Sennheiser CX 400BT True Wireless  Headphones Review

The Sennheiser CX 400BT are square-shaped in-ears with a very plain design. Unlike the Sennheiser MOMENTUM True Wireless 2, they lack a shiny silver accent on both their controls and instead, use a simple flat gray color for their logo. These headphones are available in black or white color variants.

These headphones are fairly comfortable. Although they're lightweight and don't go too deep into your ear, they're quite bulky and stick out when you're wearing them. On the upside, they come with four sets of ear tips so that you can find a fit that suits you best.

These headphones have alright touch-sensitive controls. Touching the left earbud once plays/pauses audio while touching it twice skips to the previous track. You can also hold down the left button to lower the volume. One touch of the right earbud activates voice assistant and two taps skips to the next track. If you hold down the right button, you can raise the volume. Taking calls is easy as you can touch either the left or right earbud once to accept or end calls and touch either bud twice to reject a call. You can also remap the controls via the app.

The Sennheiser CX 400BT are very portable headphones. They're smaller than the Sennheiser MOMENTUM True Wireless 2 and can easily fit into small pockets or bags. Their charging case is also somewhat small and can fit into most pockets.

These headphones have a good charging case. It has a button to turn on the status light indicator, which shows its battery status, and there are magnets to hold the buds in place. The case is made from hard plastic, which makes it feel sturdy.

The Sennheiser CX 400BT have a great build quality. Both the earbuds and their carrying case are made of dense plastic, which feels sturdy. The earbuds also have touch-sensitive controls and the carrying case has indicator lights to display battery life, which are nice touches. However, they don't have an IP rating.

The Sennheiser CX400BT are stable enough to wear for moderate physical activity. They don't move around too much but they could fall out with more intense movements. Luckily, they don't have any cables that could get snagged and pull them out of your ears.

• Sennheiser CX 400BT True Wireless headphones
• 4x Ear tip options
• Charging case
• USB-C Charging cable
• Manuals

The Sennheiser CX 400BT have a warm sound profile. They have a slightly boomy bass but vocals and lead instruments lack detail and presence, which can hurt vocal-centric genres like audiobooks or pop music. They also lack low-bass, so genres like EDM and hip-hop may miss a bit of thump and rumble. That said, their companion app offers a graphic EQ to customize their sound to your liking.

The frequency response consistency is excellent. Like most in-ears, if you can achieve a proper fit using the included tips, the treble and bass response should be the same every time you use these headphones.

These headphones have great bass accuracy. Their low-bass is underemphasized, resulting in a lack of thump and rumble. In comparison, the mid-bass is fairly neutral and delivers punch. The high-bass is a bit overemphasized, which makes some mixes sound boomy and a bit muddy.

The mid accuracy is excellent. The low and high-mids are relatively flat and neutral. However, there's a dip in the mid-mids, which nudges vocals and lead instruments to the back of the mix.

The Sennheiser CX 400BT's treble accuracy is disappointing. There's a large dip in the low-treble, which veils vocals and lead instruments. The mid-treble is better-balanced, though, and sibilants like S and T sounds are bright and present.

The Sennheiser CX 400BT's peaks and dips performance is alright. There's a wide peak concentrated mostly in the high-bass that adds a boomy and muddy quality while a dip in the mid-mids nudges vocals and lead instruments to the back of the mix. A peak in the high-mids makes some parts of the vocals and lead instruments honky and harsh, only to become veiled and lacking in detail by the dip in the low-treble. The peak in the mid-treble also makes sibilants like S and T sounds piercing.

The stereo imaging is outstanding. The entire group delay response is below the audibility threshold, resulting in a tight bass and a transparent treble reproduction. The L/R drivers are also well-matched in amplitude, frequency, and phase response, which is important for the accurate placement and localization of objects and instruments (like voices and footsteps) in the stereo image. However, these results are only valid for our unit and yours may perform differently.

Like most in-ears, the Sennheiser CX 400BT's soundstage is poor. The outer ear needs to be activated by resonances to create a speaker-like and out-of-head soundstage. However, in-ears bypass the outer ear altogether and don't interact with it. Their closed-back design also makes them sound less open and spacious than that of open-back headphones.

These headphones don't have any virtual soundstage features.

The Sennheiser CX400BT's weighted harmonic distortion performance is great. All frequencies fall within good limits, resulting in clear and pure audio reproduction.

These are the settings used to test these headphones. Our results are only valid when listening using these settings.

The Sennheiser CX400BT have an okay noise isolation performance. They really struggle to block out low bass-range noise like bus and plane engines. They're much better at reducing mid-range noise like ambient chatter or high-pitched sounds like the hum of an AC unit.

The Sennheiser CX 400BT have a fantastic leakage performance. Leaking audio falls below the noise floor of an average office, so you should be able to listen to your music at a high volume without disturbing those around you.

These headphones have an integrated mic.

The Sennheiser CX 400BT's integrated mic's recording quality is satisfactory. While you shouldn't have too much of a problem being understood, your voice may sound a bit thin and lacking in detail.

Update 11/15/2021: These headphones have been updated to test bench 1.5. In this update, we made changes to the way we test noise handling. We now use a subjective evaluation of our audio clips. This new method has resulted in different results than what we had reported in our previous test bench. As a result, the scoring of this box has changed, and we have updated our results.

The microphone's noise handling performance is sub-par. It struggles to separate your voice from noisier environments and is best suited for more quiet spaces.

The Sennheiser CX400BT have a disappointing battery performance. Although the manufacturer advertises them to last seven hours, they offer around five hours of continuous playback time. Luckily, there are roughly two charges in the case so you can top their battery off when you're on-the-go. The manufacturer also advertises a 10-minute quick charge, which is supposed to give you an hour of playtime, but we don't currently test this feature. Unlike the Sennheiser CX True Wireless and the Sennheiser CX Plus True Wireless, they lack an auto-off timer to conserve charge when not in use.

Update 01/04/2020: We originally reported that the Sennheiser Smart Control app offered a parametric EQ. However, we don’t consider this to be a true parametric EQ since you can only adjust a single band frequency. We have updated the Equalizer test results to ‘Graphic EQ + Presets’ to better represent this app’s capabilities.

These headphones have a decent companion app called Sennheiser Smart Control, which is available on Android and iOS. With this app, you can check the battery level, turn voice prompts on/off, and remap buttons. There's also a graphic EQ and you can even create your own equalizer presets.

Update 11/15/2021: We now measure negative latency values in test bench 1.5 and have extended our scoring curve accordingly. Negative latency means that your audio comes before your visuals. Previous to this test bench, we gave a score of 0 ms when the value was negative. With our test bench 1.5 results, iOS latency went from 0 ms to -112 ms, and Android went from 0 ms to -120 ms. We have updated our results to reflect these changes.

The Sennheiser CX 400BT True Wireless have passable Bluetooth connectivity. Unfortunately, they don't support NFC or multi-device pairing, which is a little disappointing. They also have very high audio latency on PC, using either the aptX or SBC codec. They have high latency on iOS and Android devices too, and your audio comes before your visuals. That said, apps and devices compensate for latency differently, and your mileage may vary with real-life usage.

The Sennheiser CX 400BT are Bluetooth-only headphones.

The Sennheiser CX 400BT are Bluetooth-only headphones and can't be used wired. They come with a short USB-C cable to charge their case.

These headphones are Bluetooth-only and aren't compatible at all with the Xbox One.

These headphones come with a charging case that holds around two charges. It uses a USB-C connection to charge. Unfortunately, it doesn't support wireless charging.