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64 Audio U4S IEM Headphones Review

Updated: Dec 18, 2023

Welcome to the unveiling review of the 64 Audio U4S IEM headphones!

64 Audio U4S IEM Headphones
64 Audio U4S IEM Headphones

"We believe that when the walls between you and music come down you do your best work and live your life in the truest way. Our vision is to redefine the connection the world has with music by leading people to experience sound in its purest form. We believe that we can make those around us great by being great at what we do. Our mission is to inspire musicians and music lovers to unearth a deeper connection with music through extraordinary-sounding products and meaningful relationships."

We are 64 Audio

Disclaimer: 64 Audio Distributor "Zur Trade" kindly provided U4S IEMs for this review.


The 64 Audio U4S in-ear monitors (IEMs) are an audio enthusiast's dream, equipped with a powerful quad-driver setup to deliver a stunning auditory experience. Boasting precision engineering and exceptional craftsmanship, the U4S promises to transport you into a world of immersive sound. In this review, we delve into the extraordinary features and performance of these IEMs.

64 Audio’s U4s is a universal fitting counterpart to the custom-fitting A4s released in 2021. This four-driver Universal In-Ear Monitor (UIEM) features a Dynamic low-frequency driver, two balanced-armature drivers, and one patented tia™ driver, making it our fifth “hybrid” UIEM. U4s is designed with the ground-breaking patented technologies originally developed for 64 Audio flagship products to bring a wider audience closer to their music.

64 Audio, formerly known as "1964 Ears," was founded in 2010 by Vitaliy Belonozhko, an audio engineer. Initially targeting the professional audio market, the company later expanded its focus to include enthusiastic music lovers. What sets 64 Audio apart from conventional in-ear monitor brands are the various in-house technologies they offer, such as tia™, APEX®, LID™, and 3D-Fit™.

In summary, 64 Audio offers a range of impressive IEMs with their own proprietary technologies.

Design and Build Quality:

The 64 Audio U4S exhibits a compact, ergonomic design that fits snugly into the ears. Crafted with meticulous attention to detail, these IEMs boast a durable, high-quality construction that ensures longevity.

As they say in the company 64 Audio, the Slate Blue shell color was influenced by some of the world’s most beautiful blue-dial timepieces while the unique faceplate pattern is inspired by Muonionalusta meteorite fragments. Both the faceplate and shell’s illuminant properties give it a unique tone in varying lighting conditions.

The aluminum housing is lightweight, sturdy, and expertly crafted. The matte corn blue housing with black marbled faceplates gives the IEMs a stylish look, and depending on the choice of Apex modules, there may be golden accents as well.

The lightweight shell and comfortable fit make them ideal for extended listening sessions, whether you're on the go or unwinding at home. The detachable cable system enhances convenience and allows for easy replacement, should the need arise.

The U4s incorporates several of 64 Audio's core technologies, which are also found in their more expensive models. One such technology is tia™ or Tubeless In-Ear Audio. Unlike traditional in-ears that utilize sound tubes within the housing, 64 Audio takes a different approach. They employ proprietary open drivers and specially designed sound chambers, eliminating the need for sound tubes and the resonances and distortions commonly associated with them. This unique design results in a natural and full sound, reminiscent of that produced by loudspeakers.

Follow [the white rabbit] ------> THE STORY OF TUBELESS IN-EAR AUDIO

Another noteworthy technology implemented in the U4s is APEX®, which stands for Air Pressure Exchange. With this technology, 64 Audio addresses a common issue encountered with in-ears. When the ear canal is sealed off by an IEM and air pressure is generated by the drivers, it can cause strain on the eardrums over time. The APEX® system utilizes various damping modules to alleviate this issue. Air Pressure Exchange is a patented vent that releases air pressure in a sealed ear canal all while retaining standard IEM levels of isolation, enabling musicians and music enthusiasts to listen more comfortably for longer. This venting alleviates listener fatigue and allows for a much more realistic soundstage. APEX® comes in three module variations for U4s: mX, m15, and m20 [Apex Technology Isolation: -20dB, -15dB, -10dB]

It's also convenient that each of your in-ear monitors is specifically designed to work with a particular apex module. This ensures an optimal and tailored experience for each model. Additionally, the ability to replace the standard external apex modules allows users to have flexibility in choosing their preferred level of isolation and sound characteristics.

Standard Configuration - Wide Nozzle Silicone Tips/m15 Modules.

m20 Apex Modules:

Using the m20 apex modules with U4s increases sub-bass response while keeping the mid-bass clean. Works great for overly bright mixes, as it tilts the response slightly towards the low end. While this module slightly shrinks the soundstage width, it works great for synth-heavy or modern pop music that doesn’t contain much acoustic spatial information. The m20 module also unlocks some of the missing sub-information in older rock recordings. (increase by 1db- 2db at 40hz-20hz).

m15 Apex Modules:

The m15 apex®module features a combination of dual ambient ports (larger than the single hole in the m20) and modified internals to achieve -15dB isolation. This module is designed for an additional -4dB cut at 20Hz as compared to the m20, making it the middle ground between bass attenuation and isolation.

mX Apex Modules:

Widest imaging with a reduction in noise isolation. Sub-bass response is greatly diminished, giving you the ability to attenuate to your preferences based on music type and mix. It is ideally used for recordings that do not capture much sub-bass content while containing a lot of spatial information. Because of the reduction in sub-bass response, it creates a slight tilt towards the high-frequency band. (reduction by 1db-10db at 350hz-20hz).

The new m12 Apex Module:

Releasing exclusively with U4s are 64 Audio, its m12 apex modules. They will eventually be made available for purchase as an accessory on the website and included with some of the other UIEMs in the future.

The m12 modules were developed for those who felt that the bass drop between m15 and mX was too drastic. Perfect for tracks where the bass is just slightly overpowering the rest of the spectrum. Increases mid-range punch. (reduction by 1db-4db at 130hz-20hz).

Additionally, the U4s features LID™ (Linear Impedance Design), ensuring consistent sound performance across different sources regardless of the output impedance of the playback device. This technology guarantees a uniform listening experience, eliminating any variations caused by different source impedances. Linear Impedance Design (LID) is a patented circuit that corrects the non-linear electrical resistance of the multiple driver sets comprising this 4-driver IEM. Varying source amplifier output impedance in devices like smartphones, body packs, DAC/amps, and DAPs typically results in varying frequency responses across them, depending on the increase in resistance. Restoring proper interaction with the source and preserving the desired sound signature enables a consistent reliable sound.

The U4s also incorporate an electrical low-pass filter as part of its design. This replaces traditional acoustic dampeners, which can introduce unwanted distortion due to vibration and air turbulence. By eliminating unwanted frequencies before they reach the driver, the electric low-pass filter reduces distortion, resulting in cleaner audio reproduction. A crossover circuit replacing traditional acoustic dampers which can introduce unwanted distortion through vibration and air turbulence.

Overall, the 64 Audio U4s boasts a range of impressive technologies and innovations, ensuring an enhanced listening experience. The incorporation of tia™, APEX®, LID™, and the electric low-pass filter contributes to the U4s' exceptional sound quality and overall performance.

Sound Quality:

At the heart of the 64audio U4S lies a quad-driver configuration, with 1 tia High Driver, 1 High-Mid Driver, 1 Mid Driver, and 1 Dynamic Low Driver (per ear) working in perfect harmony to produce unparalleled sound quality. The result is a meticulously balanced sound signature that caters to audiophiles and professionals alike.

The frequency response of U4s follows proven targets for fidelity and preference. It’s powerful, enveloping, and deep while avoiding being muddy; this kind of bass can only be produced by a dynamic driver. U4s comes short of being classified as warm, rather it is like a tight and agile stand-mount speaker with a well-implemented sub. The key to this is how the low-frequency rise starts low enough to keep out of the mid-bass.

Mids are revealing and present, contributing greatly to its wide stereo imaging and separation. U4s’ perfect balance of presence and body lends itself well to every type of music and volume preference.

High-frequency response toes the line between neutral and detailed. Mixes that are typically “bright sounding” translate nicely without being harsh or fatiguing.

The Bass response is deep, impactful, and well-extended, providing a satisfying thump without overpowering the midrange and treble. This delicate balance ensures every instrument and vocal shines through clearly and effortlessly. The mids are lush and detailed, creating a captivating and intimate soundstage for vocal-focused tracks.

The Treble is crisp, energetic, and detailed, adding a touch of sparkle to the overall presentation. The U4S handles high frequencies with precision, avoiding harshness and sibilance, making it a joy to listen to even at higher volumes.

Instrument separation and imaging are impeccable, allowing the listener to pinpoint the position of each sound source with remarkable accuracy. The soundstage is expansive, presenting an immersive experience that draws you deeper into the music.


The 64audio U4S IEM headphones stand as a testament to the brand's 64audiocommitment to audio excellence. With a quad-driver setup, expert craftsmanship, and a remarkable sound signature, these IEMs capture every nuance of your favorite tracks and elevate your listening experience to new heights. Whether you're a professional seeking precision monitoring or a music lover looking for a high-fidelity companion, the U4S will undoubtedly leave you impressed. Embrace the world of sonic brilliance with the 64audio U4S IEM headphones.

Yet, the U4s are far from being limited to a specific niche. Regardless of one's musical inclinations, the 64 Audio U4s shines brilliantly. It emerges as a true delight when considering its relatively affordable price point in comparison to its capabilities.

With its impeccable sound quality, unmatched craftsmanship, top-tier materials, and comprehensive accessory package, the U4s unquestionably takes the lead as my foremost recommendation in the approximately €1,000 price range. For audiophiles seeking an IEM that doesn't compromise on any front, the U4s is a resounding testament to 64 Audio's dedication to delivering excellence in every sense.

Whether you're a music enthusiast, a recording artist, or a live performer, the U4S will exceed your expectations with its impeccable performance across different genres. From classical symphonies to heavy metal anthems, these IEMs deliver a well-rounded audio experience, highlighting the unique characteristics of each musical style.

With full confidence, I can name the 64 Audio U4s a little brother of my favorite IEMs - 64 Audio NIO.

Overall, the scope of delivery for U4's universal in-ear monitors appears to be comprehensive, offering a variety of accessories to enhance your listening experience, comfort, and maintenance of the product. The inclusion of multiple ear tip options, different Apex modules, and cleaning tools demonstrates a commitment to catering to the user's preferences and needs. The premium leather case and cable clip also add to the overall convenience and value of the package.

The cable of the U4s deserves special mention. It is softer, more elegant, and does not have a memory effect like the previous cable. Even the fact that the cable comes with an Unbalanced SE 3.5 mm connector instead of a Balanced 4.4 mm Pentaconn, I do not consider it a big problem.

The round leather case provided with the U4s is well-made and offers good protection.

In terms of wearing comfort, the U4s cases are relatively small and not very deep.

Unlike some other UIEMs, they do not have an anatomical shape. However, the included SpinFit silicone tips provide a good fit. The teardrop shape of the housing with large radii ensures that there are no uncomfortable points of contact, and the cable routing over the ear adds stability and comfort.

Technical specifications:

1 tia High Driver – 1 High-Mid Driver – 1 Mid Driver – 1 Dynamic Low Driver

10hz – 20kHz Frequency Response

107 dB/mW Sensitivity @1kHz

Integrated 4-way Passive Crossover

11Ω Impedance @1kHz

Apex Technology Isolation: -20dB, -15dB, -10dB

[ Source: ]


  • U4's Universal In-Ear Monitors: This is the main product, the in-ear monitors themselves. These are presumably high-quality earphones designed to provide excellent sound quality and a comfortable fit.

  • 64 Audio Premium Leather Case: The included premium leather case is a nice addition, providing a protective and stylish way to store and transport the in-ear monitors when they're not in use.

  • TrueFidelity Foam Ear Tips (S, M, L): Foam ear tips offer a snug fit by conforming to the shape of the ear canal. Different sizes (small, medium, large) ensure you can choose the one that best suits your comfort and sound isolation needs.

  • SpinFit Silicone Ear Tips (S, M, L): Silicone ear tips provide a different type of fit and feel compared to foam tips. They also come in various sizes for customization.

  • Silicone Ear Tips (S, M, L): These are likely standard silicone ear tips, which provide an alternative to the foam and SpinFit tips. The multiple sizes cater to different users' ear canal sizes.

  • 3.5mm Black Premium Cable: This is the audio cable that connects the in-ear monitors to your device. A premium cable suggests higher quality materials that can potentially lead to better audio transmission.

0.23 Ohms Total Impedance

7 x 7 x 4 Multi-Twist

Silver Plated OCC Copper Wire

26 AWG

Terminated in 3.5mm

  • Apex Modules: m20, m15, m12, mX: Apex modules are a technology by 64 Audio that helps with managing air pressure within the ear canal, potentially enhancing the listening experience. These different modules likely offer varying levels of sound isolation and pressure relief.

  • Cable Clip: A cable clip is a small accessory that helps keep the audio cable in place, preventing it from getting tangled or pulled while in use.

  • Cleaning Tools: Cleaning tools are essential for maintaining the hygiene and longevity of your in-ear monitors. They likely include brushes or tools to remove earwax and debris.

  • Round Stickers: These could be decorative or functional stickers. They might allow you to personalize your case or the monitors themselves, or they could serve as identifiers for different sets of equipment.

Author - Alexander Y [ALEXKUBIK]


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