Finding a really good pair of on-ear headphones can be a challenge. While they're more compact than over-the-ear models, they often aren't as comfortable -- they sit on your ear after all -- and don't measure up sound-wise to their over-the-ear brethren, which also tend to do a better job sealing out ambient noise (at least the closed-back models do anyway). So it's easy to get excited when you come across an on-ear headphone like the Beyerdynamic T 51 p that not only features top-notch sound but is sturdily built and comfortable to wear.
Made in Germany, the $289 T 51 p is a dead ringer for Beyerdynamic's earlier T 50 (now discounted to $199), but the newer model has upgraded innards and sound. The lightweight (174 grams) mostly metal design doesn't have a hinged headband, but the ear cups fold flat for easy storage in the supplied, well-designed carrying case.
If you're looking for a headphone that has an inline remote and microphone, Beyerdynamic makes the T51i, which has a 3-button Apple remote control and microphone (that model costs about $10 more). However, this "p" version is intended for the serious audiophile who doesn't want to ever lose focus on the music. Impedance is rated at a highish 60 ohms, but the T 51 p sounded great plugged into both mobile devices (iPod Classic, iPhone 5S, and Samsung Galaxy S4) and a Schiit Asgard 2 headphone amp at home.
The earcups are padded with memory foam and they sit your ears snugly without applying too much pressure. The headband conforms to your head nicely and doesn't leave a big gap in the temple region like a lot of headphones do. It's a comfortable headphone to wear for hours at a time without experiencing listening fatigue. That said, the cold metal design isn't for everyone and those with larger heads may end up with the metal part of the headband resting too close to their heads for comfort.
Isolation from external noise on the New York City subway was decent, though not up to the standards set by noise-canceling headphones and some over-the-ear models. Then again, the T 51 p sounds better than any of those headphones and it doesn't need batteries (all noise-canceling headphones do). While most audiophile headphones come with one-year warranties, the T 51 p's runs two years.
The sound balance is slightly warmer than neutral, but detail and resolution are excellent. We found it pretty versatile -- it doesn't lend itself to one genre of music over another -- and for such a compact headphone its bass is surprisingly robust. It's not overdone or thick, but if you crave a rich low-end the T 51 p won't disappoint.