64 Audio U4s Review

64 Audio U4s Review

In this feature, we review the 64 Audio U4s which is a hybrid dynamic, BA, and tia driver universal IEM using proprietary APEX technology. It is priced at $1099.

Disclaimer: The sample sent to us is a purchased unit and does not have to be returned. Headfonics is an independent website with no affiliate links or status. We thank 64 Audio for their support. 

You can click here to learn more about 64 Audio products previously covered on Headfonics.

Note that this article follows our latest scoring guidelines, which you can read in more detail here.

64 Audio U4s Review
64 Audio U4s Review
The 64 Audio U4s is a huge step up on the previous U4 SE. The introduction of the dynamic driver and the tia highs not only improve the dynamic range and the level of headroom in its staging performance considerably but also gives more expensive models in the 64 Audio lineup such as the Duo and the U6t a serious run for their money.
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Excellent bass response
Clean design lines
Comfortable fitting
Lacks a little bit of lower mids presence
Reader's Score

I actually picked up an old 64 Audio U4 SE a few years back from a local trader not expecting much but was pleasantly surprised by the more balanced performance compared to the original bassier U4.

However, this was launched several years and in the intervening years, we have had quite a few new universal IEMs from 64 Audio but nothing quite like these older models in terms of the target market.

The U4s could well be 64 Audio’s 2023 answer to that and the first affordable 4-driver update in their universal series. Priced at $1099, it’s the cheapest of their line-up and sits just below the Duo launched in 2021.

And, like the U6t and upwards, the U4s comes packed with a lot of technology from their higher-end models that should add a lot more value to its performance.

Tech Highlights

Driver Configuration

The 64 Audio U4s is a hybrid quad-driver universal in-ear monitor. Inside are three different types of drivers, a single 9mm dynamic driver for the lows, a balanced armature driver for the low-mids, another again for the mid-highs, and 64 Audio’s patented tubeless tia driver for the highs.

Gone is the use of a more traditional acoustical damping system inside the U4s. We now have an integrated 4-way passive crossover that uses an electrical low-pass filter drawn from the U18s to eliminate unwanted frequencies and lower distortion before they reach the driver.

The U4s has an impedance rating of 11Ω @1kHz and an SPL of 107 dB/mW @ 1kHz @ 1mW (94mV). A fairly low load and not an overly sensitive SPL rating but you can find out how that stood up to our testing on page 2 of this review.


For those new to 64 Audio’s tia technology, tia stands for tubeless in-ear audio and is a key unique selling point not just for the U4s but for a lot of their monitors right up to the U18s and the Fourte

At the heart of tia is an open BA tubeless acoustical design. It is pitched as being far more open in sound than a traditional sealed and tubed BA driver. With the diaphragm free of obstruction, the contention is that a lot of the vibrations and resonances you get with closed or tubed designs will be gone.

No more tubes and no more dampers leave more room for the chamber and a switch to a single wide-bore nozzle. Wide bores produce a more natural sound to my ears than individual channels delivered in tubes right up to the tip.

The only difference between the entry-level U4s and flagship implementations of tia technology is the lack of a dedicated tia acoustical chamber.

64 Audio U4s Review

M12 apex

The U4s also uses 64 Audio’s apex, (Air Pressure Exchange) module technology designed to vent air pressure during listening and potentially reduce listening fatigue and long-term hearing damage.

Each module has an opening at the top combined with multi-cell studio-grade TPE material inside that varies in size and composition to produce different audible and sound pressure effects.

Normally you get a standard set of 3 filters from the highest isolating -20dB M20 down to the least, the -10dB MX. The -15dB M15 is the traditional middle child and comes installed in the U4s shell out of the box. 

You still get all those but 64 Audio has also decided to throw in a new module called the M12. No surprises here for guessing that this filter isolates down to -12dB and acts as a bit of a halfway house for those who felt the M15 to mX bass drop was too aggressive.

The M12 is a more subtle reduction and also focuses on retaining a bit more mid-bass punch at the same time.


The full name for this feature is Linear Impedance Design. Those of you familiar with Custom Art’s FIBAE technology will get a fairly quick understanding of the bigger aim of LID from 64 Audio.

Basically, LID is a patented circuit that ensures that whatever the impedance level is on any given output you plug the U4s into it should not have a low-Z skew and should play true to its correct tuning.

That means for those holding a DAP or amplifier with their output impedance ranging above 1-2Ω there will not be an impedance mismatch and the U4s should sound accurate or ‘true’.

64 Audio U4s Review


Form Factor

64 Audio follows a very disciplined design language these days so it’s no surprise the U4s follows the exact same design language as the U6t with a compact matte aluminum shell, smooth cornering, and the signature raised connector stem to the rear. 

I would classify the U4s as slightly on the narrower but deeper side compared to the Duo shells and a heck of a lot sturdier and heavier than the simpler resin shells from the U4 SE.

The only real difference in the form factor between the U4s and the U6t is the venting port to the rear of the U4s shell specifically for the dynamic driver. The U6t is an all-BA design and thus has no similar vent.


What is very different, however, is the aesthetics and this is usually where each monitor is given its own unique look. This time we have a very attractive-looking blue slate finish to the aluminum shell combined with a glossy ‘black and grey’ patterned face plate nestling beside the apex filter slot.

The U4s plate design is apparently inspired by the Muonionalusta meteorite which crashed into Northern Scandinavia about a million years ago with fragments popping up all over the place. It is a fine octahedrite or iron meteorite known for its very distinctive triangular design patterns.

You will note I put black and grey in parentheses because light or the angle of the light can give it a slightly different visual effect going from a monotone piano black similar to the U6t plate to a much busier almost triangular patchwork of blacks and greys. 

64 Audio U4s Review

Comfort & Isolation

Despite not having one of those ear-friendly aggressively contoured resin shells, I have always found the universal form factor of the 64 Audio monitors to be quite comfortable.

The U4s is no exception. It is relatively lightweight with a reasonably long nozzle so depending on the type of tips you choose you will not find too much of the shell pressing down on the various bumps and valleys inside your ear’s concha basin.

The longish connector stems might also be helping keep the cable up and away from the sides of your ear where sometimes pressure from the cable can develop.

64 Audio U18s


The supplied tips are pretty much the same lineup as the U6t and the high-end U18s and use that cool spidery tip holder tray that also fits neatly into the lid of the carry case.

You get 3 types of tips, a foam variant called TrueFidelity in small, medium, and large as well as a set of single-flange silicone tips in the same sizes. The 3rd set is a SpinFit CP145s in 3 sizes which I personally found to isolate the best as well as provide the highest level of comfort.

The foam tips felt a bit more secure in my ear compared to the softer more pliable silicone tips but neither the silicone nor the foam tips did that well for noise attenuation on their own. If you want to hammer down more on background noise I suggest using the foam tips in combination with the M20 apex module. 

Each set of tips brings its own unique sound signature so if you are looking for a slightly narrower staging quality combined with a softer set of highs then the foams will bring that to the table.

Personally, I prefer the slightly cleaner tone of the SpinFits with the M15 module. There was a degree less isolation but they offered the most balanced sound signature of the three without a loss in bass dynamics.

64 Audio U4s Review

Stock Cable

The U4s stock cable seems to be very similar to the Premium cable that came with the U6t, perhaps even the same one.

This is a 48″ 4-wire 26AWG silver-plated OCC copper wire cable with an officially rated 0.23Ω resistance level or 0.05Ω lower than their previous stock cable that came with the likes of the Nio

Aesthetically, the cable is relatively beefy with a braided finish using a soft shiny black external jacket. This particular sample is terminated with a branded straight 3.5mm TRS jack as well as an elongated and branded black aluminum splitter with a matching cinch that replaces the older rubbery disc/cinch.

The 2-pin connectors are fairly large but with the angling, it does a good job of guiding the cable out and over your ears without any undue pressure.  The cable uses a soft springy coating near the connectors to help shape the cable around the back of the ear but also help deaden the cable for microphonics.

The U4s stock cable is low for memory retention and performs quite well in terms of low microphonics or physical noise traveling up the wire during use. The chin cinch also has enough tightness to hold its position so it will not accidentally slide up and down when you are wearing them.

64 Audio U4s Review

Packaging & Accessories

The U4s comes in 64 Audio standard universal monitor packaging which is the company’s signature black and white graphical layout with the monitors front and center.

Inside you have the customary introduction from Vitaliy under the lid with the case, monitors, filters, cable, and new carry case on full display inside a contoured foam base.

Aside from the mentioned tips and tray holder, the accessories also include a collar clip for the cable and a small cleaning brush tool. You should also be getting that new M12 filter with your U4s accessories and it is not hard to spot as the only gold-colored filter in the tray.

The ‘Premium Leather’ carry case is a rounded high-quality simulation leather and black stitching design. It is soft but strong on the outside with the distinctive 64 Audio logo embossed on the front.

The inside is well-padded and fairly spacious with plenty of room to fit the drivers, cable, and some accessories. It is too big to be easily pocketable which smaller plastic containers or soft pouches could offer but it will have no issues being thrown into small bags on the go.

Click on page 2 below for sound impressions and pairings.

Click on page 3 below for our selected comparisons.

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