The first pair of standalone earbuds I bought was the Sennheiser CX 400-II. It was 2009 and at the time, it seemed preposterous to spend money on an accessory that was bundled for free with every phone or iPod. But I'd heard Sennheiser's audio quality was worth the splurge and I wanted to see for myself. I was not disappointed. For years, the CX 400-II were my buds of choice thanks to their superb comfort and even more superb sound.

Sennheiser is now reviving that same brand with the CX 400BT, a pair of true wireless buds for 2020 with the same distinctive attribute of 2009: awesome sound quality. At $200 with several missing features, these aren't for everyone, but if you want to enjoy your music without any of the fluff, the CX 400BT are for you.


Battery life 7 hrs, 13 hrs extra with the charging case
Connectivity Bluetooth 5.1
Charging USB-C, 1.5 hrs for a full charge, 10 mins for 1 h of playtime
Audio 5 — 21,000 Hz, 7mm dynamic driver, codecs (SBC, AAC, aptX™)
Dimensions Charging case: 59 x 33.8 x 42.3 mm, 37g
Earbuds: 6g
Colors Black, white

The Good

Sound These are the best-sounding true wireless buds I've tried.
Controls They're customizable in the app, and give neat feedback through different tones for double and triple taps.
App You can customize the equalizer and easily switch your buds' connection between multiple phones and computers.

The Not So Good

No independence The right earbud is the master. You can't use the left one alone as only the right one works in mono mode.
Missing features Besides the lack of ANC, these don't have proximity sensors, a transparency mode, wireless charging, or an IP rating of any kind.

Hardware, design, what's in the box

Sennheiser intends for the CX 400BT to be a less expensive alternative to the flagship Momentum 2 true wireless buds, keeping the same signature sound profile but dumping Active Noise Cancellation. As a result, the two pairs share a lot of similarities including their design and materials.

The CX 400BT follows the same large — really large — bud design, but instead of it tapering into a circular outer touch panel with the Sennheiser logo, it goes for the squircle shape. The black plastic feels sturdy. It's matte on the buds' circumference, but the touch panel uses a shiny material that collects fingerprints. They aren't too visible, though, and a simple wipe on my shirt or pants removes them.

Since there are no ear stabilizers, the buds rely on a correct choice of tip size as well as your ear's cartilages to stay in place. You simply insert then twist them a little to make sure they're fitting snugly. They stick out a lot, but they won't fall or budge with regular movement. There's no IP rating or sweat resistance, so you shouldn't wear when exercising or sweating. That's clearly not their intended use case.

For most people, the buds should be comfortable to wear for several hours without any issue. I have small ears and a rather large tragus, though, which the CX 400BT push against quite firmly. I can wear them for an hour or 90 minutes, but after that, they become painful. I wish this wasn't an issue because listening to these is a pleasure. The only buds that really fit my ears' shape and don't cause any mounting pressure against my tragus remain the Plantronics BackBeat Pro 5100, which don't sound as good as the Sennheisers and have a Micro-USB port. I can't have it all, sadly.

The charging case is made of the same sturdy plastic as the buds. It's hefty, though it only holds 13 extra hours of battery life, and flips open and shut with zero wiggle. The buds snap in place very satisfyingly and won't be dislodged unless you pull them out. There's a USB-C port on the back and a button to check the battery level through a small LED light. The case doesn't support wireless charging, so you can't plop it down on a pad for a convenient fill-up.

In the box, you get the buds and their charging case as well as a short USB A-to-C cable, four different sizes of buds (XS, S, M, and L), and the various manuals.

App, controls, and features

The CX 400BT are surprisingly very light on features. Besides the lack of IP rating and Qi charging, the buds lack a few common options at their price point. The most annoying of these is the absence of proper independence between the two buds. The right one acts as the master and is the only one that can connect to your devices. The left one doesn't, so you can't pop it out of the case and use it for a call without also taking out the right one.

They also don't have any proximity sensors. Music doesn't automatically pause when you take them out of your ears, nor does it resume when you put them back in. You have to tap the touch panel to do that.

On the upside, the Sennheiser Android app provides several neat options that aren't very common among true wireless buds. The first is an equalizer with multiple personalizable presets to let you set the sound profile exactly how you prefer.

The second is customizable touch controls for music. You can choose what happens on each bud with a single, double, and triple tap, as well as a tap and hold. Options include playback, volume, and power controls, plus launching your device's voice assistant. The buds play a different feedback tone when you tap repeatedly, a small but very smart touch to let you know you've triggered a double or triple tap, or a tap and hold.

The CX 400BT don't support Bluetooth Multipoint (i.e. they can't connect to two devices at the same time), but they can remember several paired devices and connect to the last used one by default. If you're listening to music on your phone and want to switch to your computer or tablet, you don't need to fiddle with your devices' Bluetooth settings, you simply open the app and choose one to connect to. I wish all Bluetooth buds and headphones had this.

And finally, the app also lets you disable the voice prompts or switch to simple tones, update the buds' firmware, and factory reset them.

Sound quality and battery life

By far, the highlight of the CX 400BT is their sound. Their soundstage is wider than any in-ear buds (wireless or not) I've tried, and gets close to that of a decent pair of over-ear headphones. They're very balanced too, with pumping bass, rich mids, and highs so crisp I started noticing nuances in my music I haven't heard before. Each instrument, vocal, and sound effect remains separate and clear. It's really something else. Plus you can always adjust the equalizer to your liking if the default setting doesn't work for you.

There's no ANC here, but by picking the correct size of ear tip, you should get enough passive noise cancellation that it won't matter much at higher volumes. Speaking of, some earbuds and headphones get too loud for their own good, these get way too loud and still remain in control of every frequency. They pack such a punch, it's a crime to use them at less than 50%. For my daily use, 60-80% was good enough, but if you like deafeningly loud music or if there's that one particular song you want to blast at max volume, you won't be disappointed at all.

I had no issues with connectivity, latency, or voice calls. People I talked to said I sounded clear though a bit far away. 2 hours of listening reduced a full battery to 70%, so Sennheiser's quote of 7 hours of battery life should be feasible. The case provides 13 hours more. I've been using these for two weeks and have yet to charge them.

Left to right: Anker Soundcore Liberty 2 Pro, Sennheiser CX 400BT, Plantronics BackBeat Pro 5100. 

Should you buy it?

Sennheiser CX 400 BT

Maybe. Sennheiser set out to build a cheaper alternative to the Momentum True Wireless 2 without compromising on sound quality, and this is the end result. Whether you're the target market for it depends on how much you adhere to that sound-first philosophy. The sound balance, layers, and power of the CX 400BT is exceptional, and deserves the $200 Sennheiser is asking for them. At their infrequently discounted price of $150, they're almost a steal.

But if you're looking at the spec sheet and features and thinking there's no way you're parting with two Franklins to get a pair of buds with no ANC or transparency, no wireless charging, no IP rating, no proximity sensors, and no real standalone mono mode, then these are clearly not for you. The few extra features they do provide (equalizer, customizable controls, easy connectivity and switching) are all geared toward making the listening experience better, nothing else.

In a crowded price range that's filled with excellent and feature-packed competitors like the Sony WF-1000XM3 (ANC, built-in Assistant), Plantronics BackBeat Pro 5100 (supreme comfort), Anker Soundcore Liberty 2 Pro (Qi charging, in-ear stability), not to mention Jabra, Samsung, B&O, etc..., it's as difficult to recommend the CX 400BT as it is not to recommend them. I wish they packed a few more extras, but I also want everyone to experience their sound to understand why they're relatively high-priced without any bells and whistles.

Buy it if

  • You want true wireless buds with excellent sound and you don't want to splurge for the Momentum 2.

Don't buy it if

  • You listen to podcasts more than music — save some money and get a sub-$100 pair of buds.
  • You want to wear them while exercising — without IP rating or ear stabilizers, these aren't suited for that.
  • You need extra features, or want to use the left bud in mono mode for calls.

Where to buy