Sennheiser CX 400BT True Wireless Review

Wireless earbud side by side

I’ve been anticipating the new true wireless earbuds from Sennheiser for quite a while. Their latest venture is the brand new CX 400BT, a follow up to their popular CX 350 which has now gone completely true wireless much like their Momentum line. Sennheiser has brought the CX line into the true wireless era and with that, bring as many features as possible to expand the capabilities of their earbuds. More than anything, Sennheiser wants the music to be yours. With the CX 400s customizable touch functionality and their Smart Control companion app, Sennheiser wants these buds to be personal to you. Let’s see how they achieve that, and of course how these guys actually sound. 

Earbuds box contents

What You Get

Most of what makes the CX 400 a product of value comes with what’s within the earbuds themselves, so the packaging only includes the materials you really need. The earpieces themselves are already sitting in their tiny charging case, with its USB-C charging cable resting in its white box. The only other accessories you’ll find are 3 additional pairs of silicone ear tips. 

Earbuds in charging case

Look and Feel

I want to touch on the size of this charging case, because how the case fits in your pocket is important since they’ll be taking up space in your pockets. The CX 400 has one of the smallest charging cases I’ve come across. It fits in your pocket well and sits nicely next to a wallet. The earpieces themselves take a larger look than I had expected. They take on a rounded square shape that’s unlike a lot of true wireless earbuds I’ve come across. Nothing about their size became obtrusive though and felt secure resting in my ears. The CX 400 is available in black and white, and both models have a pleasing aesthetic. ‘

Earbud housing


Design and Functionality

Things get interesting for the CX 400 in terms of their functionality. These buds have customizable controls that you can play with in Sennheisers smart control app. In the app, you have access to a graphic and parametric equalizer. You can play with the sound until you find a good spot and save it as a preset, of which you can have multiple. The graphic EQ is very simple, you have a fader for your bass, mids, and treble. It would have been nice to have a fuller EQ to only boost specific bands, but the option is more built for consumer-level use, going for user-friendliness rather than analytical. The parametric EQ is also streamlined but does take some getting used to. You adjust the EQ buy moving a white dot around the spectrum, and it kind of plays by its own rules at that point. You can create a v-shaped signature if that’s more your style, and you can do that pretty easily, but you might find yourself just switching to the graphic EQ to perform simpler functions. 

We can’t go any further without touching on the CX 400s customizable interface. Sennheiser has opted to feature simple tap gestures for its touch interface, which is highly preferable to me personally. On factory settings, the left earbud will play/pause, rewind, and decrease volume, while the right bud will activate voice assistant, skip track, and increase volume. In the smart control app, you can switch these actions around however you like. I switched the voice assistant tap gesture to the left bud, and the play/pause to the right since it was just more natural for me that way. Switching gestures simply takes going into the app, taping which option you’d like to switch, then taping the replacement action, and you’re all good. This interface is seamless, and the touch-sensitive controls themselves work like a charm.

The last thing I want to mention is the main component going on under the hood of the CX 400, the driver. The component that’ll do the heavy lifting for this earbud is a 7mm driver. I was expecting something a little bigger, considering the wider nature of the housing design, but we’ll get more into sound quality soon.

Single bud


Thankfully, the connection and pair are fantastic, taking full advantage of Bluetooth 5.1. With the CX 400, you’ll get superior bandwidth quality and range, as well as the latest CODECs. They will support AAC, aptX, and SBC. Sound quality barely dropped for me at all, I was even able to walk into a basement with the CX 400 still connected to my iPhone sitting on my desk upstairs. The buds exceed in superior connections quality that’ll rival most true wireless systems. 

Battery Life

Total playback time on the CX 400 will last about 20 hours, combining the 7 hours of life you’ll get from the buds, and the 13 hours you’ll get out of the charging case. An hour and a half will charge the buds completely, and only 10 minutes will get you a good hour of use. 20 hours is pretty substantial for a pair of true wireless earbuds and should last through workdays, and long commutes. 

earbuds charging


True wireless systems aren’t exactly tentpoles for good soundstage, but I was pleasantly surprised by some of the CX 400s properties in this regard. I was pleased with the amount of separation going on in these buds. It’s nothing of substantial-quality, but for a true wireless system, it makes a statement. Spatial imaging can be easily perceived and felt, as panning information stays true to space, avoiding a stage where sound elements fight for the middle. There’s some depth, but the CX 400 likes to stick to the surface. This is a more relaxed signature than I’m used to hearing in true wireless which surprised me. It lends itself to warmer textures, making contemporary genres very satisfying to listen to. American Football’s self-titled album made good use of these textures and properly captures the melancholy of that album. The CX 400BT doesn’t re-invent the wheel for soundstage but is still a proper change of pace compared to its rivals.

Low End

These lows have a soft, natural response, that lacks in depth but makes up for it in smoothness. I wouldn’t call it buttery smooth, but the relaxed nature of the stage sets the lows in a nice position that never becomes overblown. It’s a more restrained bass that responds to dynamic range effortlessly. Drum toms sound especially clear, and bass guitar sections can be analyzed distinctly. The track “How Do You Know Its Not Armadillo Shells?” by Hot Mulligan shows off an incredibly polished bass timbre that adds some excitement to these buds, just in a more subtle manner. 


With the exception of a noticeable low-mid boost, the mid-range stays flat for the most part. There’s a fun thumpy timbre to hard rock breakdowns and synth odysseys that keep energetic tracks on their feet. They create a warm timbre that also plays nicely to folkier tracks, and the mellow output gives the timbre a more effective breath. The shoegaze emo album “Peripheral Vision” by Turnover jested smoothly in the mids, and clean, chorus effected electric guitars sounded gentle and velvety. Vocal registers are clear and concise with good separation and balance well with complicated instrumentals. Podcasts and audiobooks will also sound clear, and the isolation works to block out some outside sounds to bring greater focus to vocal ranges.  


Lacking some significant fundamental frequencies and sufficient gain, but some elements are still left for the highs to delve into. Some detail is retained, especially in some sprinkly hi-hats, but you can tell from only a few minutes of listening that this isn’t a critical range for the CX 400BT. Boosting the highs using the EQ in the smart control app, revealed some missing sizzle in the highs, so I would recommend using it if you want to get the most out of this range.


My time using the CX 400BT was very enjoyable. I appreciated all the customizable options and warm sound signature, as well as their flawless connection and accessibility. For $199.95 this is a solid true wireless from Sennheiser, and a sure buy if you’re looking for a reliable pair. 

Pros and Cons 

Pros: Custom controls, warm timbre, great connection quality

Cons: Lacking highs, weak output

Sennheiser CX 400BT available at Audio 46

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Alex S. is a sound designer and voice-over artist who has worked in film, commercials, and podcasts. He loves horror movies and emo music.