Sennheiser CX 400BT review

The Sennheiser CX 400BT might not seem like the most thrilling true wireless earbuds right now. Apple’s AirPods 3 are on the way, and the active noise-cancelling Momentum True Wireless 2 are a more premium option from Sennheiser’s own stable.

The CX 400BT do not have ANC, but on a closer look, there’s actually quite a lot here for the discerning music fan. For one, these earbuds dropped in price – rather quickly, too – from $200 to $130, putting themselves on equal terms with the current 2nd-gen AirPods. Challenging Apple is no small effort, but as this Sennheiser CX 400BT will explain, a combination of excellent sound quality, competitive battery life and a healthy range of features proves enough to win out.

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Sennheiser CX 400BT review: Design

The Sennheiser CX 400BT have a boxier shape than the more rounded Momentum True Wireless 2, but they’re still featherlight with a robust matt plastic finish. They don’t stick too far out of your ears either -- no ugly stalks here.

There are no buttons on the buds, just a flat section for touch control inputs. Other than a silvery Sennheiser logo on each of these pads, the buds are all-black for an understated look.

Overall, these are a well-designed pair of earbuds, though they’re not quite the complete package. There’s no waterproofing, splash-proofing or sweat resistance, so you’d have to be careful about using them for serious exercise. This is, arguably, the single biggest omission, considering how much the CX 400BT cost. Even the Creative Outlier Air V2, which are only $70, have sweatproofing.

While the CX 400BT don’t stick out enough to offend the eyes, they do tend to pick up wind noise on blustery days. Fortunately, you can drown this out by upping the volume.

The CX 400BT come with a neat little charging case, which is taller than most competing cases, but also narrower, so it has no trouble fitting in a pocket. The lid snaps shut with a satisfyingly forceful magnet clamp, too.

On the back of the case is a USB-C charging port (the cable comes in the box) and a tiny-but-handy power indicator button. Press this, and a status LED will glow green, amber or red depending on how much juice is left in the case.

Sennheiser CX 400BT review: Comfort and fit

You can adjust the Sennheiser CX 400BT's fit with four pairs of swappable silicone eartips: extra-small, small, medium and large. I found the pre-fitted medium tips a tad too big for my dainty ears, but the small size tucked in perfectly. The CX 400BT adopt a simple push-and-twist approach for securing themselves in place, and yet they remained snug and secure when I was jogging, doing jumping jacks and, I’ll admit, headbanging while practicing bass guitar.

It’s also worth noting that the AirPods come with only three tip sizes, so you’ve got a slightly better chance of securing the perfect fit with the CX 400BT.

Again, these headphones are also very light, at just 0.2 ounces per bud. I happily wore them for hours at a time without feeling any discomfort.

Sennheiser CX 400BT review: Setup

Getting the Sennheiser CX 400BT ready is child’s play. The different tips pop on and off with ease, and pairing is just a matter of holding down on the touch sensors for a few seconds, then selecting the headphones on your source device.

The Sennheiser Smart Control app isn’t technically essential. Ignore it at your peril, though, as it unlocks most of the CX 400BTs’ most interesting features. This is available on both iOS and Android, and gets up and running fairly quickly, with a clean UI and self-explanatory settings.

Sennheiser CX 400BT review: Controls

Like most true wireless earbuds, the Sennheiser CX 400BT support touch controls: tap the left bud once to play or pause, double-tap the right bud to skip forward, hold to raise or lower volume, and so on.

These controls work well enough, and I especially appreciated how the flat surface of the touch-sensitive area contrasted with the more rounded sides. This helped me stay sure of hitting the right spot by touch alone.

What’s special about these controls is that you can customize them in the Smart Control app. While there are fewer total options than you’d get with the Momentum True Wireless 2 (which also offer noise-cancelling and transparency modes), it’s still a very welcome feature. I was personally fond of mapping the skip functionality to both double-tap and triple-tap input, so I didn’t have to worry about being so precise with my tap count.

Sennheiser CX 400BT review: Sound quality

The Sennheiser CX 400BT use Bluetooth 5.1, with support for the high-quality aptX codec, as well as the standard SBC and AAC codecs. That’s the same set of specs as the $270 Momentum True Wireless 2, and the CX 400BT even use the exact same 7mm dynamic drivers.

It’s an open attempt, in other words, to copy and paste the more expensive earbuds’ sound signature into a cheaper package. And to Sennheiser’s credit, it works. The CX 400BT sound excellent.

Even with no ANC to cut out distractions, the frequency balance, lack of distortion at high volume and abundance of detail are all easy to appreciate. I could clearly pick out all the different vocal layers on Tally Hall’s “Ruler of Everything”, and challenging rock tracks kept their distortion and overdrive effects intact without melting into mush. The choruses of Biffy Clyro’s “Mountains” sounded suitably titanic, while Nothing But Thieves’ “Trip Switch” transitioned between delicate and anthemic as well as could be expected from such dinky drivers.

Because the CX 400BT favor a neutral EQ mix -- more on this shortly -- some music, especially poppy, dancey tracks and electronic songs, benefit from the addition of some extra bass. But there’s still a very healthy amount of low-end punch, giving tracks like Bishop Briggs’ pulsing “Jekyll and Hide” the impact they need.

The CX 400BTs’ wide soundstage and ability to represent every single part in a mix make them a great pick for live recordings and classical music, too. There are no outright problems, like the boomy bass and recessed highs described in our AirPods review.

The Bluetooth connection is also rock solid, able to hold through several walls without degradation. 

Sennheiser CX 400BT review: Features

The Sennheiser CX 400BT may lack the ANC of the Momentum True Wireless 2, but this is still a very modern pair of earbuds, with some very neat tricks. Voice assistant support is one. By default, a single tap of the right bud will summon whichever digital helper your phone supports, be it Google Assistant or Siri, and the built-in microphones allow you to give verbal commands or make queries.

There’s also a “built-in equalizer”, which, in theory, sounds something like the Adaptive EQ feature on the AirPods Pro. In fact, it just means you can personalize the EQ using the Smart Control app. This isn't a disappointment, though, because the software provides a wonderfully granular level of control.

There are two ways to set the EQ. One uses a simple set of three sliders: one apiece for bass, mids and treble respectively. The other is a “visualizer” tool wherein you can drag a marker on a graph, with the frequency range on the X axis and prominence on the Y axis. It’s quite fun to push the marker around and, after a few seconds, hear how the sound shifts, even though I ultimately settled on a neutral profile with just a touch of extra bass.

Another party trick is the ability to pair the earbuds with up to eight source devices at a time, and simply select the one you want to listen to in the app. This is, as it sounds, pretty convenient, at least for music. The first time I added a laptop to the list, I couldn’t switch back to my phone without manually pairing the headphones again. But after that I could use the app to swap between the two relatively smoothly.

Unfortunately, there’s a bigger problem: Using this switching feature seems to knock the sound out of sync with videos, which isn’t an issue for the CX 400BT when you pair them manually. That’s not much of an issue for audio-only listening, but when I wanted to watch a video on a different device, I had to disconnect and re-pair the buds to get the audio back in sync.

Sennheiser CX 400BT review: Battery life

Sennheiser promises up to 7 hours of playback per individual charge, with repeated use of the charging case pushing this up to a maximum of 20 hours before the whole setup runs dry.

The latter figure is lower than the 24 hours offered by the 2nd-gen AirPods, but the CX 400BT themselves can go for longer without interruption. I got 5 hours and 17 minutes of non-stop music playback from the CX 400BT before they needed to go back in the case. That's nearly half an hour more than the AirPods’ 4 hours and 49 minutes.

That’s still, you’ll note, well short of the marketed 7 hours, even though I stayed within the 25-50% volume range. But over five hours is good by general wireless earbud standards, so it’s hard to be too grumpy about this performance.

The same goes for recharging speed: the CX 400BT produced a respectable 54 minutes of playback from 10 minutes of charging. This is one area the AirPods have them beat, though, as Apple’s buds can provide up to 3 hours of listening time from 15 minutes of charging.

Also, I wish the battery could be measured on a finer level than 10% increments. If you’re running low it can be hard to tell if you’ve got a full hour left or less than 30 minutes.

Sennheiser CX 400BT review: Call quality

Taking phone calls on the CX 400BT technically works fine, in the sense that I could hear and understand the person I was calling, and they could hear and understand me. There’s no need to raise your voice or otherwise speak unusually to ensure the microphones are picking you up.

Still, don’t expect particularly gleaming sound quality. While clarity is decent, it all sounds somewhat metallic, like you’re using an old landline phone.

Sennheiser CX 400BT review: Verdict

While they’re not without a few blemishes, it takes only a few minutes of listening to music on the Sennheiser CX 400BT to almost forget about the faults. These are, for $130, more than a match for the AirPods, with outstanding sound quality and long-lasting comfort.

And, while not every one of the many bonus features entirely works, it’s great to see so many tools and customization possibilities stuffed into a tiny pair of wireless headphones. Wait for the AirPods 3 if you really want, but if you can live without ANC, the CX 400BT deserve to be much more than a stopgap.

• More: Read our full Sennheiser Momentum 3 Wireless review