The ear buds are glossy black with white branding down the back, and the nozzles are covered with a metal mesh. The buds have a plastic leader that encases the cord for about a quarter inch, but no flexible cord guard. After the neck split, there's a little more than a foot of cord before the plug. There's also an extension cord included, which brings the total cord length up to about 52 inches (about 4 feet, 4 inches).
Inside the AH-C351's packaging, you'll find some fancy foam cutouts that hold various goodies in place. Among such treasures are: the AH-C351s themselves, two additional sizes of sleeves, an extension cable, and a carrying pouch.
For a pair of in-ear headphones, the AH-C351s seem solid. Like all in-ear headphones, the cord is fairly thin. While it feels solid, there still isn't a lot of insulation protecting your wires. The flex point toward the plug aren't all that great: they're really loose. Though they'll do a fine enough job, the loose fit could let sand or other hard particles in, which could hasten wear and tear. Also, while the ear buds have a plastic leader protecting the cord, it doesn't end in a flexible shield. This isn't ideal, because the plastic provides a hard edge that the cord can easily bend at. The neck split seems sturdy, and shouldn't rip, even if both ear buds are pulled forcefully in opposite directions. If it won't break the cord itself, then it probably won't affect the neck split. Again, while none of the issues mentioned here will outright disintegrate your headphones, they're all things that can reduce the total lifetime of your headphones.
One other thing small issue to keep in mind involves the sleeves. Though they're not so loose they fly off your headphones at arbitrary intervals, they do have a weaker grip on the nozzle than we'd like. If you're straightening out your cord after shoving it in your pocket, make sure a stray length doesn't get behind the sleeve and pop it off. Sleeves are very hard to find once they've escaped.
Like virtually all in-ear headphones, aesthetics is largely limited to the color of the cord and the back of the ear bud. Denon seems to know this, because the back of the ear buds have a sheen that's absent from the sides. These headphones look as clean and sharp as they can, given how much you can actually see. Also, the black coloration is preferable for those who don't want to wear the current mainstream uniform.The AH-C351 headphones are an in-ear offering from Denon. These headphones have a very short cord, which is perfect for hooking up to an arm-worn media player. This feature, coupled with ear buds that won't fall out easily, make the AH-C351s a good option for joggers. Also, if you prefer to keep your media player in your pocket, there's also an included extension cord that stretches your connectivity to a normal length for in-ear headphones. As you can see in the graph below, the low frequencies begin with a bit of boost. By the time it gets into the area we analyze, it continues its plunge downward, enjoying a brief stay within the realms of the limits. From there, it dips way below the bottom limit, where it eventually rounds off and juts back into acceptable territory. From that point on, the graph becomes a violent rollercoaster of unevenly emphasized frequencies. No one likes that kind of roller coaster. Now, even though the middle tones are underemphasized, the line remains fairly smooth until then (those minor jitters aren't perceptible to the human ear). The main problem is with high frequencies. When you have really steep lines, it means neighboring notes/pitches/frequencies can have a noticeably different emphasis. The sharp peak toward the end might mean spoken sibilance or cymbals can sound either normal or quiet, with the chance of suddenly shifting from one to the other.
For the most part, we didn't see any glaring problems with distortion. There's minimal distortion throughout, but it only jumps above 0.5 percent twice. These two spikes, which happen toward the high middle frequencies, are more than twice as big as the rest of the distortion blips. Even so, the overall distortion is very low. Evenly spread, minimal distortion is arguably better than a stretch of 0 percent with a gigantic, lone spike in it, since -- assuming you can even notice it -- it's consistent level will let you get used to it. A giant spike, on the other hand, will be jarring every time. Assuming you don't have ridiculously good hearing, or are an audiophile with OCD, you should have no problems with distortion.
The Denon AH-C351 headphones have really steady tracking. For the low frequencies, the right side is ever-so-slightly louder than the left, but it shouldn't be noticeable. Toward the middle, the emphasis shifts to a max of two decibels louder on the left. Again, with decibel levels so low, this change probably won't be perceptible. After the left side gets its chance in the sun, the volume moseys back to the right side for a bit, before it suddenly freaks out at the 10kHz range. Headphones will typically get erratic toward the high end, so this scribble isn't particularly shocking.
What we look for on this are any sharp spikes, like the ones on the right, only in the middle or lower end of the graph. Again, a steep slope means notes/sounds that are similar in pitch will be emphasized differently. In the case of tracking, this shift in emphasis will sound like an instrument has suddenly jumped from one side to another. Though many songs use this effect on purpose, if it isn't planned your playback will sound strange. Further, if you're familiar with the song, and know a trumpet doesn't teleport 50 feet to your left for every high C, it'll absolutely bug the heck out of you.
We found the AH-C351 headphones are capable of outputting 129.51 decibels (sound pressure level) before the high volume made distortion noticeable. This is a great result, if a bit excessive. Anything from 120 and 130 dB can cause permanent damage to your hearing over a sustained period, and anything louder than 130 is super duper danger zone. Bumping the sound up that high won't let you hear that airplane crashing down on you, or the 40-foot monster rushing towards you. Since we're really sick of reading about freak accidents that somehow involve the use of headphones, we strongly caution against listening to anything that loud.
Compared to other in-ear headphones, the AH-C351s didn't block out external noise all that well. It could block out about 10 decibels of bass, which is fairly minor, but it did improve from there. As per usual, the middle range was where these headphones blocked out the most noise. Toward the higher end, however, the headphones stopped blocking out as much sound, which is strange. Apparently this block of frequencies is just a weak spot in the AH-C351s' aural armor. In any case, these headphones protect better than just about any on-ear or over-ear, but not quite as well as most other in-ear options.
Even though these headphones might not be the best at isolating you from the outside world, they're great for keeping your music private. Sitting a few feet away from the headphones in a silent room, the pink noise -- which typically sounds like airplane ambiance -- sounds like a whisper. Good job, Denon.
This score is entirely our opinion, based on an hour of wearing the headphones. As is our standard comfort section caveat, we highly recommend trying out the headphones yourself if at all possible. We wore the AH-C351s for an hour, and found them to be comfortable. We ended up taking them for a jog, using our oh-so-trendy arm holstered media player, and were quite pleased - the ear buds stayed in and the cord didn't flap around. We didn't like using the extension cord as much, just because the plug-to-jack connection made the cord bulky and just a tiny bit heavier than we're used to. That being said, it's nothing we didn't get used to by the end of our first hour.
We didn't really notice much of a difference between the one-hour mark and the six-hour mark of wearing these headphones. This is impressive for in-ears, which can typically irritate your ears over periods of extended use. In this case, however, we think the comfort comes from the sleeves, which aren't particularly rigid. If this theory is correct, it might account for the lack of isolation we found in testing: a loose fit could mean a poorer seal, or thinner material, which would allow more sound in. Our crackpot theories aside, the AH-C351s can definitely be worn for extended periods of time.
With the extension cable, these headphones are about 52.25 inches long, or a little over 4 feet, 4 inches. This is a typical length for portable headphones, as it can typically jog from your ears to your pocket, and into your concealed media player. Of course, this length is with the extension cord; the length without it is slightly more than 2 feet. You might recognize this length as being ridiculously short for a cord. The reason for the short stretch of cable is to aid joggers who have their media players strapped to their upper arm, as is the current fashion (also, no one wants their iPod smacking them in the thigh while they run).
All the jacks and plugs are 3.5mm, or 1/8 inch. This probably won't be a problem, since we doubt you'll be plugging these in to the1/4-inch-accepting stereo in your living room.
In terms of customizability, the AH-C351s don't have as many options as other in-ear headphones. There's only one type of sleeve (the rubbery bit that goes on the ear bud), which is a bit disappointing, but it does come in three different sizes.
The only other option is inherent in the extension cord. You can make these headphones a normal length, or shorten them if you prefer to wear your music on your sleeve.
These headphones are very portable. First of all, they're in-ear headphones, which means they don't take up a lot of room to begin with. Second, they're super short without the extension cord, allowing the perfect amount of room to reach that iPod strapped to your upper arm. We went on a few jogs with such a configuration, and we had no issues with a flapping cord or the buds slipping out.
Just about the only way you can clean/maintain these headphones is by removing the sleeves and cleaning them via your own methods. There aren't any cleaning tools, removable nozzle guards, or parts you can easily disassemble.
These headphones do not require batteries, which lets them pick up some easy points.
The Denon AH-C351 headphones give you a lot for what they cost. Their audio quality is above average overall, which is more than enough for the people who would buy these -- audiophiles probably wouldn't consider in-ear headphones anyway. These headphones are great for media player users, especially joggers, who will appreciate not having to tuck excess cord into their arm holster. Really, if you're looking to upgrade your iPod headphones, the AH-C351 headphones are definitely worth the $50 investment.
The Denon AH-C351 headphones are definitely a pair of in-ears to consider. They offer good audio quality, a great wear experience, and look good as well. We had no problems with the headphones not being comfortable, but having a mandatory extension cord can sometimes be annoying. Also, we would've liked to see better isolation from a set of in-ear headphones. Keeping in mind these headphones aren't perfect, they certainly offer a lot of value for the $50 investment.
If you're looking for top-of-the-line audio quality, these headphones probably aren't for you. If you're just looking for a good quality upgrade to a pair of packed-in headphones, or need a pair you can exercise with, you should check out the AH-C351s.
Meet the tester
Mark Brezinski is a senior writer with seven years of experience reviewing consumer tech and home appliances.
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