Fit and Comfort

Seen above is the right channel of the 64 Audio U4S installed into an artificial ear mold, and I am using the size M black bore silicone ear tips included with the IEMs here. This was 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 ergonomic design of the shells shaped for the ear concha works out well for most ear types, especially given the smaller size of the shells compared to most IEMs on the market. In general, the gentle curves all around the shells make for at least two to three points of contact with the ear to help support and keep the IEMs in place. The longer nozzle also results in the the ear tips go in far enough to make for a better fit and seal, although the company's modular Apex technology will ultimately decide how good the seal turns out to be. The nozzle being quite thin is also a plus point for those with smaller ear canals, and the Apex modules help make the U4s extremely comfortable over long listening sessions, as any built-up air pressure in the ear canals is released in a controlled fashion. As such, there is no sense of physical fatigue with a good fit even if these aren't the most isolating IEMs. The angled connectors on the cable also work very well, as does the pre-formed memory-wire formation, which has the cable naturally go over and behind the ears as seen above. Pair this with the cable clip and cinch, and you now have a securely installed set of IEMs you may not want to remove, especially given these barely come to ~6 g each.

Audio Performance

Audio Hardware

Seen above is an exploded wireframe illustration to show the various components that come together to make the 64 Audio U4s. We do not get a lot of information about the exact drivers used although it is evident that the U4s uses a hybrid driver setup involving one dynamic driver and three balanced armature drivers in a 4-way crossover. The dynamic driver caters to the lower frequencies with one each of the two "standard" BA drivers handling the mids and the mid-high frequencies. Then we get to 64 Audio's patented Tia driver that is present in the nozzle itself and this happens to be the only in-nozzle BA driver implementation I've been happy with to date. It does so by opening up the balanced armature driver to have an unobstructed diaphragm that helps eliminate unwanted resonances in addition to avoiding sound tubes and/or dampers. The 4-way crossover has a proprietary electronic low-pass filter in use to further reduce lower frequency distortion and minimize the need for acoustic dampers. In addition to Apex and Tia, the 64 Audio U4s also features its patented LID (Linear Impedance Design) technology allowing for sources with varying output impedances to not impact the sound signature of these IEMs. I tested this with the U18t before here so you can see how important it is. Overall, the U4s manages to squeeze in all these drivers and audio tech into a small form factor that is also quite easy to drive courtesy a relatively low rated impedance of 11 Ω (@ 1 Khz) and average sensitivity of 107 dB/mW (@ 1 kHz). You can thus easily listen to the set using a decent dongle, let alone a more powerful portable DAC/amp such as the Questyle M15 I was mostly pairing this with.

Frequency Measurement and Listening

I will mention that I have a general preference for a warm-neutral signature with a slightly elevated bass, smooth treble range, 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 that 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 a capable and transparent DAC/amp such as the Questyle M15 used here. 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 instead of 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 64 Audio U4s in its standard configuration of the m15 modules and the wide bore silicone ear tips. 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 practically identical across the entire useful 20 Hz to 20 kHz range! This is excellent driver matching from what I understand is a randomly chosen sample out of the many being made ready for retail sale, so it bodes well for the U4s as a whole when it comes to channel imbalance issues, or lack thereof here. Measurements taken after 100 hours of testing, which included these playing a mix of various songs as well as white or pink noise and sine sweeps, showed no difference. There was no perceived burn-in effect thus, and none was measurable, either. The response with the anthropometric pinna in place matched the ideal scenario in the coupler very well too and this is an indicator of how good the seal was when installed in the artificial ear.


Here is the average frequency response for both channels of the 64 Audio U4s 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. With the U4s, things are all the more complex because Apex modules are involved. As such, I have four different graphs, which can all be examined separately on a per-channel basis for the U4s's frequency response with the m20, m15, m12, or mX modules installed.

Given the standard configuration of the U4s is with the m15 modules installed, let's first briefly discuss how the other three modules change the sound signature relative to what you get with the m15. As evident from the plots above, the degree of isolation is highest with the m20 modules and the least with the mX modules. You can also try to listen to the U4s without any Apex modules installed but that would come off without any meaningful isolation or bass response. The m20 modules increase the sub-bass response over what you get with the m15 modules—it may not feel like much but that extra tilt towards the lower frequencies can help even out overly bright mixes while aiding those who love electronic music. It's not my favorite of the bunch since I prefer a more balanced sound leaning towards a touch of brightness anyway. The mX modules flip things the other way round and are better suited for music genres without a lot of sub-bass emphasis. These are also good for music monitoring where imaging is critical given the clearer mids reproduction. I found it useful for instrument separation with classical music in addition to R&B and jazz as a whole. What might be the best synergy with the U4s are the new m12 modules, so much so that I personally think the U4s with the m12 modules is one of my favorite IEMs ever. It makes the sub-bass peak now on par with the ear gain (~7 dB relative to 1000 Hz) to make for a more balanced sound signature. This also helps address complaints from those who thought the bass response gap between the m15 and mX modules was too high while adding some heft to the mid-bass that I think benefits the U4s further. I also found it helping with a slightly larger soundstage compared to the m15 modules while also minimizing the somewhat plasticky timbre present there. Let's just say 64 Audio made a good decision developing these new m12 modules and including them exclusively with the U4s at this time.

So while I personally preferred the U4s/m12 combo, I do see why 64 Audio has the m15 modules pre-installed. The overall sound signature of the U4s can be seen as neutral with a bass boost, and we get ~9.5 dB of bass boost with the m15 Apex modules here relative to the lowest point at ~500 Hz. Bass extension is excellent here and note also how it hedges the line between being a warmer tuning and a more sub-bass focused one. I do appreciate some mid-bass energy here for instruments beyond just those in synth music, especially with bass guitars and snares coming off quite strong here. This doesn't make for a muddy or boomy bass at all, and there is also no bleeding into the mids. The dynamic driver used here in lieu of some of the subwoofer BA drivers I've seen in the 64 Audio U12t/U18t also helps add weight to the tones here, albeit I would have liked to see more macro-contrast between leading and trailing ends of tones. Mids are among the more accurate reproductions here of any IEMs I've tested to date, especially for those caring more about instruments than vocals. I mentioned before how instrument separation can be somewhat influenced by the choice of the Apex modules but it's really a minor tweak on what is otherwise an excellent set in this regard.

Vocals are more a miss for me, however, especially as we lead to the upper mids. Female vocals come off slightly blunted with higher-pitched notes sounding hollow. Male vocals can be forward facing but require a deep fit so be sure to play around with different ear tips. The mids is also where you will start to note how the U4s has a plasticky timbre to some instruments, especially brass guitars and violins. I suspect some of this could be the result of the ear gain peaking at 2 kHz rather than 3 kHz, which itself means some notes don't get emphasized. This makes the U4s have a more colored tonality than purely neutral, although not enough to detract from an actual music monitoring and mixing use case if you so desire. Imaging is excellent with the m15 and m12 modules and soundstage is aptly wide enough to not be an issue or a particular strength. I suppose the strong point for me was the treble response here which comes off very well extended without being overly harsh, sibilant, shouty, or fatiguing in any sense. Treble-sensitive folks can turn to the m20 or m15 modules to help where others who prefer music genres as I do would go with the m12 modules to have a bright-neutral tilt which is still on the detailed side. The Tia driver adds some air to help appreciate cymbals in particular and overall I had a great time with the set for listening to my classical music collection in addition to new-age jazz and even pop music. The U4s isn't the most resolving set on the market anyway, but it's overall a safe tuning that should work well for most people and can be customized further using the four Apex modules.


The 64 Audio U4s takes over the spot of least expensive UIEMs from the 64 Audio Duo, which in turn means these two sets are priced close to each other and merit comparison. In practice, the two are very different in use case and sound signature. The Duo is a semi-open set with a more V-shaped tuning and meant for a niche application to where I'd consider it more experimental and a product that is best demoed before making any purchase decisions. The bass response does feel more impactful with the Duo but otherwise it does come off slightly muddy and you also lose out on the flexibility of the Apex modules with the U4s, let alone the smaller size making for a more comfortable fit. I'd easily recommend the U4s over the Duo for most people and have certainly used the U4s far more than the Duo. The other set seen here is the audio community's darling from 64 Audio in the form of the U12t—a 12 BA driver set from a few years ago that has lasted the test of time in being one of the safest and more universally appealing sets for $2000. The U4s costs nearly half as much and offers you a very similar sound signature, if not arguably slightly better in a few metrics, and also has the added benefit of the currently exclusive-to-it m12 Apex modules and a proper dynamic driver-based bass response. There are a few places the U12t still wins out though, including being slightly more resolving, but then again neither would be winning any detail awards from me anyway. I'll have more to say about the U12t in its own review later, and perhaps the new m12 modules will make me change my mind there, but for now I have to say the U4s are my favorite 64 Audio IEMs I've tried to date and it just happens to be the least expensive one too!


The price range that the 64 Audio U4s belongs to is a highly competitive one and will have a few other exciting offerings coming up soon, if not already announced by the time this is live. There are plenty of established products in the market already, including the ThieAudio Monarch MKII that uses a tribrid driver configuration to have won many awards and recommendations since its launch in late 2021. I had the opportunity to listen to it for an entire evening all to myself and was left with mixed thoughts. In particular, I thought the bass response of the Monarch MKII was more balanced than on the original ThieAudio Monarch but the use of a DD + BA here meant there wasn't as much body in the mid-bass as I'd have liked. The Monarch MKII is also not as comfortable to use or get as secure a fit to where the overall more impressive mids may not be as well appreciated by everyone. Then there's the recently reviewed Custom Art FIBAE 5, another tribrid set that also provides for a customizable bass response with the use of Knowles dampers. It's further away from neutral than the U4s is to where I'd almost call it an audiophile's V-shaped tuning, and the treble response can be a bit spicy for most people. While I do like the FIBAE 5, I have to give the win to the U4s again for it's flexibility and generally more appealing tonality. The brand reputation also plays a role here, as does the wider retail availability of the 64 Audio offering. This last point plays a role when pitting the U4s against one of the very best sounding IEMs at any price range in the form of the Symphonium Audio Helios. Indeed, the treble response of the Helios is a masterclass in how to execute treble extension and a smooth tonality where almost everyone else falters. It also uses four drivers—all BA—in a 4-way crossover with electronic filters to make for an engaging bass response that fools many into thinking there's a large dynamic driver inside. The issue with the Helios is its large shells and nozzle that doesn't help with a secure fit for all, let alone a comfortable one for longer periods of use. There's also a bass tuck which can hurt those who primarily listen to rock music. I understand there's a limited edition Helios SE in the works to help address these issues which may well change the $1000 IEM field again, but for now I will place the 64 Audio U4s right up there as one of the benchmarks other IEMs to be compared to.