Is it time to try another Earbud

The U-2 earbuds herald Moondrop’s return to the earbud market after a four-year hiatus.

Note: I would like to thank Shenzhenaudio for providing the Moondrop U-2 for the purposes of this review. It currently retails for very reasonable $34.99. If you are interested in more information about the Moondrop U-2 check it the details on Moondrop’s website and its obviously available over at shenzhenaudio

Are these worth the wait? Read on to find out.

Are Earbuds Audiophile quality?

Audiophiles and Earbuds have a troubled past, where Earbuds have mostly fallen into that “there are fine for podcasts” category but shunned for their quality. But when implemented correctly, earbuds can you close to open sound ’nirvana’ that we love from our open back headphones. So, in my opinion, Earbuds are to IEM’s what open back headphones are to closed backs, they naturally have a wider soundstage and obviously feel more spatious.

But they are typically not without their “problems”, slight movements in your ear can dramatically change their sound signature and very little research has been carried out of measuring earbuds, effectively meaning that graphs of earbuds should not be compared with IEM’s or headphones. So while I have some hopefully interesting measurements later in this review but should interpret them with caution.

Though Moondrop have implemented some ’tricks’ with the U-2 to help stability and placement, so lets get into the details.


Moondrop waited a few years before deciding to reenter the earbud market with to provide an ‘audiophile’ quality open sound and a unique design:


As you can see cleerly in the images above, the U-2 adopts a ‘paper clip’ design making a very iconic look to the U-2. Using the paperclip design allows the cable to be held out from your ears, and also it is incredible lightweight and limiting any ‘pull’ of the earbud and microphone sound in your ears from the cable touching the side of your face.

Before delving into the sound lets have a look at the packaging:


The Moondrop U-2 earbuds come in a simple package with a clear outside: box-clear2.jpeg

Taking off the plastic reveals the U-2 with their ‘iconic’ paper clip design: Zoom in.jpeg

The back of the box contains some inforation and the typical anime character: back-of-box.jpeg

Inside you get a simple bag a bunch of foam ear-tips and a couple of cards of information: box-contents.jpeg


Design and Build Quality

The U-2 earbuds internally according to Moondrop feature a 14.8mm compressed long-stroke composite paper driver inside an ABS cavity material featuring an N52 high-performance magnetic circuit and a CCAW lightweight voice coil. 473d1db6.jpeg

I found the U-2 extremely lightweight (probably more to do with the paper clip design) but slightly bigger than the previous ear buds that I reviewed the FiiO FF1: Comparisons2.jpeg

Crucially I found the longer extension provided by U-2 design kept the cable away from the side of my face and less prone to being displaced as I moved my head around.

With the foam tips applied the U-2 while bigger feels more premium: Comparisons1.jpeg

Cable and Connectivity

The U-2 is equipped with a twisted silver-plated cable and a 3.5mm standard single-ended plug: cable.jpeg

It is reasonable quality, but I did find it gets tangled pretty easily with the paper clip design causing the wire to get wrapped around the ‘clip’. I would also have liked if it could be detached.


I found the U-2 with the foam eartips extremely comfortable and stable in my ears and I wore the for hours while working mostly forgotting I was even wearing them. Without the foam eartips they were prone to move and as you will see below did not sound good, so I only briefly used them without the foam eartips.

Sound Quality

So, as you will see in the measurement section, earbuds are very dependent on how you wear them. I personally found the U-2 to be excellent (for an earbud), you get a good midrange and a great open soundstage. The bass quickly rolled off which is typical of earbuds, so no sub-bass and bass was only present when I used the foam ear tips. They sounded poor and lifeless without the foam.


The Moondrop U-2 for an earbud delivers some nice ‘rich’ bass which is surprising for an earbud. I found the midbass ‘rich and full’ and compared to the FiiO FF1, I found the bass more controlled and less ‘boomy’ on the U-2. It provided a solid foundation for the rest of the music without being either boomy or anemic.


The midrange frequencies are clear and articulate, allowing vocals and instruments to shine with natural timbre. The precise tuning and advanced driver technology ensure that the midrange is free from muddiness, making the U-2 suitable for vocal and accoustic centric music.


The lower treble was crisp without being harsh to my ears, adding a sense of air and detail to the overall sound signature. There was some upper treble ‘sparkle’ which might affect some people but I like the extra detail it provided.

Soundstage and Imaging

If an earbud placement suits your HRTF and it ‘sits’ right at the suit spot in your ears, you can get an amazing soundstage from these earbuds. Moondrop by using such a large driver have tried to maximise the chances of getting this suit spot. If you do get this appropriate placement, the U-2 will provide a very open and wide sound stage more akin to an open back sound than any IEM.

The imaging was ok for me, but I think the ‘midbass’ boost does muddy the clarity needed for good imaging, still there was reasonable separation between instruments.

Specifications and Measurements

Some specifications from Moondrop:

  • Frequency Response: 20Hz - 20kHz
  • Impedance: 30Ω±15% (@1kHz)
  • Sensitivity: 116dB/Vrms (@1kHz)
  • Total Harmonic Distortion (THD): ≤0.1% (@1kHz, 94dB)

But let me now get into why I had a subtitle on this review “Measurements tell you everything and nothing about the U-2”.

Frequency Response

So I typically measure all my IEM’s using my 711 clone coupler (and typically verify my measurements with some others published to make sure they are roughly in line ) and with headphones I use my clone KB501X soft pinna and I have 2 separate databases (based on Cringraph) for IEMs (here)[] and Headphones (here)[]
with usual targets available.

But the research and configuration of both a 711 coupler and Soft Pinna are not representative for Earbuds e.g. the distance to the microphone and hence the Pinna ‘gain’ in 711 coupler is totally different for Earbuds and similarly the targets for the Overear headphones with a Soft Pinna take into account the effect of the soft ear shape but again with earbuds this is wrong as it sit in much closer to your ear and are not affected by your outer ear shape.

So where does that lead you when reviewing earbuds with measurements? It means you can only very roughly judge the relative sound and cannot compare with either the in-ear targets or any overear targets especially related to the pinna ‘gain’ regions that we typically look at.

So the other problematic thing with earbuds is placement and these set of measurements might give you an idea of the problem with earbud placement: u-2 measurements mess.jpg

So this ‘mess’ of different measurements where when I measured different sittings of the U-2 within both my Kb501X (which are the measurement with much more of a treble rolloff) and on my 711 Clone coupler (which are the measurements with the more ’normal’ level of treble).

Here is a picture of 2 of the sittings: U-2-in-pinna.jpeg IMG_0423.jpeg

Also, the measurements where the bass rolls off significantly is when I measured the U-2 without the foam cover. This matched my subjective opinions of the U-2 and like my previous earbud review of the FiiO FF1 (where I subtitled that review “Never wear this earbud naked”). I have the same opinion about the U-2, for a decent bass performance you need to use one of the Foam covers: IMG_0424.jpeg

So to pick a representative measurement I picked one that most matched what I heard subjectively from the U-2 and that is this measurement:

u-2 nearest my Subjective.jpg

This is also on my measurement DB and I have compared it with a Truthear Zero Red and the FiiO FF1: comparison graphs.jpeg

So I felt it had a better more balanced sound than the FiiO FF1 which as you can see in this comparison has more midbass and is less balanced overall. Of course comparing the U-2 to the Truthear Zero Red is totally unfair based on what I mentioned in the introduction to this section, but I feel if you use the foam pads you will get some midbass and mostly a balanced sound from the U-2. As you can also see Moondrop have added that treble boost you can see at the 12K region which does at least give you treble ‘sparkle’ (maybe too much for some) to the sound signature.

So ultimately, my measurements really just show that your HRTF (i.e. ear shape) and how the U-2 earbud sits in your ear can greatly affect the performance you get from the U-2 but when it works for you (as it does for me) it provides an excellent sound (for an ear bud) with a spatious sound.


Distortion is totally under control and won’t be a problem with the large driver inside the U-2: R Moondrop Bare U-2 Distortion.jpg


I have given the U-2 a rating of 4, the sound from earbuds can be extremely subjective with different ear shapes causing dramatically different sound signatures. But for me, I found the Moondrop U-2 provided a good job of ‘balancing’ the limited bass and the typical treble rolloff you get from earbuds provide an enjoyable likable sound.


It is great to see Moondrop back innovating with the U-2 Earbuds after a few years away and while I really liked the sound from the U-2 it will definitely not work for everyone as the measurements show. I believe they are worth trying, and you may find they work really well for you. If they do you will have a comfortable, lightweight, relaxed sounding earbud that you can listen too for hours and forgot you are wearing.