath the graph at right.
What we found:
The Beyerdynamic DT 990 PROs had excellent frequency response. They boost bass ever so slightly, but the rest of the time they evenly emphasize sound. Towards the end the graph gets slightly more erratic, but manages to stay within the scoring limits. The 10kHz mark was ever-so-slightly overemphasized, but not by any great stretch. If you're looking for a dynamic response, you should look elsewhere. If you don't want your headphones tweaking your music for you, then the DT 990 PROs are an excellent option.
One thing to note about the graph at right: the left and right channels play almost the exact same emphasis. Typically one channel will be slightly louder at times or, on severely bad headphones, the two channels will veer off in wildly different directions.
How the Beyerdynamic DT 990 PRO compares:
The DT 990 PROs trounced just about all our comparison headphones. The SE-A1000s, which had a great frequency response, were the only challengers to even come close. The AH-NC732s, which are excellent active-cancellers, paled in comparison. Most of the over-ear headphones we've tested have been a bit erratic in their frequency response. While a dynamic frequency response is definitely preferred by some, the poor-scoring headphones below all underemphasized higher-end sounds by a significant amount. The DT 990 PROs don't have this problem. In fact, they really don't have any problems with frequency response.
Distortion (15.00)
How the test works:
Distortion refers to differences between the original sound file that is fed into your headphones and the sound they actually produce. We test distortion by playing back a frequency sweep and comparing the resulting sound to the original. On the graph below, the percentage of distortion runs down the Y axis while the range of tested frequencies stretch out along the X.
What we found:
The DT 990 PRO had virtually no distortion. There's not much else to say; the sound quality you'll get from these things is borderline pristine.
How the Beyerdynamic DT 990 PRO compares:
The DT 990 PRO has some stiff competition in this category, but manages to come in a very close second place. Realistically, however, the four headphones below with over 10 points would be utterly undescernable from each other. The Ultrasone HFI-2200s had a few issues with distortion, as did the Pioneer SE-A1000s. The HFI-2200s' distortion probably had something to do with their Natural Surround Sound feature creating artificial echoing in the lower end. The Pioneer SE-A1000s just had more distortion than the average set of headphones.
Tracking (5.95)
How the test works:
Tracking is a measurement of relative volume. Whenever both channels are outputting similar volume levels, they have even tracking. What we're looking for here is any point where one channel is louder than the other. This, obviously, is bad. On the graph below, when the line goes above zero it means the left channel is louder. When the line drops the right channel is louder.
What we found:
The DT 990 PROs had even tracking for the most part, but towards the high end the tracking gets a bit screwy. Tracking typically goes a bit haywire towards the high end, but typically that doesn't happen until after 10kHz. The shift isn't astronomical, so chances are you won't really notice it. Overall, the headphones did very well.
How the Beyerdynamic DT 990 PRO compares:
The DT 990 PROs had a middling tracking score relative to our comparison headphones. The Pioneer SE-A1000 only had small shifts from left to right and the Ultrasone HFI-2200s remained relatively flat until it was out of our scoring zone (lucky, that). Again, this score is based on the largest shift exhibited; emphasis that slowly meanders from the left channel to the right won't be noticeable.
Maximum Usable Volume (5.79)
How the test works:
Maximum usable volume refers to the highest decibel output the headphones are capable of without exceeding 3% distortion. Once you've hit this level, boosting the volume any more will cause significant harm to your audio quality. We arrive at this decibel threshold by running a series of distortion tests at increasing volume levels.
What we found:
The DT 990 PROs were capable of an average output: 102.33dB. This isn't a bad output for a set of open-backed over-ears, because if you're using them, you're probably in a quiet enough environment.
We award maximum points to 120dB of output, but any more than that is dangerous and we simply can't recommend it. If you even go over that limit by a tenth of a decibel, we will send a note to your parents.
Isolation (1.38)
How the test works:
What we found:
No surprises here. The DT 990 PROs are open-backed headphones, so they don't isolate particularly well. If you're going to be in noisy environments, toss on a pair of in-ears or active noise cancellation headphones. We were actually surprised at how little noise these headphones blocked out; you won't notice much of a difference between wearing them and taking them off.
How the Beyerdynamic DT 990 PRO compares:
The DT 990 PRO actually doesn't lose this comparison, since there are many other open-backed headphones listed below. Surprisingly, the DT 990 PROs blocked out more sound than their open-backed brethren. It lost to the ATH-A700s, which are closed-back over-ears. The Denon AH-NC732s are active noise cancelling headphones, so they of course trounced the competition. Again, if you're looking for headphones to take with you on your commute, do not get open-backed headphones like the DT 990 PROs. Stick with in-ears, like the Shure SE420s or active-cancellers like the Denons.
Leakage (2.52)
How the test works:
Leakage refers to how loud your muic will sound to someone standing close by. Since these are headphones and not gigantic 80s-era boomboxes, quiet headphones get the most points. We test leakage by piping pink noise through the headphones while a microphone a few inches away listens.
What we found:
Again, not a surprising result. The DT 990 PROs did not have good leakage control. If you need to keep quiet, they aren't the headphones for you. Even at a moderate volume a neighboring cubicle could hear your playback. Again, this poor score is relatively meaningless for a pair of over-ears: they're meant to be used in a personal listening environment, where disturbing others is not a concern. If you want to keep your music private, your best bet is a set of in-ears.
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