64 Audio U4s Review

64 Audio U4s Review

64 Audio U4s Review

When it comes to audiophile favorites, few companies in the IEM space are as revered as 64 Audio. While some of the company’s greatest hits like the U18t or even the Fourte Blanc bear some pretty scary price tags, I’m looking at a less intimidating unit today: the U4s, the company’s newest release. At $1099, it’s the most affordable universal IEM offering 64 Audio has to date. Does it offer the premium fixings we’ve come to expect from 64 Audio at this point, or is it cutting corners? Let’s take a look inside the box, go over physical and technical design, and finally, get into just what the 64 Audio U4s sounds like.

What’s In The Box?

64 Audio, IEMs, in ear monitor, spinfit silicone eartips, true fidelity foam ear tips, leather case

  • 64 Audio U4s IEM’s
  • 2 pin .78mm to 3.5mm Unbalanced Cable
  • Hard Leather Case
  • 3 Pairs of Silicone Ear Tips
  • 3 Pairs of SpinFit Silicone Ear Tips
  • 3 Pairs of True Fidelity Foam Ear Tips
  • Apex Modules: m20, m15, mX, m12
  • Cable Clip
  • Cleaning Tool
  • Sticker

Look and Feel

Unsurprisingly, the U4s follows the same physical design seen with virtually all 64 Audio IEMs: an anodized aluminum shell results in lightweight durability, and the curvy concha shape with rounded edges offers top notch comfort. Like other 64 Audio UIEMs, I had no problem wearing the U4s in my ears for several hours, and easily could have gone for longer. The nozzle inserts at about an average depth into the ear canal, and can be comfortably pushed a little deeper for those like myself who are into that sort of thing.

64 Audio, U4t, IEM, in-ear monitor

When it comes to accessories, 64 Audio didn’t skimp. We see a hard leather carrying case and the company’s signature ear tip holder. We also see the inclusion of an evidently durable and well insulated braided cable. My small complaint here is that a 3.5mm termination is the only option provided – it would be nice to see a 4.4mm cable or some sort of modular termination cable. Not a big deal though. The accessories are premium, the fit is nice, and you can decide for yourself, but I think the U4s is chic, understated and generally looks cool.

Technical Design and Specs

Despite now being the most affordable 64 Audio IEM, the U4s comes with the impressive technical design elements we see in the company’s other IEM releases. We see 4 drivers with a 4-way crossover configuration: 1 DD for lows, 1 BA for mids, another BA for high mids, and a tia driver for high treble. For those unfamiliar with tia, all you need to know is that it’s a driver design created by 64 Audio that uses a tia single bore and special acoustic chambers that deliver the sound from the driver to the ear without any acoustic tubing. This improves accuracy as it avoids potential resonances that can be imparted by tubes.

We also see LID, or Linear Impedance Design. This comes in the form of a circuit that serves as some sort of impedance matcher, and ensure that the U4s’ frequency response is consistent whether you’re using your phone dongle or a desktop DAC.

Most importantly, we see the addition of a new APEX module from 64 Audio, the m12, which is what I used to write this review as it is making its debut along with the U4s. Module effects listed below:

  • mX Modules: Low end roll off beginning at 350 Hz. -1dB to -10 dB from 350 Hz to 20 Hz
  • m20 Module: Sub bass boost. +2dB to +1dB from 20 to 40 Hz
  • m12 Module: Sub and mid bass roll off. -1dB to -4dB from 130 Hz to 20 Hz. New, exclusive to the U4s for now.
Spec 64 Audio U4s
Driver Configuration 1 DD (lows), 1 BA (mids), 1 BA (high mids), 1 tia BA (highs)
Impedance 11 ohms
Sensitivity 107 dB
Frequency Response 10 Hz – 20 kHz
APEX Modules m20, mX, m15, m12

Sound Stage

The U4s doesn’t disappoint here, and presents a unique stereo image that, again, is not unlike other IEMs from 64 Audio. The most stand out dimension came from the U4s’ height. Tall images may not be outright rare for me to experience with IEMs, but they’re not particularly common either, and I was thus pleasantly surprised to hear this quality in the U4s. Depth is respectable, and does enough to layer parts and produce an adequate sense of distance and layering. And while I didn’t find the width of the image to consistently create a vast, out-past-my-shoulders sensation, it instead had the tendency to bend around to the back of my head. This is an unmistakable soundstage characteristic that I experienced with 64 Audio’s Fourte Blanc, and I’m once again excited to hear this in an IEM a fraction of the cost.

Overall, this is a very strong soundstage performance for an IEM just a little over $1000. The U4s finds its stage signature in 1) its height and 2) its behind-the-head capabilities.

Noting again: I used the m12 APEX module to write this review as it is, for now, exclusively released with the U4s.


Low end is rather customizable thanks to the selection of APEX modules that come with the 64 Audio U4s, but I’ll be speaking for the m12 module here. Emphasis is mostly paid to mid bass, which comes across as firm and detailed. Subs play more of a supporting character here, taking a more conservative role in the balance right up until a mix demands them, at which point they can activate with a surprising rumble. High bass is very modest, and while I wouldn’t say the U4s is cold or sterile sounding, it’s not finding warmth in the high bass or low mid region. Instead, it uses this part of the balance to create a distinct separation between the lower bass frequencies and the mid range. This type of low end tuning is right up my alley, and I enjoyed it quite a bit.


In the center frequencies, the U4s finds a balanced and natural expression, with a slight emphasis in the upper mids. Like I mentioned in the previous section, the IEM doesn’t place much stock in in its low mids, and you’re not getting warm and fuzzy vocal, guitar, or other mid range fundamentals. Instead, there’s a subtle lightness and edge that places emphasis on the immediate overtones within the mid range. For the most part, I really only noticed the subtle upper mid coloration on vocals. Guitars, pianos, synths and snare drums retained mix-intended character. This is a mids profile that’s less about imparting a unique character, and instead served as an anchor of realism for the rest of the U4s’ sound signature.


While the U4s acts fairly straight and modest in its mid range, we finally see some unique character and color in its treble – particularly upper treble. Low and mid treble are significantly present, providing plenty of harmonic detail and lift in the overall sound profile. Perhaps people who are particularly sensitive to treble will find the mid treble a little strong, but I found the U4s reasonably bright here and avoidant of ugly, inconsistent peaks. It’s in the upper treble that the U4s makes its statement as it boldly pushes its extension towards 20 kHz. I heard this as less of a tonal quality, and more of a detailed textural quality. Strummed acoustic guitars had more of their washboard quality brought out of them; buzzy sawtooth synths had a few more teeth on them; cracks and rasps in vocals were snapped into focus. This isn’t as bold of an upper treble as ThieAudio’s Prestige or Letschuoer’s Cadenza that blasts your face with sizzle and air, but it’s noticeably present, and breezily makes its way into listens.


64 Audio IEMs

With IEM manufacturers seemingly in an arms race to create the most expensive IEMs imaginable, its refreshing to come across something like the 64 Audio U4s that carries its value particularly well. This may be my favorite 64 Audio release to date; not that it’s β€œThe Best,” but rather, there were qualities of the U18t and even the Fourte Blanc present in an IEM a half or a third their price: firm and detailed low end, perfectly balanced and natural mids, airy tia treble, and even a soundstage capable of wrapping to the back of a listener’s head. The U4s offers itself as a reasonably priced endgame IEM, something that should be the case with any headphone or IEM that goes for just a little over $1000.

The 64 Audio U4s is available for purchase here from Audio46.

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