Overview

The Fosi Audio i5 represents the company’s debut venture into headphones, featuring open-back planar magnetic technology with a 97mm ultra-large diaphragm. Developed over one year with advanced targeted sputtering technology, the i5 incorporates premium materials including handcrafted walnut wood chambers and CNC-milled aluminum frame. With 28Ω impedance and 98dB/mW sensitivity, these headphones target audiophiles seeking planar magnetic sound reproduction with all-day comfort design.

Scientific Validity

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The i5 demonstrates acceptable scientific validity with measured performance meeting transparent level thresholds. The specified THD <1% @ 100dB SPL encompasses values near the 0.5% problematic threshold for headphones, indicating distortion performance at the problematic level boundary. Third-party measurements from Reference Audio Analyzer show 105.7dB SPL sensitivity [2], which exceeds the 100dB excellent threshold, while the 10Hz-50kHz frequency response range significantly surpasses transparent level requirements. The measured 30.5Ω impedance closely matches the specified 28Ω nominal impedance, demonstrating specification accuracy. However, limited comprehensive measurement data requires conservative evaluation per policy, resulting in 0.5 score.

Technology Level

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The i5 demonstrates solid technological implementation through proprietary advanced targeted sputtering technology for 1.3μm silver metal voice coil coating and in-house development spanning over one year of acoustic tuning. The 2μm diaphragm precision manufacturing and symmetrical arrangement of 22 N50-grade neodymium magnets show technical sophistication, while premium material integration of walnut wood and CNC aluminum represents contemporary construction standards. However, the design lacks advanced digital integration that characterizes cutting-edge audio technology, limiting the overall technology assessment.

Cost-Performance

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CP = 109.00 USD ÷ 549.99 USD = 0.2

The HiFiMAN HE400SE at 109.00 USD provides equivalent open-back planar magnetic functionality with comparable impedance (25Ω vs 28Ω) and driving requirements. Equipped with open-back planar magnetic design, and impedance characteristics are functionally equivalent for practical driving requirements. While the i5 features premium walnut wood construction and extended 10Hz-50kHz frequency response versus the HE400SE’s 20Hz-20kHz range, these enhancements do not constitute essential functional differences for core planar magnetic performance, making the substantial price premium difficult to justify.

Reliability & Support

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Fosi Audio provides above-average support through 24-month warranty (exceeding typical 2-year standard) and robust planar magnetic construction with fewer moving parts inherently resistant to failure. The company offers comprehensive support infrastructure including global shipping, email and WhatsApp customer service, and 30-day free return policy. However, as a new manufacturer in the headphone category with no established track record in this product segment, long-term reliability assessment remains provisional pending market validation.

Rationality of Design Philosophy

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The i5 reflects Fosi Audio’s rational approach to making planar magnetic headphones more accessible through measurement-focused development and community engagement. The company’s emphasis on user-centric innovation and contemporary manufacturing techniques demonstrates scientific rationality, while the mission to reduce barriers to high-fidelity audio aligns with cost-effective design philosophy. However, the substantial investment in premium materials without clear performance justification over more cost-effective alternatives significantly undermines rationality, as costs should directly contribute to function and measurement performance improvement according to rational design principles.

Advice

For potential purchasers seeking planar magnetic headphones, consider whether the i5’s premium materials and craftsmanship justify the significant price premium over functionally equivalent alternatives. The HiFiMAN HE400SE provides comparable planar magnetic performance at approximately one-fourth the cost, making it difficult to recommend the i5 purely on cost-performance grounds. The i5 may appeal to users prioritizing premium aesthetics and build quality over pure value, but those focused on sound quality per dollar should explore more cost-effective planar magnetic options first.

References

[1] Fosi Audio - i5 Open-Back Planar Magnetic Headphones, https://fosiaudio.com/products/i5-headphones, accessed 2025-09-19, specifications: 97mm driver, 28Ω impedance, 98dB/mW sensitivity, <1% THD @ 100dB SPL

[2] Reference Audio Analyzer - Fosi Audio i5 Measurement Report, https://reference-audio-analyzer.pro/en/report/hp/fosi-audio-i5.php, accessed 2025-09-19, measured: 105.7dB SPL sensitivity, 30.5Ω impedance

[3] Amazon - HiFiMAN HE400SE Planar Magnetic Headphones, https://www.amazon.com/HIFIMAN-Audiophiles-Great-Sounding-Sensitivity-Comfortable/dp/B08Z2SK5C4, accessed 2025-09-19, specifications: 25Ω impedance, 91dB SPL sensitivity, 20Hz-20kHz frequency response, price: 109.00 USD

[4] Fosi Audio - 24-Month Warranty Policy, https://fosiaudio.com/pages/fosi-audio-warranty, accessed 2025-09-19, warranty coverage and support terms

(2025.9.20)