64 Audio U18S Review - Yin to the Yang

Review written by @Precogvision

I. Introduction 

Releases from 64 Audio seem like a calculated decision not dissimilar to the actual sound that characterizes their IEMs. Case in point? The original 64 Audio U18T, a groundbreaking feat of IEM design sporting 18 BA drivers, the highest driver count of any IEM in the world at the time. But it was released in November of 2016 which is going on five years ago at the time of this publication. Forget a world where IEM “updates” and “Gen 2s” are released at a breakneck pace, that’s a long time by any standard! 

This year, 64 Audio has finally followed up the U18T with their U18S. The U18S promises “resolution and presentation that reveals details previously gone unnoticed. The space between you and the performance is removed; the difference between hearing the best reproduction and experiencing it in real time”. Those are some bold words, but we all know marketing is marketing, so allow me to break down how the U18S actually stacks up in-practice.

This unit was provided for review by Headphones.com. As usual, what follows are my honest thoughts and opinions to the best of my ability. 

II. Source & Driveability 

All critical listening was done off of an iBasso DX160, A&K SP1000M, and iFi Micro iDSD Black Label with lossless FLAC files. I used the stock cable and the stock Spinfit tips. The U18S is a fairly easy IEM to drive, and hissing was a non-issue expectedly. 

III. The Tangibles

If there’s one area the 64 Audio IEMs have fallen short consistently and that I’ve cited to no end, it’s in their presentation and accessories. But thankfully, it looks like we’re finally seeing some steps in the right direction with the company’s U18S. Included are the following accessories: 

• TrueFidelity Foam Eartips (s/m/l)
• Silicone Eartips (s/m/l)
• Spinfit Eartips (s/m/l)
• 8-Braid Silver Cable
• MX/M15/M20 Apex Modules
• Simulation Leather Case

The cable that comes with the U18S is their new, 8-Braid Premium Silver cable. The tactility on this cable is a massive - and I mean massive - jump over the company’s standard premium cable. Of course, that’s not to say the Premium Silver cable’s a great cable, but more closely to say that the bar was just that low! To this end, the Premium Silver cable still sports the pesky memory wire that plagued previous iterations.  

New for 2021, the U18S sports a shell and faceplate that is distinct from the company’s past models. Using a pair of calipers, I measure the width of the U18S’s shell at 0.54’’ relative to the U12T’s width of 0.48”. Because of this, the U18S does feel a tad more insecure in my ears especially when making abrupt movements, although I did not experience any issues with overall comfort. The faceplate itself is quite cool and sports a milled pattern that is inspired by “natural formations”. Y’all know by now that I’m not too picky about this stuff; I think it strikes a good balance between being understated and aesthetically pleasing. 

IV. Technology 

64 Audio utilizes several patented, unique technologies in their IEMs. Here’s a breakdown for your convenience: 

Apex - When one listens to an IEM for prolonged durations, sound waves in a sealed canal build up and lead to discomfort; Apex serves to slowly release some of this pressure, thus extending listening time. The way this is achieved is simple but clever. A vent is present to equalize pressure; viscoelastic open-cell foam is placed in the vent to both retain the low-end frequencies and control the release of air. Within the context of the various Apex modules - modules you can swap between certain 64 Audio IEMs - vents are simply added to the module. The M20 has a single vent, the M15 has two vents, and the MX likely has three vents. The trade-off is a minor loss of isolation in that order. This technology works. I can wear the 64 Audio IEMs significantly longer than other IEMs with minimal fatigue. 

As for what these modules do sound-wise, they predominantly affect the bass regions. The MX module, aside from sloping off the sub-bass, also results in a bump at 4-5kHz on measurements for reasons unbeknownst to me. On a psychoacoustic-level, the changes are more widespread. When there is less bass, one’s perception of the midrange and treble is brought forward. As a result, staging has a tendency to expand, and layering - the perceived space between instruments - is more pronounced. 

LID - Linear Impedance Design. Different audio sources pack different impedances, and the way these impedances react with the various drivers in an IEM can alter the frequency response. This is part of the reason why you’ll notice that IEMs sound different between sources. As I understand it, LID essentially consists of L-pads to flatten the impedance curve, thus maintaining consistent frequency response between sources. This way you’ll always be getting the intended sound signature of your IEM. The U18T does not feature this technology; the U18S does. 

Tia - The tia driver is 64 Audio’s proprietary, unlidded balanced armature. Whereas most balanced armatures pass sound through a tube, the tia driver is mounted directly to the spout of 64 Audio’s IEMs. By bypassing ancillary frequencies and firing directly into the ear, this should theoretically translate to reduced resonance and cleaner, more extended highs. In practice, I hear all the 64 Audio IEMs as exhibiting strong peaks for air anywhere from 15-17kHz. This phenomenon adds to exceptional microdetail up top, and I suspect it plays a strong role in the unique staging that the 64 Audio IEMs exhibit. Depending on the extent of one’s hearing, some listeners might find these peaks more difficult to hear but can still benefit from the unique texture present to the tia sound. 

V. Sound Analysis 

Frequency response taken off of an IEC-711 coupler. There is a resonance peak at 8kHz. As such, measurements after this point should not be considered entirely accurate. 

Avid readers will know that I have covered most all of the 64 Audio IEMs in-depth except the U18T. And well, I may or may not have been avoiding it for personal reasons. The U18T leans brighter and more mid-bass heavy than 64 Audio’s other IEMs, paving way to a more mainstream signature. A solid signature, mind you, albeit one that comes off as somewhat uninspired to this reviewer’s ears.  

By contrast, the U18S is most closely the antithesis of the U18T. Those who enjoyed the U18T likely need not hear the U18S; however, if you’re someone like myself, then this is an IEM you might want to give a listen. In fact, I would say that the closest point of comparison is actually 64 Audio’s own U12T. But the U18S’s ear compensation at 2kHz does not rise to the amplitude of the U12T, and the 5kHz lower-treble peak has been smoothed over some. What does this mean? Well, to put it as simple as possible: The U18S is a darker, more relaxed U12T.

Given that the U12T’s tonal balance is nothing short of exemplary, that can hardly be a bad thing, but let’s dig into the juicy details. 

Bass

I’ve gone back and forth on my Apex module of choice with the U18S. Of course, I defaulted to the M20 module just because I’m no basslet and it’s what I run in my U12t. But over prolonged listening, I found that it was skewing tonal balance undesirably; more specifically, treble sounded too dark to my ears. So I think I’d recommend sticking with the M15 module if one wants a more airy presentation; those who want a more relaxed presentation are best suited with the M20 module. Either way, you get a largely sub-bass oriented curve with a slight tinge to the lower-midrange frequencies; I have no bones to pick on the basis of tonality. 

But many IEMs have fallen short when pressed for the intangibles. Luckily, the U18S is no slouch, and I would say it’s on the better end of BA bass. And it better be given that it’s got more BAs dedicated to bass than most IEMs in full! Transient attack is expectedly excellent with good tightness. Decay sounds more grainy than the U12T to my ears - more textured as a result, perhaps - but not sounding as natural either. Slam is expectedly milquetoast with the U12t having a noticeably more organic timbre and better sense of physicality. Overall, the U18S’s bass is good but not great in my opinion. 

Midrange

Relaxed, gentle, and not necessarily in a “warm-and-gooey” sense. There’s the Twitter review summary of the midrange. More specifically though, you have your fairly neutral lower-midrange and the upper-midrange has been brought up a tad relative to the U12t. I think the balance struck here is excellent, probably dead-neutral. That said, the ear compensation (the frequencies from ~1-3kHz that are elevated to account for IEMs bypassing the ear) does not rise to my ideal amplitude, thus lending to a more distant midrange presentation. Ultimately, some listeners might find the U18S’s midrange “dull” - myself included - but I think one would be hard-pressed to call it bad. 

Treble

Everything I’ve outlined thus far is fine; by most accounts, even within the realm of the ideal. But I think the treble is where some listeners might find the U18S stumbles. It sounds about equal parts impact and crash - so fairly linear going from 5kHz to 10kHz - to my ears, but the amplitude, the quantity, is not quite what I’d consider appropriate. Furthermore, the frequencies after 10kHz, as is typical with the 64 Audio IEMs, exhibit a dip up until the tia peaks for air somewhere close to 15kHz. 

While I hear this characteristic ~15kHz “zing” with the M20 module, even as a younger listener I find myself defaulting to the M15 module to maximize perceived treble energy. This is not to say the U18S’s treble is rolled-off - it still extends very high up - but rather, that the sensation of air will likely not be there for a large demographic of listeners without use of the MX/M15 module. Compounding this issue is that with the MX/M15 module, it borders on too zingy to my ears! 

Technical Performance

Let’s take a brief detour back to the U18T. Despite the U18T sporting a mind-boggling 18 BA drivers, I did not find it to be a particularly detailed IEM. I mean, of course it had the technicalities to play with most of the IEM world’s heavy-hitters. But against the likes of 64 Audio’s own U12T, Tia Trio, and Tia Fourte? I’d beg to differ. 

Thankfully, this does not hold true for the U18S. Heck, I might hear even more detail than the aforementioned IEMs; the U18S can play at the top for detail, no problem. Notes on the U18S seem to lean more textured with added grit relative to the U12T. I would not say notes have that “wetness” to them that reminds me of the Tia Fourte; however, there is a minor sense of micro-detailing going on with the U18S as a result of the aforementioned texture. While this might seem like a backhanded compliment, considering I don’t think most IEMs have any micro-detail to speak of, that’s actually fairly commendable. 

And of course, this wouldn’t be a 64 Audio IEM without the terrific imaging. The recession after ~8kHz contrasted to the spike at ~16kHz lends to some unique treble reverb; by extension, the way the stage is “imaged” around the listener. The U18S leans more toward the U12T side of things, maintaining excellent positional cues and some of that elusive center image distinction. Although this is something I generally refrain from talking about, I do hear more of that elusive blackness of background that the U18T simply lacked to my ears. This is in the sense that instruments appear to “pop” and “sink” into the darkness of the U18S’s stage; I’m again inclined to partly attribute it to the aforementioned treble contrast. 

Unforgiving Explication 

There’s really nothing the U18S does wrong in my listening library, or that I can immediately discern as a dealbreaker. I’ve called out IEMs for this before - the Vision Ears Erlkonig comes to mind - but similarly, if I had to cite a major gripe about the U18S, it would be that it’s just too safe. The magnitude of the ear compensation and the lack of treble energy are pushing it, lending the U18S’s presentation to dullness. Yeah, it’s still detailed - really detailed - but there is a lack of intensity to its presentation that immediately stood out to me.  

In this vein, we come to the topic of dynamics. The U18T was distinctive to me as being one of the flattest IEMs in the 64 Audio lineup. And alas, flat does not do justice here; I say this in the sense that the U18T had near-zero dynamic range and sounded, well, pretty dead. The U18S “fixes” this by doing a complete 180, but seems to have strayed too far: Macro-dynamic swings have a downwards-compression in which they’re scaled with too much pianissimo for my tastes. Tonally, I would suggest that this compression is somewhat attributable to the aforementioned, recessed ear compensation and darker treble. If I had to pick my poison between the U18S and U18T for this metric, though? U18S every time.  

VI. The Verdict

If any other company had put out the U18S, no doubt my assessment would have been a whole lot less critical. But I think it’s important to consider context. 64 Audio is distinctive to me as being one of the finest IEM companies in the world; I think it’s only fair that I hold them to a higher standard. The U18S’s true competition lies not with the U18T, a completely different flavor, so much as it does with the U12T which has been tuned similarly. I have to ask myself: Does the U18S represent an upgrade over the U12T? 

And the short answer is no, at least not strictly. Unless one desires a more relaxed, darker U12T, it’s a tough recommendation; all the more so when one considers that the U12T is as laidback as many would be willing to go. There’s also, you know, the fact that the U12T’s only a thousand dollars cheaper. But credit where credit is due, the U18S is undeniably a darn good IEM that comfortably sits with its flagship peers: a worthy Yin to the Yang that is the U18T. 

VII. Reference Tracks

• Aimer - Hakuchuumu
• David Nail - Let It Rain
• Everglow - DUN DUN
• Girls’ Generation - Galaxy Supernova
• Illenium - Broken Ones
• Joe Nichols - Sunny and 75
• Keith Urban - Defying Gravity (2009)
• Sabai - Million Days
• Sawano Hiroyuki - Best of Vocal Works Remastered (2020)
• Steve Jablonsky - Arrival To Earth
• Taeyeon - My Voice (2017)
• Tiffany - I Just Wanna Dance

-Precogvision

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