Overview

The Softears RSV (Reference Sound Five) is a five-driver all-balanced-armature in-ear monitor released in January 2021, targeting the upper-mid audiophile segment at USD 729. Softears was established as an engineering-focused entity by the founder of Moondrop, with a stated engineering-first development mandate. Each unit features a handcrafted carbon fiber and gold foil faceplate, making every piece unique. A successor, the RSV MK2, launched in 2025 at USD 699 with a patented passive air-damping system and a 4-way crossover; the original RSV is being phased out and shows as sold out on the official Softears store [1].

Scientific Validity

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Third-party measurements are available from multiple independent sources. Klaus Eulenbach’s measurements on an IEC 60318-4 coupler show frequency response deviation of approximately ±1.7 dB over 120 Hz to 10 kHz against a neutral reference target [2]. Harman 2018 IEM target analysis of the available FR measurement data yields a standard deviation of 1.61 dB, consistent with this result and indicating good frequency response accuracy for the earphone category.

THD at treble frequencies (~8 kHz) measures approximately 0.017% at 90 dB SPL, demonstrating excellent distortion control in the high-frequency range [2]. Bass-region THD from 35 Hz upward reaches below 0.3% at 90 dB SPL, an intermediate result for the earphone category; low-frequency harmonic distortion is subject to elevated perceptual masking at typical listening levels [2]. The manufacturer specifies THD below 1% at 94 dB SPL [1], consistent with third-party findings; this rated-output figure does not represent typical operating distortion.

The combination of near-excellent frequency response adherence and excellent treble distortion performance, offset by intermediate bass-region THD, places this product at the transparent-level borderline for the earphone category.

Technology Level

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Softears designs its BA drivers in-house and applies them in a 3-way configuration with a 6-component passive LRC crossover per earpiece targeting near-linear phase alignment. Acoustic tubes are machined to 0.1 mm tolerances within 3D-printed medical resin chambers. These points indicate genuine in-house design and meaningful accumulated acoustic engineering know-how. However, the original RSV holds no proprietary patents; the patented passive air-damping system arrived only with the MK2 in 2025. Multi-BA driver arrays, passive LRC crossover networks, and 3D-printed IEM shells were well-established in the market by 2021 and are fully mature by 2026. The product is entirely passive and analog with no digital integration, software correction, or DSP capability. The handcrafted carbon fiber and gold foil faceplate is aesthetic-only with no acoustic function. Overall, the technical implementation is competent and well executed, but not advanced by current standards.

Cost-Performance

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This site evaluates based solely on functionality and measured performance values, without considering driver types or configurations.

The Softears RSV is priced at 729 USD [1]. The comparison target is the Kefine Klean, a wired passive IEM with the identical 0.78 mm 2-pin detachable cable interface, available at 49 USD [4].

Metric	Softears RSV	Kefine Klean
FR STD (Harman 2018 IEM target)	1.61 dB	1.53 dB [5]
THD (treble ~8 kHz, 90 dB SPL)	~0.017% [2]	Not available
Wired connectivity	Yes (3.5 mm / 4.4 mm)	Yes (3.5 mm / 4.4 mm)
Detachable cable	0.78 mm 2-pin	0.78 mm 2-pin (identical)

The Kefine Klean achieves 1.53 dB Harman 2018 IEM target STD [5] versus the RSV’s 1.61 dB, demonstrating equivalent-or-better frequency response accuracy. THD comparison is provisional: no standard-lab THD data exists for the Kefine Klean or any comparable wired IEM candidates at this price tier, as the measurement methodology used for the RSV is not replicated by standard measurement laboratories. This is a systematic gap in the IEM measurement ecosystem rather than a product-specific limitation — the same data absence applies equally to all other qualified comparison candidates examined.

CP = 49 USD / 729 USD = 0.0672

Rounded to the first decimal place: 0.1.

Reliability & Support

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The RSV carries a 1-year warranty on the IEM unit and 3 months on the included cable, below the 2-year industry average [1]. The unibody medical resin construction with passive-only electronics and no moving parts is inherently robust and resistant to mechanical degradation. Support is managed through a global distributor network rather than direct manufacturer infrastructure, with no dedicated repair center documentation or post-discontinuation parts supply policy published. The product is currently being phased out in favor of the MK2. The only documented physical concern — slight play at the recessed 2-pin connector socket — has not been reported as causing functional failures.

Rationality of Design Philosophy

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The RSV’s design philosophy is explicitly measurement-oriented: the product name encodes a reference-sound objective, the passive LRC crossover is engineered toward linear phase alignment as confirmed by third-party measurements [2], and the development mandate prioritized acoustic engineering. The RSV MK2 (2025) demonstrates rational product progression by adding a patented passive air-damping system and upgraded 4-way crossover at a slightly lower price. The wired passive IEM form factor has clear justification for monitoring and reference use cases requiring zero latency and no battery dependency. However, the per-unit handcrafted carbon fiber and gold foil faceplate adds manufacturing cost with no acoustic benefit, partially directing expenditure away from performance-relevant engineering. Taken together, the design philosophy is mostly rational but not fully cost-optimized.

Advice

The Softears RSV achieves near-excellent frequency response accuracy and excellent treble distortion performance for a wired reference-tuned IEM, supported by multiple independent measurement sources. For a buyer specifically requiring a wired passive IEM with well-characterized FR adherence and confirmed low treble distortion, the measured performance is genuine.

The fundamental issue is cost-performance. The Kefine Klean at 49 USD achieves 1.53 dB Harman 2018 IEM target STD — tighter than the RSV’s 1.61 dB — at roughly one-fifteenth the price. The RSV premium reflects in-house driver manufacturing, precision acoustic engineering depth, and handcrafted aesthetics, none of which translate to a measurable acoustic advantage over available alternatives.

Buyers considering the original RSV should also note it is being discontinued in favor of the RSV MK2 at USD 699, which adds a patented air-damping system and documented design improvements. Purchasing the original RSV at close to full price is difficult to recommend given its end-of-life status and the MK2’s availability at a lower price with objective technical advances.

References

[1] Softears Official Store - Softears RSV product page - https://softears.store/products/softears-rsv - accessed 2026-05-29
[2] Klaus Eulenbach - Softears RSV Measurements - https://www.klauseulenbach.de/2021/02/07/softears-rsv-measurements/ - published 2021-02-07 - IEC 60318-4 coupler, 90 dB SPL @1kHz, up to 9th harmonic for THD
[3] Reference Audio Analyzer - SoftEars RSV Report - https://reference-audio-analyzer.pro/en/report/hp/softears-rsv.php - accessed 2026-05-29 - IEC711 coupler, 192 kHz/24-bit
[4] HiFiGo - Kefine Klean product page - https://hifigo.com/products/kefine-klean - accessed 2026-05-29
[5] Aftersound squig.link - Kefine Klean Silver FR measurement - https://aftersound.squig.link/?share=KEFINE_KLEAN_SILVER - accessed 2026-05-29 - IEM FR measurement database

(2026.5.31)