Headphone Reviews

Sound Quality
Wow. Just great ... for a sealed headphone. Many in the headphone enthusiast world are going to bitch and moan: "But the Denon AH-DX000 sound so much better!" And they already have in this thread, but I don't think they're really getting it: these are sealed headphones! I agree that the Denon AH-D2000 is a slightly better sounding headphone with a more refined and properly articulate treble, but the Denons (D2000, D5000, and D7000) are far from sealed. I love recommending the D2000 to audio professionals because they are articulate and clear, but I always have to warn that they really don't seal very well...hardly at all, in fact. So the fall-back position for a really good sealed headphone seems to be the slightly biting Audio Technica AT-M50 or somewhat woolly Shure SRH840. Lately I've been turning to the Skullcandy Mix Master as a top-end sealed can, but it's not very refined. None of them really turn the trick.

The AKG K550, on the other hand, is a very well balanced sounding headphone, and, to my ears, bests the above mentioned cans in terms of overall performance. They're nice and tight in the bass, where I find the SRH840 woolly, and the Denons a bit loose. The midrange clarity is very good and quite even for a sealed can, presenting the upper mid-range/low treble beautifully and giving voices their full throat. The best of the bunch here.

The only place they stumble is in the treble's upper registers where they're a bit "zazzy" and indistinct. While the high notes aren't biting, they're also not well enough enunciated to bring out the natural textures of cymbals and brushes on the drums. If you're a mixing or mastering engineer that can live without good isolation, the more refined treble of the Denon D2000 is the way to go, though they tend to be a tad bright up top. The more expensive Denons are, well, quite a bit more expensive, but somewhat better sounding. (The diminishing returns curve is hard at work here.)

Some will want a comparison with the AKG K701--a bass shy, but otherwise excellent headphone. I think the K550 is a remarkably different beast, bringing far more heft to the low notes, a similarly good mid, and not quite as competent treble, but overall a significantly more balanced and enjoyable listen for me.

Measurements

Some significant changes below 100Hz in the raw frequency response measurements show that these headphones have some difficulty achieving a repeatable seal. I think actual performance is worse than the measurements in this case because I worked fairly hard in achieving a seal each time. The difficulty, I think, is the measurement head is quite a bit smaller than most male heads because it's designed to be an average of both male and female heads, and I think that did come into play with these cans.

Normally, I might not work so hard at it, but I think AKG worked very hard to make a sealed headphone that didn't sound like a sealed headphone--and I think AKG succeeded. The price that needs be paid by the listener for AKG's effort is to get the seal right.

The spatially averaged and HRTF compensated (Head Related Transfer Function) frequency response to 1kHz show a headphone with a slightly warm tilt, and slightly uneven sound. Nonetheless, this is a good result for a sealed headphone, and, to my ears, sounds even better than shown. The downward slope from 1kHz to 5kHz is desirable, and right in line with what I'd like to see. The subsequent peak at 9kHz is observed with most headphones, and, though maybe slightly larger than I'd like, is probably fine. I didn't hear much of a biting or strident sound with these cans. The curve above 9kHz is remarkably free of peaks and valleys--not sure how AKG managed that, but it's probably a good thing.

Impedance and phase response plots show a headphone with little in the way of impedance changes, and are significantly free of artifacts that would indicate slip-shod design.

The 30Hz square wave shows a slightly uneven waveform top, which is also seen the the frequency response plot from 200Hz and below. The rising 90Hz THD+noise plot below 100Hz also indicates some modest difficulty keeping the bass tight. Compare the THD+noise at 90dB with the Denon AH-DX000 line, and you'll see the tightness of bass indicated here bests all but the D7000.

The 300Hz square wave shows a moderate second peak, and the impulse response shows a good bit of ringing subsequent to the initial peak. I did have trouble with the treble on these cans, and feel this measurement echoes my experience. If there's a weak point with these cans, this is it. Thankfully, there's no painful harshness here, just some indistinctness in the highs.

The isolation of these cans is good for a sealed headphone. Here is where these cans shine relative to the Denons. With broadband isolation at -14dB these headphones will isolate well, and with 55mVrms to achieve 90dBSPL will make for a reasonably good headphone for portable use--aside from the nine-foot cable being a bit odd in that application.

Overall, I'd say these cans sound somewhat better than they measure when a good seal is achieved.

Summary
On AKG's website they say:

With the K550s, AKG engineers have struck a masterful balance between the noise-isolating qualities of closed-back headphones and the spacious, dimensional sound of an open-back design--creating reference-class headphones ideal for private listening both at home and on the go.

Marketing-speak notwithstanding, I have to say AKG has indeed produced a sealed headphone with the performance usually found only in open headphone designs. This is the best sounding truly sealed headphone I've heard to date.

Achieving a good fit and seal is an absolute must with these cans as the sound quality achieved is very sensitive to a proper fit. If your head is small, you will have to bend the headband to get enough clamping pressure for best performance.

This is a very nicely styled and constructed headphone, which is also very comfortable for long listening sessions

Though they aren't quite as refined sounding as the Denon AH-D2000, they deliver slightly better bass, and somewhat better upper-mid and low treble performance, but they provide much better isolation.

At this point I'd like to give a big shout-out to Head-Fi member CantScareMe, who was among the first U.S. headphone enthusiasts to give these headphones a try. In the now quite long Head-Fi thread on the AKG K550, he chronicled his experience with these cans, and in his words one can read the difficulty and effort he put into finding his way through getting a good fit, which eventually netted him a very satisfactory listening experience. He then produced a terrific and well worth reading user review of the K550 here. Great job, dude!

The K550 will now be my go-to recommendation for audio pros and headphone enthusiasts alike needing both good isolation and solid audio performance. These will find a place on my Wall of Fame due to very good overall performance on all fronts. Highly recommended.

Resources after the video!!!

Resources
Head-Fi review and thread on the K550.
AKG U.K. K550 product page; hidden K550 page on U.S. site.
Very comprehensive and insightful history of AKG at Funding Universe
AKG K550 brochure.