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While the Beats do have good detailing, they don't have the best audio quality out there. They do have a good frequency response for the most part, with a slightly emphasized, well-rounded bass. It does have a strange lack of emphasis in the high-middle frequencies, however, which could make instruments in that range sound as though they quickly fade in and out. The Beats had a fair amount of distortion in the middle frequencies, which trail off into the higher end.
The beats also had fairly low isolation for a pair of active noise-cancelling headphones: there was very little difference between the active noise cancellation and just wearing the headphones with cancellation turned off. Also, the Beats tend to leak quite a bit. This is a bad trait for a pair of noise-cancelling headphones to have, since the active cancellation creates the illusion of isolation. This could lead to the listener thinking their playback is leaking far less than it actually is.
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About our testing:
Our testing rig is comprised of the same hardware and software manufacturers use to tweak their headphones. The hardware is HATS (head and torso simulator). HATS is shaped like the top half of a person, sans arms, and has powerful microphones in its ears. The software is SoundCheck, which we use to analyze the data captured with HATS. For more information on our tests, read this article.
Frequency Response (5.69) 
How the test works:
We arrive at our frequency response score by measuring how much emphasis the headphones put on each frequency. First we put the headphones on HATS. Next, we play back a frequency sweep, which runs through all the frequencies between 100 Hz and 20 kHz. Each frequency is fed into the headphones with the same power level, which means, ideally, the headphones would output each frequency at the same decibel level. HATS listens to what's actually coming out of the headphones and reports back to SoundCheck. The graph below shows how the headphones changed the original frequency sweep by adding or subtracting emphasis. The green line represents the left channel (ear cup) and the red is the right. The dotted black lines are the limits both lines should fall between. The bottom of the graph represents the frequencies we played through the headphones, and the left size of the graph represents the decibel level HATS measured.
What we found:
As we speculated in our first look article, the Beats have a relatively flat bass response that's bumping against the top limit. This means an even-sounding bass that's slightly emphasized. You can also see there was an odd plummet towards the high end, wehre the right channel dips below the bottom limit. Towards the high end, it looks like the Beats pick and choose different frequencies to emphasize, leading to an up-and-down scrawl that eventually drops off towards the extreme high end. This series of peaks might make high-pitched sounds seem like they fade in and out slightly, but it's nothing severe enough for most listeners to notice.
Overall, if it weren't for that weird plummit in the high-mids, the Beats would've had a great frequency response. As it stands, they're still above average.
How the Monster Beats compares:
Compared to most other active noise-cancellers we've reviewed, the Beats have one of the more stable frequency responses we've seen. They give a good boost to bass, but that seems to be a trend: the NC500Ds, QuietComfort 3s, and even the Aurvana X-Fi headphones (once you turn the volume up to compensate for their weak response curve) all add a steady emphasis to the bass. The Beats probably wouldn't be great for purists who want their music to come out of the headphones untampered, but their response curve is, for the most part, good.
Distortion (2.03)
How the test works:
To get our distortion score, we play a known sound file through the headphones which contains frequencies between 100 Hz and 10 kHz. We have HATS listen to the sound that comes out of the headphoens, and then compare the original sound file to what HATS picked up. Distortion refers to any difference between the original sound file and what ends up going into your ears. On the graph below, the green and red lines again represent the left and right channels respectively; the bottom line also denotes the various frequencies we tested. The left size of the graph represents the percentage of distortion present at any given frequency.
What we found:
When we initially listened to the Beats without music, we noticed the noise cancellation added a noticeable amount of noise. This is probably what that hump around 1 kHz represents. Regardless of the source of the distortion, there's quite a hump present in the mid-range frequencies that gradually trails off towards the high end. Also notice the right channel bump towards the high end: this corresponds to the sudden drop in decibel level present in our frequency response graph above.
How the Monster Beats compares:
Active noise-cancelling headphones typically don't have the lowest distortion. Since active noise cancelling involves injecting your playback with extra soundwaves, their relatively low performance isn't surprising. This being said, the Beats have more of a distortion problem than any of our comparison headphones. These are definitely not headphones for purists.
Tracking (3.34)
How the test works:
We test tracking by playing a frequency sweep through the headphones and again assign HATS with the task of listening intently. This time HATS is monitoring the decibel levels of each channel. The data bounces back to SoundCheck, which provides us with our graph. What this graph depicts is which ear cup was louder at each frequency and by how much. For perfect tracking, both channels would be playing at exactly the same levels. On the below graph, when the blue line rises above the zero line, it means the left channel is louder; below the zero line and the right channel is louder. The debcibel levels run down the left side of the graph, which the frequencies we tested span the bottom of the graph.
What we found:
The Beats have tracking problems. Things to look for that are bad include sudden, dramatic shifts in volume. Such shifts are often jarring to the listener. Again, you can see the frequency response graph's results present here: the first big shift towards the left channel corresponds to the sudden drop in the right channel's volume from the frequency response graph.
How the Monster Beats compares:
Well, the Beats didn't have the worst tracking out of all the headphones we've chosen to compare them to, but they came close. The biggest downfall of the Beats is that random drop it has in the right channel. That shift will be more noticeable than any shift on any of the comparison graphs, with the sole exception of the QuietComfort 3s' quick left-right-left change-up around the same frequency.
Maximum Usable Volume (9.07)
How the test works:
The maximum usable volume test is essentially a series of distortion tests. Each iteration we increase the volume, looking for the point at which distortion levels reach 3%. At this point, the distortion is noticeable and annoying.
What we found:
The Beats were capable of a pretty good audio level: 116.11 decibels. We award maximum points for 120 decibels, but being less than four decibels shy isn't that much of a difference. Those who like their music loud should be just fine with the Beats. Of course, one should take caution with anything over 100 decibels, since it's potentially harmful for your poor little ears. Remember kids, headphones can't make you deaf: they just help careless people make themselves deafer faster.
Isolation (4.65)
How the test works:
Isolation refers to the headphones' ability to seal a listener off from the outside world. We test this by placing the headphones on HATS, then bombarding both with pink noise (sounds like the inside of an airplane during a flight). HATS keeps track of the decibel levels of whatever noise makes it through the headphones, and SoundCheck pops out another helpful graph. We perform this test with active cancellation off and on, with the former being green and the latter blue.
What we found:
The Beats don't have the best noise cancellation. Typically the strong suit of active noise cancellation is in the bass frequencies. In this case, however, the active cancellation doesn't do much. It accounts for less than ten extra decibels blocked out in the bass frequencies. Anything higher-pitched than 1 kHz will get blocked out about regardless of whether the active cancellation is on. The active cancellation actually creates noise towards the high end, causing it to perform worse than if you'd just left cancellation off.
How the Monster Beats compares:
The Beats have really poor noise cancellation for a pair of noise-cancellers. The active cancellation accounts for such a tiny bump that it's hard to justify its inclusion -- especially since the mandatory battery life so limits the Beats' functionality. In any case, just look at the differences between the Beats and other headphones below. The only pair of headphones with weaker isolation were the Sennheiser HD 555s, and they have semi-open backs ("open back" means, instead of being totally enclosed with plastic, the outside of the ear cup is open to the air, letting sound in/out more easily).
Leakage (3.33)
How the test works:
For leakage, we play pink noise through the headphones when they're on HATS. We have a microphone set up six inches away, and record any of the noise that leaks out.
What we found:
The Beats are not headphones to take to the library. We were listening to music at a moderate level and someone a few seats away who was listening to their own music -- granted, they were using iPod headphones which aren't particularly good isolators -- and they could hear our playback clear as day. Typically people think of isolation and leakage as the same thing: if you can't hear anything from around you, you assume no one around you can hear your playback. This isn't true for active noise-cancelling headphones, because the cancellation happens artificially, within the ear cups. Keep this in mind, because if you're listening to your music at a moderate level on the bus, chances are the person next to you will be able to hear every note.
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