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About our testing:
Our testing rig consists of hardware and software used by manufacturers. Our hardware is a head and torso simulator, or HATS, which is pretty much what it sounds like it is. It looks like a mannequin from the waste up (minus the arms) and uses high-sensitivity microphones in its anatomically correct ears. Our software is SoundCheck, by Listen, Inc., which is an electroacoustic analysis program that lets us produce and examine soundwaves. For our testing, For more information on our tests, read this article.
Frequency Response (2.28) 
How the test works:
For this test, we put the headphones on HATS and play back a frequency sweep (100 to 20,000 Hz), where each tone is being played back at a known decibel level. HATS listens to the playback through the headphones and sends back information on what it heard. Soundcheck uses the data to figure out how much emphasis the headphones put on each frequency. The left side of the graph is the decibel level, the bottom axis is the range of tested frequencies, and the dotted lines are the limits. If you'd like more info, click the orange icon above.
What we found:
The Denon AH-NC732's frequency response has some issues towards the higher third of the graph. The bass is emphasized, but not emphasized too much. At around 7kHz, the graph takes a huge plunge. This is the frequency you'd want to equalize up if you like drums, so this dip might take some of the punch out of drum-heavy songs. After that initial downward spike, the graph rescues itself for a very short period of time, only to fall back below the lower limit.
One thing you should look out for on these graphs are sharp lines. That means that an instrument straddling that frequency range can have a dramatically different emphasis for different notes. The sudden dip down around 7kHz and the subsequent zig-zagging means that things toward the higher end might seem a bit inconsistently emphasized.
Again, the AH-NC732s have a fine response for the lower frequencies, but tend to fumble higher ones.
How the Denon AH-NC732 compares:
The average score for this section is presently 4.01, so the Denon AH-NC732s are a bit worse than average. They aren't the worst of the headphones below, but that's not saying much: the QC3s fall off earlier and further.
Looking at the non-QC3 comparison headphones below, it's obvious why the AH-NC732s underperformed. Those that do stray outside the limits go nowhere near as far off their mark as the AH-NC732s (again, with the exception of the QC3s). The 6isolators have a very flat response and only dip slightly outside the limits in a very small area. It's intersting to note that most of the active cancellers (the AH-NC732s, MDR-NC500Ds, and QC3s) follow more or less the same trend of a good bass response but an underemphasized higher end. The exception to this rule is the Creative Aurvana X-Fi, so the previous three headphones can't blame their poor performance on some flaw inherent in active cancellation.
Distortion (15.90)
How the test works:
This time we run the same frequency sweep through the headphones, only this time we're measuring any differences between the original soundwave and what HATS ends up hearing from the headphones. Any differences are represented in the graph below as a percentage of distortion present at any given frequency. Anything below 3% is negligable. Again, we play a frequency sweep through the headphones, this time from 100 Hz to 10 kHz. Since the frequency sweep is a known sound wave, we can measure the difference between it and what HATS ends up hearing. This difference is what's known as total harmonic distortion. Lower levels of distortion will only annoy an audiophile, but even a layperson's ear will find anything over 3 percent to be noticeable. In the graph, the left side represents the percentage of distortion, and the bottom of the graph represents the frequency spectrum we tested. If you'd like more info, click the orange "i" above.
What we found:
The Denon AH-NC732 headphones have virtually no distortion when noise cancellation is turned off. As you can see, the line barely ever raises above zero for the entirety of the frequency spectrum.
When it's turned on, however, it's an entirely different story. That bump in the middle almost reaches 3%, which is almost distracting. If we were to score based on this performance, it would've gotten a 1.16, which would have been the worst distortion score we've assigned by a little over half a point.
Unfortunately, this distortion spike is just something you'll have to deal with if you're listening to an active-canceller. Subpar sound quality comes easily when the headphones are injecting soundwaves into your music that doesn't belong there. What we'd recommend doing is shutting off the noise cancellation when you don't need it. Chances are, if you do, you're in a loud enough environment that the distortion will seem like a natural byproduct of the ambient noise.
Of course, this means the AH-NC732 has the distinction of having both the best and worst distortion scores, if only for now.
How the Denon AH-NC732 compares:
Of course, the AH-NC732, as our current high score topper, blows away the competition. As good as the other headphones here are, they all have more distortion than the AH-NC732s.
Tracking (3.22)
How the test works:
To test tracking, we analyze our frequency sweep playback and measure the relative decibel level of each ear cup. The graph below shows any differences in the relative volumes of the left and right channel. If the left channel is playing louder than the right, the line will swing up; if the reverse is true, the line will dip down.
What we found:
The AH-NC732s had a rocky tracking test. It doesn't have any huge swings throughout the main part, but this is a normalize graph and that short downward spike after 1kHz looks like it might've actually plunged down a bit further than this graph shows. Also, towards 10kHz the graph bounds upwards, meaning the volume swings dramatically to the left channel. After that point it gets a bit scribbly, but that's typically what happens on the high end. While there aren't any drastic pulls either way on this graph, it's far from smooth. This being said, even the drastic swings are less than a 10 decibel shift, which isn't particularly noticeable.
How the Denon AH-NC732 compares:
The Denon AH-NC732 has a wobbly tracking graph. Just by eyeballing the tiny little graph thumbnails below it's apparent it didn't do too well. Again, we don't score on the lower or higher end of the graph since the data isn't 100% accurate. The only pair of headphones that are comparably rocky are the Aurvana X-Fi headphones, but even they don't jump to the height that the AH-NC732s do.
Maximum Usable Volume (6.81)
How the test works:
This test is actually a series of distortion tests. Our distortion test above plays back a frequency sweep at a set decibel level. On this test, we keep bumping the volume up until we reach 3% distortion. This level of distortion would be noticeable and annoying.
What we found:
The Denon AH-NC732s are capable of outputting 106.62 decibels, which isn't great. Most headphones can output 110-120dB, which is what we award maximum points for. Anything louder than that is damaging. Reaching 106dB isn't bad, but it might not be enough for those who like it loud.
Isolation (7.14)
How the test works:
For this test, we throw the headphones on HATS, then blast them with pink noise (low frequencies are played louder than high frequencies to keep everything at about the same energy level). HATS listens for the sounds that get through the headphones and their fancy noise cancellation. On the graph below, the blue line is when active cancellation is switched on and the green line is when the active cancellation is switched off.
What we found:
The main selling point of the AH-NC732s is their active noise cancellation. Denon touts it as capable of cancelling out 99% of external sound, which is a figure vague enough to essentially be meaningless. Interestingly enough, contrary to the norm in such oversteps of marketing, the AH-NC732s aren't bad. In fact, they have decent cancellation for a set of active-cancellers. They're currently our second best set of active-cancellers, after the Creative Aurvana X-Fi headphones.
Like other active-cancellers, the AH-NC732 mainly blocks out bass frequencies, and like other active-cancellers, the cancellation actually creates a bit of noise towards the middle frequencies. Again, this isn't as good as a good set of in-ears, such as the Etymotic Research 6isolators, but they represent the current upper crust of active cancellation.
How the Denon AH-NC732 compares:
For the majority of those reading this review, this section is the most important one. Exactly how well does the AH-NC732s' noise cancellation do, compared to other competitors - or, for that matter, how well does active noise cancellation even work to begin with? If you find yourself asking that question, then we certainly have an exciting array of graphs for you. Again, the blue lines are active cancellation, green lines are passive. No blue line means no active cancellation.
First of all, good ol' earplugs seem to beat noise cancellation in terms of overall isolation, as evidenced by the 6isolators. If you're looking for some spot reduction on bass, however, then active cancellation is something to look into. In this regard, the AH-NC732 headphones score between the QuietComfort 2 and 3. The QuietComfort 2s form a poor seal with the head (the padding is fuzzy and about as air-tight as a stuffed bear) so we weren't surprised to see the AH-NC732s win that match-up. It's very similar to the QC3s in terms of forming a tight seal with your head (both headphones use similar material on their pads), but the QC3s block out a bit more bass.
While the AH-NC732s and their noise cancellation aren't the worst out of the headphones we've reviewed, they aren't as good as the Bose QuietComfort 3 headphones, and both underperform versus a pair of in-ear headphones.
Leakage (5.88)
How the test works:
To test leakage, we put a microphone a set distance away from our headphone-outfitted HATS and then play pink noise through the headphones. The microphone then reports everything it hears back to SoundCheck.
What we found:
The AH-NC732 aren't the quietest headphones ever - on the contrary, they actually leak a bit more that you'd think, given their cancellation abilities. This being said, the AH-NC732 has the second-highest score in this category for a non-in-ear headphone (the Sony MDR-DS6000 currently occupies first place). This achievement doesn't say all that much for the AH-NC732, but it also doesn't say much for non-in-ear headphones in general. If you like listening to your music really loud, someone sitting next to you will probably hear it. If you're in a library or quiet office, everyone will glower at you.
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