64 Audio U4s Review - Carving Out a New Sweet Spot


Lower-driver-count IEMs have always been a fascination of mine. Executed correctly, they reflect as an illustration of efficiency and what a company is able to achieve with what is often perceived as an inherent bottleneck. But these IEMs are becoming a lost art with few manufacturers catering to them, partly due to limited interest from audiophiles. Take a look at the Head-Fi forums, for example, and you'll see that 64 Audio's flagship models and the latest $6K behemoth IEMs are all the rage. But their lineup is actually quite diverse, and they have more than a couple IEMs on the more affordable end of the spectrum.

The U4s ($1100) is one of them. Originally only available in the CIEM form of the A4s (which I had mixed thoughts on), it has been ported to a universal format, making it the cheapest universal IEM in 64 Audio’s lineup. Let’s take a closer listen at what it brings to the table and whether or not there’s any differences from the CIEM version.

This unit was loaned for review by 64 Audio. As always, what follows are my honest thoughts and opinions to the best of my ability.

Source & Drivability

All critical listening was done off an iBasso DX300 and iPhone 13 Mini with lossless files. The stock 3.5mm cable was used. The U4s requires very little power to drive; at around 20% volume on my iPhone, I had no trouble hitting my usual listening volumes of ~70dB. Hissing was not an issue. If you'd like to learn more about my listening methodology, test tracks, and general beliefs in audio, then I would encourage you to check out this page.


  • 2-pin 0.78mm cable
  • Leather puck case
  • Apex modules (MX/M12/M15/M20)
  • Wide-bore silicone tips s/m/l, Spinfit silicone tips s/m/l, Foam tips s/m/l
  • Cleaning tool and sticker

The U4s comes with 64 Audio’s new assortment of standard accessories: pretty much everything you’d need to hit the ground running. The puck case is friction fit with a faux leather wrap and felt lining inside to protect the IEM. I like this case. The included cable is the brand's black premium cable. Overall, this cable is somewhat prone to kinking and not quite as pliable as I’d like (interestingly, I find the white version to be better on this front) but manageable.

Moving to the U4s itself, the shell is milled aluminum and coated dark blue while the faceplate is gray/black with a meteorite-like texture. It has the teardrop ergonomic that characterizes 64 Audio’s other IEMs. Personally, I found comfort with the U4s to be excellent - I was able to wear it for hours at a time - but this is always subjective. It’s also worth noting that the U4s has a small vent at the back of the shell which is used to port the IEM’s dynamic driver. I found that I had to place the U4s slightly differently in my ear than their other IEMs (which I normally deep insert) to avoid blocking the vent.

Technical Breakdown

Much of what follows below is taken from my extensive breakdown of the A3t.

To understand the tia system, it helps to first understand how a standard BA configuration works. Usually, we see there are two mediums for which frequencies are carried from a BA driver: 1) a slot, a spout, or a hole at the driver itself, and 2) a connecting tube which then carries the frequencies to the ear. Ostensibly, this is how the other two BAs in the U4s are configured. But the disadvantage of using this typical configuration, particularly with a tweeter, is that these frequencies lose energy over the distance of the tube, resulting in less output (poor treble extension) and added resonance when the frequencies reach the ear.

The tia configuration circumvents these limitations by “opening” a BA, which is to say removing the aforementioned slot, spout, or hole at the driver and allowing the frequencies to radiate from the inner diaphragm of the driver itself. But this still doesn’t address the limitation of travel distance, so the next step - at least with a BA tweeter - is mounting the driver directly at the nozzle of the IEM itself. Then what do you do with the wire connecting to that driver because there's only so much room at the nozzle? If you're 64 Audio, you run it through the tube that connects to the orifice of the Apex module (see below) and have it pop out from a split of airtight plastic. Collectively, these differences translate to superior treble extension in their IEMs, albeit not without idiosyncrasies I’ll delve into later.

Speaking of Apex, there are a couple other patented technologies that 64 Audio uses in their IEMs. The Apex technology is basically a more fancy form of venting an IEM. While the patent encompasses different forms of venting (see the Trio and Fourté, which have the technology but do not have the cylindrical module seen on most of their IEMs), this is usually achieved via the visually distinctive Apex modules.

The Apex modules are cylinders composed of aluminum that are plugged into an orifice at the faceplate of the IEM which, in turn, is connected directly to the nozzle via a tube. Ridges are present around the module and outfitted with O-rings to achieve an airtight seal. Air travels through a filter and viscoelastic foam within the module to control the exchange of air pressure; air exits the module through a small hole that rests between the finger notches of the module. This is also a simple but clever solution to mitigating wind noise, as the module can be inserted in the IEM at an angle to have wind pass by.

The Apex modules are aluminum cylinders composed that are plugged into the faceplate of the IEM

There are two prime advantages that come to mind when considering the Apex system. First is the pressure-relief it provides. With a traditional sealed IEM, sound waves build up against the eardrum, causing discomfort over longer periods of listening.

Naturally, Apex relieves some of this pressure, thus preventing the onset of this reflex. But the less obvious advantage of the Apex system is its usefulness as a tuning parameter, as it introduces a known source of leakage which can be controlled. Incidentally, BA earphones are also more prone to involuntary leakage (relative to dynamic drivers). In theory, the leakage from the Apex port overwhelms this involuntary leakage and allows for tuning superior bass extension. This is more relevant in the case of 64 Audio's UIEMs than their CIEMs; either way, it's likely partially responsible for the “special sauce” many listeners would attribute to their BA bass.

Of course, no system is perfect, and here the Achilles heel is isolation. It’s a vent, so it simply cannot match the standard -26dB isolation created by a sealed IEM. This is reflected in the naming of the modules; the M20 module provides -20dB isolation and so on and so forth for the M15 and MX modules. However, this limitation is not necessarily glaring in practice, and I would suggest that isolation with the U4s’ stock M20 module is adequate to protect hearing in most any loud environment.

Finally, LID is 64 Audio’s technology for counteracting differences of sensitivity between individual drivers and how they interact with a given electrical load. Traditional multi-BA IEMs will usually exhibit changes in frequency response depending upon the output impedance of a source; the (in)famous Campfire Audio Andromeda is an example of this. It sounds brighter with higher OI sources, vice versa with lower OI sources. LID ensures the U4s’ frequency response is consistent regardless of source OI. In truth, though, there isn’t necessarily a “one method solves all” approach for achieving linear impedance and the approach must be adapted based upon the individual IEM. Their pending patent for LID more or less confirms this, as it is broad and outlines multiple methods (for which I’ll spare covering in the interest of brevity) for achieving linear impedance.

Perhaps a less considered advantage of LID is that it is ideal for source comparisons because it removes the impedance variable from the equation. Assuming that one believes in differences of sound between sources, any differences that a listener observes between sources using the U4s should be the result of source coloration alone.

Sound Analysis

The frequency response below was taken using a clone IEC-711 coupler. Measurements after 8kHz should not be considered entirely accurate. If you would like to compare the U4s to hundreds of other IEMs I have measured, then please see here. 

Before talking about the U4s, let’s take a step back to the 64 Audio Nio, one of their hybrid IEMs derived from the “OG” N8 IEM. In my early days in the hobby, I fondly recall this being one of my favorite IEMs: simply pounding bass, a lush, thick midrange and reasonably spacious sounding to boot. It was impressive enough for me to fork over my own hard-earned cash on a pair. But time went on; I eventually sold my set due to finding it somewhat too sloppy in the transients, too tame overall. You might imagine my surprise to finally hear the U4s and find it to be pretty much everything the Nio was - and possibly more.

Like the Nio, the short reason why one considers the U4s is the bass response. It is sub-bass focused with a dash of mid-bass that slopes off just past 300Hz. This keeps it fairly clean; in fact, more so than the Nio. Nonetheless, a common criticism of the Nio is its somewhat “dirty” bass transients and ability to articulate texture past 200Hz. This sentiment still applies to the U4s wherein it sports a more pillowy characteristic to bass attack. But for a sense of authenticity to bass notes, the U4s certainly delivers with a meaty thickness and plenty of decay. Think of cookies: there are crunchy cookies with a lot of texture and there are softer cookies with more chew; the U4s is closer to the latter. Out of the few IEMs at a kilobuck with noteworthy bass, this is one worth indulging in.

It certainly doesn’t hurt that there’s a decent amount of fine-tuning afforded by the Apex modules. The U4s basically has three distinct flavors of bass. The M15/M20 modules produce the most bass and the character that established the Nio as a noteworthy basshead IEM. The MX module will produce nearly dead-neutral bass with a slight roll-off. Then, new for 2023, the M12 module will produce something of a happy medium; there’s something for everyone now. Between choosing from the MX to M20 modules, one can expect the U4s to go from brighter to darker sounding accordingly.

Interestingly, while using the MX and M12 modules does clean up the U4s’ bass, it retains a slight softness to its bass attack. This is an interesting phenomenon that I believe stems from other parts of the U4s’ frequency response: specifically the midrange tuning. They have a few variants of midrange tuning (some less desirable than others) that they use in their IEMs; thankfully, the U4s follows what is their safest formula. This is realized through flat lower-mids from 400Hz to 1kHz, then a more reserved pinna compensation and presence region. Consequently, the fundamentals of vocals are maintained while female vocals, especially sopranos, are presented further back in the mix and more reserved. This could be good or bad depending on individual preference. But for me, it’s a boon given that I listen to a lot of more shouty, possibly shrill music (read: K-Pop).

Moving back to the Nio, one of my main qualms was its treble response. Although it made use of 64 Audio’s tia driver, I couldn’t help but find it a tad dark in the upper-treble and, in tandem with its massive bass shelf, somewhat plodding overall. This doesn’t apply to the U4s. The U4s is more U-shaped than L-shaped (a term usually used to describe a bassy and dark IEM like the Nio), meaning that it has more energy concentrated in the treble. While the U4s’ 5kHz peak might look possibly concerning on paper, it has the benefit of more crunch from 6-10kHz, and more upper-treble energy, to balance out that initial peak. Snares, cymbals, and other instruments sound zingy, vibrant, and have excellent definition.

Granted, this is not quite a natural sounding treble response. Those with more sensitive ears might find the U4s too bright, possibly pesky at louder volumes. For this reason, I certainly prefer the M12/M15/M20 modules for the benefit of masking some of this brightness. And treble aside, there are a number of other tuning decisions that some listeners might be predisposed to take issue with like the more muted upper-midrange. But ultimately, these are all nitpicks. I have a predilection for IEMs that are more colored and that are colored tastefully. As far as I’m concerned, there are few IEMs on the market that walk this line as masterfully as the U4s does. 

As an aside, I did say that the Nio was one of my favorite IEMs in the past and the personal preference curve I made a couple years ago illustrates this. My preference curve is basically the U12t with some added mid-bass, courtesy of my experience with the Nio, and more mid-treble. While I don't necessarily subscribe to my own target these days (frankly, I didn't put much thought into it when I made it), it's pretty neat that the U4s is the closest IEM to realizing it. 

Technical Performance

If there’s one thing that I’ve learned evaluating IEMs, it’s that extension is paramount to achieving technical excellence. Thankfully the U4s has bass and treble extension in spades. Notes have a generous sense of weight to their transients due to the strong bass boost and thicker midrange. Even though midrange notes have a more distant presentation, details are still easily honed in on due to the strong upper-treble extension which accentuates midrange reverb trails. The U4s also sports remarkably pleasant (if not necessarily natural) timbre.

Imaging on the U4s is also decent even if it stumbles somewhat in sharpness of instrument positioning and layering. I suspect this strength is attributable to not only its Apex venting system, but also the contrast between its more recessed 3-4kHz region and strong upper-treble boost. This creates a fairly open presentation and decent sense of center imaging. Comparatively, full-BA IEMs like the 64 Audio U6t and 64 Audio U12t have superior imaging due to more contrast across the frequency spectrum. Their peaks at 5kHz and 15kHz, and dips at 3-4kHz and the mid-treble, are more emphasized. This pushes instruments out further on the stage with more space between, making individual instruments easier to pinpoint. 

Dynamics on the U4s are praiseworthy. This is in the sense that the more reserved upper-midrange might encourage listeners to turn up the volume to hear vocals more upfront in the mix. This increases the loudness of the bass and treble, creating a greater sense of contrast (see the Fletcher-Munson effect where sound becomes more V-shaped at louder listening volumes). The U4s’ treble response is also not peaky to the point of which it sounds wholly unnatural; this translates to a tasteful sense of excitement up-top. Collectively, the U4s is not conventionally snappy or sharp to the way it articulates dynamic swings, but rather more impactful with a good sense of air being pushed behind notes, especially in the bass.

Compared to the A4s CIEM

The frequency responses of the U4s and A4s cannot be strictly compared 1:1 because of differences in insertion depth on the coupler. The A4s was measured using a specialized attachment provided courtesy of 64 Audio. 

Fortunately, courtesy of 64 Audio, I happen to own the CIEM version of the U4s: the A4s. This puts me in the unique position to write about some of their differences.

Interestingly, the bass response of the A4s is slightly cleaner (with less mid-bass) than the U4s. But this is a benign consideration compared to their discrepancies in treble. My suspicion is that the increased insertion depth of the CIEM models shifts the resonance peaks of the tia driver, to the extent of which it creates a more “feathery” treble response. More critically, though, I find that it negatively affects timbre on the CIEM versions and sounds plasticky and smoothed over. This is an observation that I’ve found consistent between not only the A4s, but also my A3t and a couple of A12t units that I have force-fitted and measured.

I wouldn’t rule out 64 Audio’s CIEMs entirely. After all, they do have the benefit of increased isolation and comfort, plus way more customization. To be clear: if you prioritize sound quality, the universal versions are the way to go.

The Bottom Line

The question that I have to ask myself when assessing an IEM’s competitiveness is whether it fulfills a spot in its price category that was previously missing, or whether it usurps existing competition. In this case, I think the U4s does both.

There are few to none IEMs in its price bracket, minus its own 64A brethren like the Nio, that sound similar. In fact, the Nio itself might not be the best comparison for value, since the market has made great strides since its release. And it's clear that 64 Audio has responded: the U4s effectively undercuts the Nio by a third of the price with comparable (again, if not better) sound quality. It's not hard to see why this fantastic release has my unanimous stamp of approval.

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