Overview

The 64 Audio U4s represents a premium universal in-ear monitor featuring a 4-driver hybrid configuration with the company’s proprietary tia (Tubeless In-Ear Audio) high-frequency driver, complemented by three balanced armature drivers and one dynamic driver for low frequencies. Priced at 1,099 USD, the U4s incorporates 64 Audio’s signature technologies including LID (Linear Impedance Design) for source impedance correction and APEX (Air Pressure Exchange) modules providing user-controllable isolation levels from 10-20dB. The hybrid design targets audiophiles seeking advanced driver technology and customizable acoustic performance.

Scientific Validity

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The U4s demonstrates moderate scientific validity with available third-party measurement data from TechPowerUp revealing specific performance characteristics. Available specifications include 10Hz-20kHz frequency response, 107 dB/mW sensitivity, and 11Ω impedance. Third-party measurement shows a 9.5 dB bass boost relative to 500Hz, indicating colored frequency response that deviates from neutral tuning [1]. For IEM category standards, the measured frequency response shows significant deviation beyond ±3dB within the critical 100Hz-16kHz range, which impacts transparency compared to well-tuned reference IEMs achieving ±1dB deviation. Critical metrics including THD, SNR, dynamic range, IMD, and crosstalk lack comprehensive independent verification for complete scientific assessment. The APEX modules provide 10-20dB isolation depending on module selection. While the available measurement data confirms substantial frequency response coloration that affects neutrality, the limited scope of published measurements prevents full evaluation against established IEM performance benchmarks for transparency and accuracy.

Technology Level

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64 Audio demonstrates significant technical innovation with multiple proprietary technologies representing advanced development in the IEM category. The tia driver eliminates traditional balanced armature limitations through an open design that removes sound tubes and dampers, enabling unobstructed diaphragm movement. LID technology corrects non-linear electrical resistance across multiple driver sets, ensuring consistent frequency response regardless of source output impedance. APEX modules provide user-controllable air pressure exchange with four isolation levels (mX, m15, m20, m12). The integrated 4-way passive crossover optimizes each driver’s performance within dedicated acoustic spaces. While these innovations demonstrate substantial technical advancement, the limited availability of comprehensive measurement data prevents full validation of the claimed technological superiority compared to established industry solutions.

Cost-Performance

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This site evaluates based solely on functionality and measured performance values, without considering driver types or configurations. The U4s at 1,099 USD offers user-facing functions including APEX modules providing user-controllable isolation levels (mX: -10dB, m15: -15dB, m20: -20dB, m12: -12dB) and LID technology for source impedance correction.

Comparator selected: Etymotic ER2SE (wired IEM; passive isolation; neutral FR target; widely measured; same essential user function set). Its manufacturer-listed isolation is 35–42 dB, which is higher than U4s equipped with APEX modules (mX: -10dB, m15: -15dB, m20: -20dB, m12: -12dB). Third-party FR measurements place ER2SE close to a diffuse-field/neutral target. Therefore, for core axes relevant to IEM fidelity (FR linearity; passive isolation), ER2SE qualifies as equivalent-or-better from the listener’s perspective. [2][3]

Cheapest verified market price (ER2SE): 149.99 USD (official store; sale pricing excluded for CP).
U4s price (denominator): 1,099 USD.
CP calculation: 149.99 USD ÷ 1,099 USD = 0.1364 → 0.1 after rounding to one decimal place. [1][2]

(Notes: APEX isolation figures are manufacturer-published per module; ER2SE’s isolation is manufacturer-published and repeatedly validated in practice by deep-insertion Etymotic designs. FR evidence for both models relies on third-party coupler measurements.) [1][2][3]

Reliability & Support

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64 Audio provides standard 2-year limited warranty coverage for universal-fit earphones with global manufacturer support infrastructure. The company covers return shipping for in-warranty products and maintains dedicated customer support. With a 15-year track record since 2010, 64 Audio has established credibility in professional and consumer IEM markets. However, the complex 4-driver hybrid system incorporating multiple proprietary technologies (tia, LID, APEX) presents inherently higher failure risk compared to simpler single-driver designs. The sophisticated crossover network and modular APEX system introduce additional potential failure points. While company history and support infrastructure demonstrate competence, the complex multi-technology design moderates overall reliability expectations.

Rationality of Design Philosophy

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64 Audio demonstrates strong technical innovation with proprietary technologies providing measurable functional benefits. The tia driver technology addresses genuine acoustic limitations of traditional balanced armatures, while LID corrects real-world source impedance variations that affect frequency response across different sources. APEX modules offer practical user control over isolation levels with direct acoustic impact. The design philosophy incorporates scientific approaches to solving specific technical problems while also including subjective elements focused on “inspiration” and aesthetic considerations. The 1,099 USD pricing reflects the cost of unique proprietary technologies not available elsewhere at lower price points. While the company pursues innovative approaches and demonstrates continued technological advancement, the balance between scientific innovation and subjective marketing elements moderates the overall rationality of the design philosophy.

Advice

The 64 Audio U4s targets audiophiles seeking APEX modules providing user-controllable isolation levels and LID technology for source impedance correction. The APEX modules offer four isolation levels (mX: -10dB, m15: -15dB, m20: -20dB, m12: -12dB) that directly affect frequency response and soundstage characteristics. LID technology corrects source impedance variations ensuring consistent performance across different audio sources. Potential purchasers should understand that products with higher passive isolation performance (e.g., Etymotic ER2SE with 35-42dB isolation) are available at significantly lower prices. Users specifically requiring APEX module configurability or LID technology may find value in the U4s, while those prioritizing maximum isolation per dollar or neutral frequency response may find better value in alternatives such as the ER2SE.

References

[1] TechPowerUp. 64 Audio U4s In-Ear Monitors Review - A New 1000 USD Benchmark? - Fit, Comfort & Audio Performance. https://www.techpowerup.com/review/64-audio-u4s-in-ear-monitors/4.html. Accessed 2025-11-22. IEC711 audio coupler, 1/12th octave smoothing, m15 Apex modules.

[2] Etymotic. ER2SE Earphones. https://etymotic.com/product/er2se-earphones/. Accessed 2025-11-22.

[3] In-Ear Fidelity. ER2SE FR graph page (711-type coupler). https://crinacle.com/graphs/iems/etymotic-er2se/. Accessed 2025-11-22.

[4] 64 Audio. U4s In Ear Monitors

Quad Drive Hybrid UIEM

U4s 64 Audio. https://www.64audio.com/products/u4s. Accessed 2025-11-22.

(2025.12.3)