There's a lot that can go wrong with noise-cancelling wireless headphones compared with a regular pair of wired headphones.
Even by themselves, each component is difficult to get right. Wireless connectivity can be short-range or buggy, and battery limitations mean the threat of your headphones suddenly dying on you is constant.
On top of that the noise-cancelling offered by different models of headphones can vary drastically, with the cancellation levels on cheaper sets barely surpassing the passive isolation offered by simply having a well-padded pair of cans covering your ears.
Finally there's the risk that all the effort and space dedicated to delivering decent battery life and wireless connectivity means there's little budget or materials left to offer a pair of headphones that actually sound great to listen to.
For the most part the Sennheiser PXC 550s do an admirable job of balancing this diverse set of requirements. They're not hampered by poor battery life or connectivity, and they don't sound half bad, even when connected over standard Bluetooth rather than the enhanced Bluetooth aptX.
There are however, a couple of niggles, as we'll see. So are the PXC 550's good points good enough to (no pun intended) cancel out the bad ones?
The Sennheiser PXC 550 headphones aren't bad looking per se, but they're certainly very conservative in appearance.
The two earcups are made of matte plastic, with a simple oval metal band on each which houses the microphones the headphones use for noise cancellation.
The bottom of the right earcup contains a couple of controls. A single button deals with both switching between the headphone's four equalizer modes and bluetooth pairing duties, while a slider enables you to choose between two different kinds of noise cancellation, or turn noise cancellation off entirely.
Also included on the bottom of the right earcup is a micro USB charging port, and a 2.5mm jack if you'd like to use the headphones in a wired configuration. I'd prefer to have seen a more standard 3.5mm jack, but thankfully 2.5mm connectors aren't exactly proprietary.
You might be confused by the absence of buttons on the headphones to control volume and track skipping, but that's because Sennheiser has opted to include touch controls on the right earcup.
Simply swipe up or down to increase or decrease the volume respectively, swipe left or right to skip forwards or backwards, or tap to play and resume your current track. We'll look at how well this control scheme works in the next section.
One other notable feature is the use of earcup rotation to turn the headphones on and off. Typically when storing headphones or hanging them around their neck users will rotate the earcups so the headphones sit flat, and this action turns the Sennheiser PXC 550s off.
It's a great little feature, and means you rarely leave the headphones on to waste battery when you're not using them. As a result I found that battery life was never an issue with the headphones, even when I used them on the daily commute for several days in a row without charging them.
Sennheiser advertises that you should get 30 hours of use from these headphones without needing to recharge them, and although I didn't precisely time the battery life I never found the PXC 550s running out of battery when I needed them.
One unfortunate omission, however, is the absence of an audio battery indicator. There are a series of lights on the underside of the right earcup, but these are easy to miss.
By way of contrast, the Bose QuietComfort 35s alert you to their battery level every time you turn them on, which means you're always aware when they're running low on power.
I started my listening tests by spinning up the recent release from the Red Hot Chili Peppers, The Getaway.
I was impressed with the way the PXC 550s handled the Chili's multi-layered sound. Vocals and instrumentations were nicely separated, and the soundstage was pleasant and broad.
Undoubtedly the headphones excel when it comes to the mids and trebles. The guitar work in Dark Necessities had a lovely clarity, and Anthony Kiedis's vocals were rich and full-bodied.
They're just as strong at the bass end of the spectrum, although these headphones don't give undue influence to bass notes. It's nicely balanced and crisp, but if you like your bass heavy then you might find these phones lacking.
Listening to Saigo no bansan by Mouse on the Keys, I wsa very impressed with how the PXC 550s dealt with the complex drumming. Every hi-hat beat had a lovely crispness to it, and this was mirrored across the entirety of the kit.
Throughout the course of my testing I also listened to a number of podcasts. This wouldn't be worth mentioning were it not for the four EQ modes, which work to boost or limit certain frequencies depending on what you're listening to.
You have a choice of 'Club', 'Movie', 'Speech' or 'Off', which leaves the equalizer at its default settings. I normally tend to steer well clear of these modes, as I find they introduce more problems than they solve, but this wasn't the case with the PXC 550s.
I found that on occasion, when using the headphones on the London Underground to listen to podcasts, their noise-cancelling wouldn't be quite enough to allow me to clearly hear the words without turning the volume up to an uncomfortably loud volume.
This problem was fixed dramatically by switching to the 'Speech' EQ mode, which boosts the mid and treble frequencies at which the human voice tends to operate. Using this mode I was able to comfortably listen to podcasts, even at much lower volumes.
Of course, this wouldn't have been necessary had the PXC 550's noise-cancelling mode worked better. It's not completely ineffective, and it deals very well with low background hum, but when you're faced with background noise like that in an underground carriage, which varies a fair amount, it starts to struggle.
Although two different levels of noise-cancellation are available, I could discern very little difference between them.
I also found that the PXC 550s had trouble dealing with a bus or plane, which would create a lot of vibration as well as noise – this would seep into whatever I was listening to, and add an unpleasant warbling effect to my music. It didn't make it unlistenable, but this wasn't a problem I had with the Bose QuietComfort 35s.
Problems with the noise-cancelling aside, my single biggest gripe with the Sennheiser PXC 550s is with the touch controls.
They just about work if you're looking to use them to pause your music, but I found them frustrating to use for anything more complicated.
Adjusting the volume normally required a fair amount of trial and error, and attempting to skip tracks would occasionally activate the volume controls instead.
I appreciate that the three buttons on the bottom of the QuietComfort 35s aren't sexy or stylish, but they're practical and, unlike Sennheiser's system, they work 100% of the time.
What stops these issues being deal-breakers is the fact that the PXC 550s sound superior to their more user-friendly rivals.
Take a classic like Bob Dylan's All Along the Watchtower. Every part of the track comes together and sounds wonderful through these headphones.
Even the fairly harsh harmonica, which can sound rough through some headphones, had a soft edge that nevertheless allowed it to cut through the mix.
Another plus is that you get good performance through either aptX or standard Bluetooth, although the latter feels a touch more washed out when compared directly with the former.
The Sennheiser PXC 550 headphones sound great. Their battery life is decent, and is aided by the fact that you can turn them off with just a twist of the earcups.
Although I wasn't impressed with the 'Club' or 'Movie' EQ modes, I found that the 'Speech' mode dramatically improved the listenability of podcasts.
The touch controls simply aren't responsive enough for more complicated actions such as skipping through multiple tracks. I used them for quickly pausing or resuming my music, but for the most part I'd rely on my music player for anything more demanding.
The noise cancellation is good for the most part, but the PXC 550s struggle slightly more than their competitors in acoustically variable environments. The earpads offer a good amount of passive noise isolation, although their faux-leather covering can get a little sweaty.
The PXC 550's greatest strength is their sound. Other wireless noise-cancelling headphones might offer a better user interface or better noise-cancellation technology, but ultimately none that I've tried have quite matched up to the sound quality of these Sennheisers.
However, there are a couple of irritations that prevent me from being able to fully and unreservedly recommend them. They're not quite deal-breakers, but there are definitely other noise-cancelling wireless headphones out there that don't suffer with the same problems.
If you value sound quality above all else then these might be the headphones for you – just be aware that you'll be sacrificing a certain amount of usability in return.