Sennheiser CX 400BT True Wireless

What they can’t do

If you compare the CX 400BT with its immediate predecessor from the Momentum series, they lack active noise suppression and Transparent Hearing features, don’t have the water and sweat resistance according to IPX and they are a little short on runtime – that’s all. Drivers, touch control and app connection are identical.

The CX 400BT are much more angular in shape, adding a little more weight and making them a little less expensive to make. However, this is hardly noticeable, especially as the feel of the CX 400 is still nice and comfortable. After attaching the appropriate silicone adapter (available in XS, S, M and L) and turning them slightly, the in-ears are firmly secured and very tightly fitted in the ear. There is already a significant degree of external noise isolation, which makes the lack of ANC noticeable.

Sound

Pairing is quick and easy thanks to Bluetooth 5.1. One criticism is that they don’t support multipoint. However, they can be paired with up to eight different Bluetooth devices, which the headphones recognise, but they can only be connected to one device at a time.

Using SBC, AAC or even aptX codecs the sound flows through the dynamic 7mm drivers. And this sound leaves nothing to be desired. Rich, velvety, soft, dynamic, wide, clear, deep and full – each of these attributes fits into the frequency response that Sennheiser specifies as 5Hz to 21,000kHz.

The bass goes deep down and creates a wide sounding base for the soft and very transparent mids that reproduce voices and acoustic instruments over a wide area with good definition. The clear highs just pour over it, openly and brilliantly, without any screeching or clanking.

In this headspace, spacious recording of complex classical music come into their own just as much as rumbling, fat guitars or delicate sound collages. Sennheiser delivers high-quality, open sound without any ifs and buts and without any major deviations into the artificial or ugly. The natural reproduction from bass to treble is more relaxed and allows you to even enjoy listening on public transport, as the CX 400BT are always loud enough.

Operation and Smart Control

On the backs of the headphones are touch surfaces which, despite the lack of tactile response, enable targeted and reliable control. In addition, a touch is acknowledged with a pleasant sound. Because of this, you are less likely to tap around without a sense of direction. Tap once on the left to start or stop playback, tap twice on the left to skip one track forward, tap twice on the right to skip one track back. Press and hold the left to reduce the volume, right to increase it. These functions can also be personalised with the free “Smart Control App”, which they recommended you download from the Apple App Store or Android Play Store, as it completes the “Device-In-Ear chain” with extended functionalities.

In the app it is possible to reconfigure tap gestures and reactions within a certain range: For example, do you want to call up your voice assistant with a tap on the right? Or not at all? It is possible to set it up in the app.

In addition, there is an easy to use equaliser, which comes in a classic three-band version or as a modern touch and slide matrix to give the individual sound tuning an appropriate stage. Here you can create and save personal presets. Furthermore, you can activate automatic call acceptance in the app, which switches incoming calls directly to your ear as soon as you take the right earphone out of the charger. When making a phone call with the CX 400BT, the voice quality was very good, thanks to the built-in microphones, and the person on the other end of the call understood me clearly and distinctly.

Charging case and runtime

The slightly rounded rectangular Sennheiser charging case with USB-C connection is just about pocket-sized. According to the manufacturer, after these in-ears have been running for seven hours, the box provides an additional 13 hours, which means a probable operating time of 20 hours – which is decent enough. It’s a pity that the case itself cannot be wirelessly charged to Qi standard.