There's no getting around the fact that the K390's noise cancellation module is rather hefty in both size and weight. Luckily, AKG built in a shirt clip to alleviate concerns raised by the latter (so the earbuds are not pulled out of the ear by the weight). However, the module measures almost 3 inches across and it's an inch tall and a bit more than 0.5 inch deep, so it's definitely not the most inconspicuous thing to have attached on your person. The top edge of the unit houses an on/off switch for the noise-canceling feature as well as a mute button that allows you to hear what's going on around you without removing the earbuds. A single AAA battery is concealed beneath a flap on the bottom side. This is also where you attach the included stereo patch cable for connecting your MP3 player or other audio source. It's nice that the cord is detachable as this provides a more clutter-free method for utilizing the noise cancellation as a standalone feature (on an airplane, for example).
As mentioned, AKG has been in the audio game a long time, so we went into the K390's testing with high expectations. Overall, we were pretty impressed by the sound offered up by the earphones, particularly the fact that it changed very little based on whether noise cancellation was activated. The K390s manage to put forth a nice, crisp response with plenty of detail on the high-end, yet they were not too bright for our tastes. Mids weren't as remarkable; though music sounded reasonably warm, it wasn't quite as rich or buttery as we like. Bass response was a bit of a mixed bag. On the one hand--if you can get a good seal with the ear--the lows are nice and tight without being overwhelming, but once you turn on the noise cancellation, the low-end loses a bit of its "oomph." Picky listeners, however, may be able to find more suitable options.
As far as noise canceling is concerned, we have some mixed reviews. Although the earphones do a reasonable job of overcoming the hum of the server in our office, the antinoise created a significant amount of audible white noise during silence. We're not sure that this is any better than listening to a jet engine or the rumblings of a chain--it's all in the ear of the beholder.