Fit and Comfort

Seen above is the right channel of the Astell&Kern AK ZERO1 installed into an artificial ear mold, and I am using the size-M silicone ear tips included with the IEMs. This is my typical combination for personal use too, and illustrates the nature of the achievable fit with these IEMs. I do have average-sized ears, and the ear mold above represents my own experiences well enough as a proxy. The semi-custom design of the shells shaped for the ear concha with ergonomics in mind works out fairly well for most ear types, but note that the angular shape of the shells makes it less comfortable with a few discrete points of contact in the concha as opposed to more continuous support as with a more semi-custom design. That said, the provided ear tips can make for a fairly deep fit that won't have the shells protruding out of the ears much, either. Those with medium-sized ears or larger will find this very handy, while it might be painful for those with smaller ears. The IEMs weigh ~7 g each, and a good fit makes physical fatigue minimal. The cable is also useful here, with the pre-formed ear hooks working well, although I would have liked to see an angled housing for the MMCX connectors to make for a steeper cable-exit past the IEMs. Passive isolation is quite good courtesy the single vent and deeper fit, but I did occasionally find myself adjusting these to not only allow for pressure equalization but also since the IEMs can be a touch cold in the ears because of the all-metal construction. Also note that light directly hitting the face plates will reflect back at various angles courtesy the design, making them shine slightly.

Audio Performance

Audio Hardware

A&K provides a handy illustration of the AK ZERO1's internals, and there is a lot to see. We knew going in that it employed a tribrid driver system, and evident now is that there are separate, specially designed 3D-printed acoustic chambers for each of the driver types. This separated setup is very rarely seen in such IEMs, with the likes of the 64 Audio Duo, Trio, and Fourte employing this mechanism costing way more. The aluminium housing also reduces unwanted resonance, adding function to the form. The lower frequencies are catered to by a single 5.6 mm micro-dynamic driver that, through some detective work, I believe is likely the AMT Acoustic ADS-SP-5.6. The mids are handled by two balanced armature drivers with custom coils, perhaps again from the AMT Acoustic portfolio. Lastly, and definitely the most interesting to me, is the use of a novel micro-rectangular planar magnetic driver for the high frequencies. This is where I first got a hint about the use of AMT Acoustic drivers, with its APM series of small-casing micro-planar drivers that have a form factor reminiscent of BA drivers quite unique in itself. The AK ZERO1 has a complex crossover network in place to make the most of the transitions here, which would otherwise be jarring if not well implemented.

It may thus seem that driving the hardware would be hard thus, and you would be right. The AK ZERO1 has an average rated impedance of 16 Ω at 1 kHz, which may not be true throughout the entire marketed 25–30 kHz frequency range owing to the different drivers and likely non-linear impedance response. What makes this a more demanding set, however, is the significantly lower than average sensitivity of 96 dB/mW, which means a portable DAC/amp or even dedicated DAP—no doubt an A&K one at that—will be a good idea. Going with one of the portable Bluetooth options we have covered before also makes this a wireless solution, and the lack of a 3.5 mm audio jack for most phones these days is another reason to consider a DAC/amp for portable use. If not on the go, space is less of an issue, but the relatively short cable might still be a potential handicap if connecting to a PC as the audio source.

Frequency Measurement and Listening

I will mention that I have a general preference for a warm-neutral signature emphasizing a slightly elevated bass and smooth treble range with detailed mids and good tonal separation. I also generally prefer instrumental music over vocals, with favored genres including jazz and classical music.


Our reproducible testing methodology begins with a calibrated IEC711 audio coupler/artificial ear IEMs can feed into enough for decent isolation. The audio coupler feeds into a USB sound card, which in turn goes to a laptop that has ARTA and REW running and the earphones connected to the laptop through the sound card. I begin with an impulse measurement to test for signal fidelity, calibrate the sound card and channel output, account for floor noise, and finally test the frequency response of each channel separately. Octave smoothing is at the 1/12th setting, which nets a good balance of detail and noise not being identified as useful data. Also, the default tuning was used for testing, and no app-based settings were chosen unless specifically mentioned. Each sample of interest is tested thrice with separate mounts to account for any fit issues, and an average is taken of the three individual measurements for statistical accuracy. For IEMs, I am also using the appropriate ear mold fitted to the audio coupler for a separate test to compare how the IEMs fare when installed in a pinna geometry and not just the audio coupler. The raw data is then exported from REW and plotted in OriginPro for easier comparison.


The IEC711 is such that you can't really compare these results with most other test setups, especially those using a head and torso simulator (HATS). The raw dB numbers are also quite contingent on the set volume, gain levels, and sensitivity of the system. What is more useful information is how the left and right channels work across the rated frequency response in the AK ZERO1, or at least the useful part of it. The left channel was separately tested from the right one, and colored differently for contrast. I did my best to ensure an identical fit for both inside the IEC711 orifice, so note how the two channels are very similar for all intents and purposes. Given reviewers such as myself get a randomly chosen single sample more often than not, the best way to test for driver reproduction and consistency is by comparing the two channels to each other, and the small differences between the two here do not really translate in practice. Measurements taken after 30 hours of testing, including with these playing a mix of various songs as well as white or pink noise and sine sweeps showed no differences, either. There was no perceived burn-in effect thus, and none was measurable. The response with the artificial pinna in place matches the ideal scenario in the coupler very well, which is also an indicator of how good the fit and seal was in the artificial pinna.


This is the average frequency response for both channels of the Astell&Kern AK ZERO1 plotted against my personal target taken from VSG.squig.link, which also gives you an idea of my personal preferences to better correlate any possible biases. The tuning of a set of headphones or earphones does not have to match my target as long as it is tuned with some direction, makes sense, and is executed well. After all, no one set will appeal to everyone, and having different options is what makes this hobby so interesting and hard to quantify. A&K didn't shy away from marketing this as the new standard for IEMs with the highest quality and most natural sound, and I also recognize that the emphasis on the treble region and vocals means the AK ZERO1 is predominantly being marketed to the East Asian market.

With that in mind, I acknowledge that the AK ZERO1 does meet most of the needs of that demanding and frankly different market. I was quite impressed by how natural the transitions came off, which is a testament to the electronic crossover build. The individual acoustic chambers also play a big role in keeping the three driver types playing mostly to their strengths, and the overall tonality of the AK ZERO1 can be described as mildly V-shaped leaning towards bright. The bass response is arguably the weakness when it comes to the technical performance, although you can see how the ~7 dB bass elevation from the mids more or less meets my personal approval. I will also quench your fears in saying the 25 Hz–30 kHz frequency response is nothing to worry about on the lower end, with plenty of sub-bass extension down past the maximum at 40 Hz. This makes the bass response just north of neutral for many; however, the micro-dynamic driver doesn't have the punch to back it up. As such, it feels more warm neutral than bassy and thus is not a set I'd recommend if primarily listening to the likes of electronic music or even rock n' roll, let alone heavy metal, where you want high volume in addition to impact. The bass region also didn't take as well to EQ as I'd like even with A&K marketing a low THD 0.7% at 1 kHz.

If something has to have a V-shaped tuning, I prefer it be gentle and mild. The AK ZERO1 is a good example of this, as the transition over to the lower mids makes for forward-facing and slightly warm male vocals that work well for old school funk as much as baritones in opera. Channel and instrument separation is decent enough, but nothing worth calling out. The two BA drivers used for the mids are working overtime here, with good imaging in an average-sized soundstage that also allows for this set to be quite good for general media consumption—movies and podcasts in particular, but also the occasional gaming session if you had to. In general, this is a set that works better for vocals than instruments, and the elevation continues from ~650 Hz up to cater to the upper mids and pinna gain compensation. Female vocals can be exciting or shouty depending on where in the frequency spectrum those voices and your own preferences lie.

The treble response is going to be divisive at best for the global audience. I am again reminded of the Japanese market having certain specific preferences that go well with the more elevated response. Kpop, baby metal, and anime soundtracks are examples of this, but I still think the 5 kHz peak is a blunder. It makes some female vocals seem unnatural and overly bright, and some other tones coming off sibilant doesn't help either. It certainly doesn't play well for classical music, without much range for instrument fundamentals and harmonics before the frequency response goes dark past the coupler resonance point. There were times thus where my face undoubtedly had a grimace listening to some songs, yet many others worked beautifully. Noting my preference doesn't generally match the intention of the AK ZERO1, I suppose it's a compliment that I still found many things to like about it and did not just outright hate it.

Comparisons

The obvious comparison has to be with the other three tribrid IEMs reviewed thus far, including the ThieAudio Monarch, LETSHUOER EJ07M, and XENNS UP, with the Monarch being so differently tuned and seemingly surpassed by the newer Monarch MKII that I decided to place the DUNU ZEN PRO here instead. It costs more than the AK ZERO1 and uses a single dynamic driver, but is tuned similarly, if not more to my tastes, while also providing increased technical performance and coming with a much richer accessory and cable experience. The XENNS UP is priced identically and a set that is more bass-emphasized at the expense of the treble response. As such, it is not really a direct competitor as much as it is complementary to the AK ZERO1, although those with specific music tastes will end up gravitating towards one more than the other. The LETSHUOER EJ07M is the overall winner in my books, at least when it comes to providing a fun and surprisingly balanced tuning backed by decent technicalities. It also costs the least of the four here and happens to be the only one available in a CIEM option.