Wireless audio has drastically improved over the last couple of years, and the astounding number of Bluetooth headsets is now being augmented with an influx of Bluetooth stereo headphones—intended not just for wireless phone calls, but high-quality, wireless music enjoyment. Sennheiser's MM 100, a $149.95 (direct) pair of Bluetooth headphones, is lightweight and comfortable, with built-in, on-ear controls that are very easy to master quickly. You also get solid audio performance, with articulate, rich bass and crisp highs, although some unfortunate distortion on deep bass tracks at high volumes keeps the rating down a bit. Still, if you don't listen to too much music with deep bass, or you listen to everything at moderate volumes, the MM 100  is worth your consideration—particularly if you're looking for a comfortable, exercise-friendly pair of headphones.

Design

A simple black behind-the-head band connects the MM 100's black-and-metallic earpieces, which have black foam, supra-aural (on-the-ear) earpads. The left earpiece houses the USB charging port, which is protected by a flip-out rubber cover. Buttons to control volume, playback, and to answer or end calls, are all located along the right earpiece's side paneling.

The MM 100's behind-the-head design is not for everyone, but I encourage the skeptics to give it a try. The earpieces have tiny spaces for the top of the ear to fit through, securing their position, and the headband just barely makes contact with your scalp. If you wear ponytails often, it might be pesky, but otherwise, this design seems ideal for the gym and other active scenarios.

Flashing lights are part of the Bluetooth design, and at least the lights on the MM 100 are not as blatant and attention-grabbing as they are on the SMS Audio Sync by 50 ($399.95, 2.5 stars) or the Outdoor Technology DJ Slims ($69.95, 4 stars). That said, those of us who wear glasses might encounter the annoyance of seeing the blue pairing status LED blink on and off—it is angled forward on the right earpiece, perfectly positioned to be picked up as a reflection inside the right lens of some glasses. Not a dealbreaker, though.

The MM 100 supports Bluetooth 2.0 + EDR, and the following profiles: A2DP, AVRCP, HSP, and HFP. A USB charging cable is included, as well as a wall socket charger that connects to the cable. Sennheiser rates the MM 100 for 7.5 hours of listening time on a single charge.

Performance

Sennheiser is known for its excellent headphones, from high-end home theater offerings to reliable, flat-response studio monitors, and even the more affordable, consumer-end models like the MM 100 provide a satisfying sonic experience.

At moderate volumes, the MM 100 sounds fantastic, with robust—but not booming—bass response providing the perfect amount of thump to kick drums and clarity to lower-register instruments. The mids and highs are never tinny, always clear, and the way the earpads seem to float on the ear also provides a sense of depth and space that in-canal earphones can only dream of. Of course, this looser, more open design also means sound leaks out—so that person sitting next to you on the bus may be able to hear your music.

At top volumes, deep bass tracks like the Knife's "Silent Shout" prove to be too much for the MM 100's drivers, and some noticeable distortion kicks in. This is not the case with typical rock or pop music that lacks deep bass, however—at full volume, PJ Harvey's "The Words That Maketh Murder" sounded crisp and clear without a hint of distortion. (Not that you should be listening at levels this high.) It should be noted that the volume on the earphones will generally operate independently of your Bluetooth device's volume—in this case, we're discussing a scenario in which they are both at maximum volume—not ideal for preserving your ears, but a worthy test of the drivers. The distortion only kicks in when the earphone volume approaches maximum, and again, only on deep bass content.

John Adams' modern classical piece "The Chairman Dances" sounds excellent on the MM 100—another piece that benefits from the sense of space the on-ear design provides. Lower-register instruments are conveyed with a very subtle low-end resonance, nothing is ever booming or exaggerated, but the large percussion hits do have a nice punch to them. The higher-pitched wooden percussion and higher register strings sound beautiful—clear, never harsh, and occupying their own unique spaces in the mix.

Button operation for volume adjustments and skipping tracks is pretty intuitive. My only minor complaint here is that it's fairly easy to rest your fingers on the track buttons when you're adjusting the volume—and if you place your resting fingers down too hard, you will both adjust volume and change what you are listening to unintentionally. Most on-ear controls offer similar obstacles, however, and the Call Answer/End button, which is also the Pairing button, is much larger and centrally located so you'll never fumble for the right control when receiving a call. Call clarity is decent, but it did seem rather quiet in some cases. My call partner could hear me clearly, but in noisier environments it was a bit difficult to hear the voice on the other end.

If the MM 100 was a wired pair, we might knock it a little more for its distortion, but for $150, this is a very solid pair of wireless headphones that are ideal for exercise. Still, if you listen to music with deep bass on a regular basis, you should consider an alternative, like the wireless Phiaton PS 20 BT ($149, 4 stars) and the Sony XBA-BT75 ($299.99, 3.5 stars), which do better on the low-end. If these models are out of your budget, the aforementioned Outdoor Technology DJ Slims  is a nice choice at just $70, but for those who don't listen to deep bass tracks at high volumes, the MM 100 is worth a listen.

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