A STAX “Pro bias” electrostatic headphone holds its diaphragm at 580 V DC, about 9.7 times the 60 V DC limit for the touch-safe ES1 class in IEC 62368-1 (STAX product specifications 2026; DigiKey, IEC 62368-1 design article 2021). That bias also sits inside the 75 to 1,500 V DC band the EU Low Voltage Directive uses to define its scope (European Commission, LVD 2014/35/EU page 2026). The diaphragm it charges can be extremely thin: Sennheiser claims 2.4 micrometres for the HE 1, and a research graphene membrane measured about 30 nm (Sennheiser HE 1 product page 2026; Zhou and Zettl, Applied Physics Letters 2013). The same electrostatic principle, run in reverse, is what condenser microphone capsules use, covered in our vintage tube microphone capsule noise statistics, and it takes a different route from the magnet-driven designs in our planar magnetic vs dynamic driver THD statistics. We aggregated data from STAX technical specifications, the IEC, UL, the European Commission, peer-reviewed papers in Applied Physics Letters, the Journal of the Acoustical Society of America and Microsystems and Nanoengineering, and archived independent measurements published by Stereophile.
A note on the word “compliance” in this article. It has two meanings here, and we keep them apart. Mechanical compliance is how easily a diaphragm yields to a force (the opposite of stiffness): a thin, low-mass membrane under light tension is highly compliant. Regulatory compliance means meeting electrical-safety rules such as IEC 62368-1 or the EU Low Voltage Directive. Sections 1, 2 and 6 deal mostly with the first meaning, and sections 4 and 5 with the second.
TL;DR
- 580 V DC bias on every current STAX full-size earspeaker: SR-X9000, SR-009D, SR-L700 MK2 and SR-X1 (STAX product specifications 2026)
- 2 mA DC: the ES1 touch-current limit in normal operation, rising to 25 mA DC under a single fault (UL Japan, IEC 62368-1 technical brief)
- 60 V DC: the ES1 voltage limit in IEC 62368-1, against 120 V DC for ES2 (DigiKey, IEC 62368-1 design article 2021)
- 75 to 1,500 V DC: the voltage range covered by the EU Low Voltage Directive since April 20, 2016 (European Commission, LVD 2014/35/EU page 2026)
- About 30 nm: thickness of a graphene electrostatic diaphragm, biased at 100 V DC (Zhou and Zettl, Applied Physics Letters 2013)
- 2.4 micrometres: Sennheiser’s own stated diaphragm thickness for the HE 1 (Sennheiser HE 1 product page 2026)
- 0.01% THD at 1 kHz and 100 dB SPL: Sennheiser’s own claim for the HE 1 system (Sennheiser HE 1 product page 2026)
- 1.26% THD or less from 20 Hz to 400 Hz for a push-pull MEMS electrostatic speaker, vs 4.4% at 500 Hz for the earlier single-sided design (Fraunhofer IPMS, Microsystems and Nanoengineering 2022 and 2019)
- 10 dB: high-frequency shelf measured on the Stax SR-007 from 2 kHz to 20 kHz (Stereophile, Comparing World-Class Headphones 2011)
- May 26, 2023: publication of IEC 62368-1 Edition 4.0, 823 pages (IEC Webstore 2023)
1. Bias Voltage: The 580 V “Pro Bias” Standard and Lower-Voltage Designs
In an electrostatic headphone, the bias is a fixed high DC voltage that charges the diaphragm, while the music signal swings the two fixed electrodes (stators) on either side of it. The striking pattern is uniformity: every current STAX full-size earspeaker specifies the same 580 V DC bias, as do its energizers, so the bias works less like a design choice and more like an interface standard between headphone and amplifier.
Research designs show how much of that voltage comes from geometry rather than physics. A graphene earphone with a 120 micrometre electrode gap ran at only 100 V DC (Zhou and Zettl, Electrostatic Graphene Loudspeaker), and a 2025 nanosheet transducer used 80 V DC (Park, Lee et al., Materials 2025). Narrow gaps cut the voltage needed for the same field strength, which is why the miniature designs in section 6 run at tens of volts rather than hundreds.
| Metric | Value | Source |
|---|---|---|
| Bias voltage of current STAX full-size earspeakers (SR-X9000, SR-009D, SR-L700 MK2, SR-X1) | 580 V DC | STAX product specifications 2026 |
| Bias voltage range of the STAX SR-003 MK2 in-ear earspeaker | 540 V to 580 V DC | STAX product specifications 2026 |
| Bias voltage of a graphene electrostatic earphone prototype | 100 V DC | Zhou and Zettl, Applied Physics Letters 2013 |
| Electrode-to-diaphragm gap in the graphene prototype | 120 micrometres | Zhou and Zettl, Applied Physics Letters 2013 |
| DC bias on the electrode of an NbSe2 nanosheet hybrid transducer | 80 V DC | Park, Lee et al., Materials 2025 |
| Best-performing AC drive tested on that nanosheet transducer (of 30, 60 and 90 V) | 90 V AC | Park, Lee et al., Materials 2025 |
Context: the 580 V figure is a manufacturer specification, not an independent measurement, and the STAX energizers in section 4 list the same DC bias. The full specification pages are on stax.co.jp.
2. Diaphragm Thickness, Electrode Gap and Moving Mass (Mechanical Compliance)
An electrostatic diaphragm is pulled almost evenly across its whole surface, so it does not need to be stiff to stay in shape. That lets designers make it extremely thin and light, which is what gives the driver its high mechanical compliance. A research graphene diaphragm measured about 30 nm thick, from 22% to 25% light transmission, roughly 80 times thinner than the 2.4 micrometres Sennheiser states for its HE 1 (30 nm is 0.03 micrometres; 2.4 / 0.03 = 80) (Sennheiser HE 1 product page).
The payoff of low moving mass shows up in efficiency rather than in a single mass figure. The graphene authors report operating currents of a few nanoamps, power consumption far below 1 microwatt, and an electrostatic efficiency that can in principle be close to 1, against below 0.1 for magnetic-coil earphones. We could not find a peer-reviewed moving-mass figure, in milligrams, for any commercial electrostatic headphone, so the comparison with dynamic and planar drivers remains qualitative (see the Data watch note).
| Metric | Value | Source |
|---|---|---|
| Diaphragm thickness of the Sennheiser HE 1 (vendor’s own claim) | 2.4 micrometres, platinum-vaporised | Sennheiser HE 1 product page 2026 |
| Thickness (estimated from light transmission) and diameter of a graphene electrostatic diaphragm | About 30 nm thick, 7 mm across | Zhou and Zettl, Applied Physics Letters 2013 |
| Efficiency of an electrostatic driver in principle vs typical magnetic-coil earphones | Close to 1 vs below 0.1 | Zhou and Zettl, Applied Physics Letters 2013 |
| Thickness of a conductive NbSe2 nanosheet used in a hybrid transducer diaphragm | About 3.5 nm | Park, Lee et al., Materials 2025 |
| Diameters of the circular push-pull diaphragms modelled and measured for an over-ear headphone | 60 mm and 65 mm | Chiang and Huang, JASA 2019 |
| Semi-major and semi-minor axes of an elliptical push-pull headphone diaphragm prototype | 32 mm and 30 mm | Chiang and Huang, Journal of Vibration and Acoustics 2022 |
Context: in the 2019 study, the 65 mm design built into an over-ear headphone produced an artificial-ear response close to the Harman preferred target curve, and the authors judged its sound quality comparable to high-end dynamic headphones (Chiang and Huang, Journal of the Acoustical Society of America 2019). That is the authors’ own assessment, not a blind listening test.
3. Distortion and Frequency Response: What Has Actually Been Measured
The low-distortion reputation of electrostatics rests mostly on manufacturer figures. Sennheiser claims 0.01% THD at 1 kHz and 100 dB SPL for the HE 1, but that is a vendor claim for the whole system, amplifier included, not an independent measurement of the headphone. Independent published numbers are scarcer and older. For context on what a “neutral” response means, see our Harman target curve preference statistics.
The independent record that does exist is useful because it contradicts the “flat by default” assumption. Archived measurements published by Stereophile found the Stax SR-007 shelved down 10 dB from 2 kHz to 20 kHz relative to the mids and bass, with a narrow peak near 10 kHz (Stereophile, Comparing World-Class Headphones: Stax SR-007). Most recent available data: Stereophile and InnerFidelity, 2011, plus a measurement sheet whose file is dated 2017.
| Metric | Value | Source |
|---|---|---|
| THD of the Sennheiser HE 1 system at 1 kHz and 100 dB SPL (vendor’s own claim) | 0.01% | Sennheiser HE 1 product page 2026 |
| Frequency range stated for the Sennheiser HE 1 (vendor’s own claim) | 8 Hz to above 100 kHz | Sennheiser HE 1 product page 2026 |
| Measured high-frequency level of the Stax SR-007, 2 kHz to 20 kHz, relative to mids and bass | Shelved down 10 dB | Stereophile, Comparing World-Class Headphones 2011 |
| Broadband isolation of the Stax SR-X Mk3 Pro, 100 Hz to 10 kHz | -11 dBr | Stereophile measurement sheet 2017 |
| Electrical impedance and phase measurements for electrostatic headphones in the Stereophile archive | Not available (not measured) | Stereophile measurement sheet 2017 |
| Frequency response stated for the Koss ESP/950 electrostatic headphone (vendor’s own claim) | 8 Hz to 35,000 Hz | Koss ESP/950 product page 2026 |
Outlier note: the archived Stereophile sheets plot THD+noise for electrostatics at 90 and 100 dB, but the published PDFs print curves, not tabulated percentages, so we report no independent THD percentage for a full-size electrostatic headphone (Stereophile SR-X Mk3 Pro measurements). The Koss ESP/950 page lists a 100k ohm impedance and 98 dB SPL/1mW sensitivity, which are also vendor figures.
4. Energizer Voltage Swing and Power
An electrostatic driver behaves as a capacitor (110 pF including the cable on STAX’s full-size models), so it needs voltage rather than current, and STAX rates those earspeakers at 100 to 101 dB at 1 kHz for a 100 Vrms input. The flagship SRM-T8000 swings 470 Vrms at 1 kHz, given as 1,330 Vp-p in the manufacturer’s specification, with 60 dB of gain (STAX SRM-T8000 specifications). That is about 15.7 times the 30 Vrms ES1 limit for AC below 1 kHz listed in the table (470 / 30 = 15.7), which is why section 5 matters for anyone building or modifying these amplifiers.
The spread across the range shows the trade-off between headroom and portability. The battery-powered SRM-D10 MK2 drops to 200 Vrms and 53 dB of gain, while the mains-powered models sit between 300 and 470 Vrms (STAX SRM-D10 MK2 specifications). Even the portable unit offers twice the 100 Vrms input used for the sensitivity rating.
| Metric | Value | Source |
|---|---|---|
| Maximum output of the STAX SRM-T8000 energizer at 1 kHz (60 dB gain, 95 W consumption) | 470 Vrms (1,330 Vp-p) | STAX SRM-T8000 specification 2026, also listed by HomeTheaterHifi 2024 |
| Maximum output of the STAX SRM-700S, SRM-400S and SRM-500T energizers at 1 kHz | 450, 400 and 300 Vrms | STAX product specifications 2026 |
| Maximum output, gain and battery life of the portable STAX SRM-D10 MK2 | 200 Vrms, 53 dB (450 times), about 3.5 hours on USB input | STAX product specifications 2026 |
| ES1 voltage limit for AC below 1 kHz in IEC 62368-1 | 30 Vrms (42.4 V peak) | DigiKey, IEC 62368-1 design article 2021 |
| ES2 voltage limit for AC below 1 kHz in IEC 62368-1 | 50 Vrms (70.7 V peak) | DigiKey, IEC 62368-1 design article 2021 |
| Lower AC voltage bound of the EU Low Voltage Directive | 50 V AC | European Commission, LVD 2014/35/EU page 2026 |
Context: the 1,330 Vp-p figure appears in the manufacturer specification reproduced in the HomeTheaterHifi SR-X9000 review of August 19, 2024. The arithmetic checks out: 470 Vrms times 1.414 times 2 is about 1,329 V.
5. Regulatory Compliance: IEC 62368-1 Energy Classes and the EU Low Voltage Directive
IEC 62368-1 replaced the older audio/video and IT safety standards by sorting energy sources into classes and prescribing safeguards for each. Its fourth edition, published on May 26, 2023, runs to 823 pages and carries a stability date of 2026, the year up to which the IEC expects the text to stay unchanged (IEC 62368-1:2023 on the IEC Webstore). The electrical classes matter directly for electrostatic headphones: ES1 needs no safeguard for ordinary users, ES2 needs at least one, and ES3 needs a double or reinforced safeguard (DigiKey, How to Design Protection Circuits Compliant for IEC 62368-1).
Voltage alone does not settle the class. The standard sets limits for both prospective touch voltage and touch current, which is why a high-voltage but current-limited supply is assessed on both counts (UL Japan, IEC 62368-1 technical brief). We found no public document stating how any specific electrostatic headphone system is classified, so the ratios below compare voltages only.
| Metric | Value | Source |
|---|---|---|
| IEC 62368-1 edition, publication date and length | Edition 4.0, May 26, 2023, 823 pages (corrected version August 2025) | IEC Webstore 2023 |
| DC voltage limits for ES1 and ES2 | 60 V DC and 120 V DC | DigiKey, IEC 62368-1 design article 2021 |
| ES1 touch-current limits, normal operation vs single-fault condition | 2 mA DC vs 25 mA DC (0.5 mA rms vs 5 mA rms for AC below 1 kHz) | UL Japan, IEC 62368-1 technical brief |
| 580 V DC Pro bias compared with the ES1 and ES2 DC voltage limits | About 9.7 times ES1 (580 / 60) and 4.8 times ES2 (580 / 120) | Derived from the section 1 bias and the DigiKey DC limits |
| Maximum rated voltage of equipment covered by IEC 62368-1 | 600 V | Element, A Complete Guide to IEC 62368-1 2025 |
| Voltage range covered by the EU Low Voltage Directive 2014/35/EU | 50 to 1,000 V AC and 75 to 1,500 V DC | European Commission, LVD page 2026 |
| Date from which the Low Voltage Directive 2014/35/EU applies | April 20, 2016 | European Commission, LVD page 2026 |
| Scheduled withdrawal date of EN 62368-1:2014 in Europe | July 6, 2024 | Element, A Complete Guide to IEC 62368-1 2025 |
Context: the European Commission’s LVD page applies the voltage range to the input or output of the equipment, and Element expects the fourth edition to become mandatory in 2026 or 2027 (Element, A Complete Guide to IEC 62368-1). The UL brief also describes ES2 contact as possibly painful and ES3 contact as able to cause injury, including fibrillation.
6. MEMS Electrostatic In-Ear Speakers: Scaling the Principle Down
MEMS (micro-electro-mechanical systems) speakers are the newest electrostatic designs, etched in silicon rather than built from stretched film. They show the mechanical-compliance trade-off in its plainest form: an electrostatic gap can only be used up to a point before the moving electrode snaps to the fixed one (pull-in), which a Fraunhofer IPMS and Arioso team put at about one third of the electrode gap (Ehrig et al., Forum Acusticum 2020). A symmetrical push-pull actuator held THD at 1.26% or less between 20 Hz and 400 Hz, where the same team’s earlier single-sided design had measured 4.4% at 500 Hz (Kaiser et al., The push-pull principle, Microsystems and Nanoengineering 2022).
Output is the open problem. The earlier design reached 69 dB at 500 Hz in a standard ear simulator, and the push-pull version 79 dB up to 1 kHz (Kaiser et al., all-silicon MEMS micro speaker, Microsystems and Nanoengineering 2019, and the 2022 paper above). A 2025 review covering performance data from 1990 to 2026 found piezoelectric MEMS speakers dominate the field, mainly because of their focus on miniaturisation and efficiency (Wittek et al., Review of MEMS Transducers for Audio Applications). For how small conventional speakers behave near their limits, see our smartphone speaker distortion and maximum SPL statistics.
| Metric | Value | Source |
|---|---|---|
| THD of a single-sided MEMS electrostatic speaker at 500 Hz (40 V DC offset, 10 Vpp drive) | 4.4% | Kaiser et al., Microsystems and Nanoengineering 2019 |
| SPL of that device at 500 Hz in an IEC 60318-4 ear simulator (1.26 cm3) | 69 dB, maximum 104 dB at 11.4 kHz | Kaiser et al., Microsystems and Nanoengineering 2019 |
| THD of the push-pull MEMS electrostatic speaker from 20 Hz to 400 Hz | 1.26% or less, peaking at 2% at 1.4 kHz | Kaiser et al., Microsystems and Nanoengineering 2022 |
| Electrostatic gap and chip size of the push-pull device | 2.5 micrometres, 10 x 10 mm2 | Kaiser et al., Microsystems and Nanoengineering 2022 |
| Typical separation gaps in electrostatic MEMS speakers | 1 to 8 micrometres | Micromachines, Review of Recent Development of MEMS Speakers 2021 |
| SPL of a 2 x 2 mm parylene electrostatic MEMS speaker at 1 cm, driven at 150 V | 113.4 dB at 7.68 kHz | Micromachines review 2021, citing Kim 2005 |
| Drive and THD of a sputtered-PZT piezoelectric MEMS speaker, for comparison | 2 Vpp, THD below 2% | Micromachines review 2021, citing Stoppel 2018 |
Context: the same Kaiser et al. 2022 paper cites earlier work by Sugimoto et al. in which push-pull driving lowered harmonics by about 10 dB compared with one-sided driving. The Micromachines review notes the 2005 parylene result was measured at 1 cm, so it is not directly comparable with ear-simulator figures.
Summary: Electrostatic Headphone High-Voltage Bias and Driver Compliance by the Numbers
| Metric | Value | Source |
|---|---|---|
| Bias voltage of current STAX full-size earspeakers | 580 V DC | STAX product specifications 2026 |
| Maximum output of the STAX SRM-T8000 energizer at 1 kHz | 470 Vrms (1,330 Vp-p) | STAX SRM-T8000 specification 2026 |
| ES1 and ES2 DC voltage limits in IEC 62368-1 | 60 V DC and 120 V DC | DigiKey, IEC 62368-1 design article 2021 |
| ES1 voltage limit for AC below 1 kHz | 30 Vrms (42.4 V peak) | DigiKey, IEC 62368-1 design article 2021 |
| 580 V DC bias as a multiple of the ES1 DC limit | About 9.7 times | Derived (580 / 60), see section 5 |
| ES1 touch-current limit, normal operation | 2 mA DC | UL Japan, IEC 62368-1 technical brief |
| EU Low Voltage Directive DC range | 75 to 1,500 V DC | European Commission, LVD page 2026 |
| IEC 62368-1 Edition 4.0 publication date | May 26, 2023 | IEC Webstore 2023 |
| Maximum rated voltage covered by IEC 62368-1 | 600 V | Element, A Complete Guide to IEC 62368-1 2025 |
| Graphene electrostatic diaphragm thickness | About 30 nm | Zhou and Zettl, Applied Physics Letters 2013 |
| Graphene earphone bias voltage | 100 V DC | Zhou and Zettl, Applied Physics Letters 2013 |
| Sennheiser HE 1 diaphragm thickness (vendor’s own claim) | 2.4 micrometres | Sennheiser HE 1 product page 2026 |
| Sennheiser HE 1 THD at 1 kHz and 100 dB SPL (vendor’s own claim) | 0.01% | Sennheiser HE 1 product page 2026 |
| Stax SR-007 measured treble shelf, 2 kHz to 20 kHz | Down 10 dB | Stereophile, Comparing World-Class Headphones 2011 |
| MEMS electrostatic speaker THD, push-pull design (20 Hz to 400 Hz) vs earlier single-sided design (500 Hz) | 1.26% or less vs 4.4% | Kaiser et al., Microsystems and Nanoengineering 2022 and 2019 |
| Pull-in limit on usable deflection of a single electrostatic electrode | About 1/3 of the electrode gap | Ehrig et al., Forum Acusticum 2020 |
| Typical electrostatic MEMS speaker gap | 1 to 8 micrometres | Micromachines review 2021 |
Methodology and Sources
- STAX product specification pages, read in October 2026: SR-X9000, SRM-T8000, SRM-D10 MK2, plus the SR-009D, SR-L700 MK2, SR-X1, SR-003 MK2, SRM-700S, SRM-400S and SRM-500T pages on stax.co.jp. All STAX figures are the manufacturer’s own claims.
- Sennheiser HE 1 product page and Koss ESP/950 product page: vendor claims, labelled as such.
- Zhou and Zettl, Electrostatic Graphene Loudspeaker, Applied Physics Letters 102, 2013 (arXiv full text).
- Chiang and Huang, Experimental modeling and application of push-pull electrostatic speakers, JASA 146(4), 2019; Chiang and Huang, elliptical push-pull diaphragm, Journal of Vibration and Acoustics 2022 (abstract via OpenAlex).
- Park, Lee et al., High-Performance Acoustic Transducers with Exfoliated NbSe2 Nanosheets and Hybrid Force Mechanisms, Materials 18(4), 2025.
- Stereophile archived InnerFidelity measurements: Comparing World-Class Headphones: Stax SR-007 (2011) and the Stax SR-X Mk3 Pro measurement sheet (2017).
- HomeTheaterHifi, Stax SR-X9000 earspeaker review, August 19, 2024, used only for the manufacturer’s 1,330 Vp-p figure.
- IEC 62368-1:2023, IEC Webstore.
- DigiKey, How to Design Protection Circuits Compliant for IEC 62368-1, Steven Keeping, May 5, 2021.
- UL Japan, IEC 62368-1 technical brief on ES classes vs SELV.
- Element, A Complete Guide to IEC 62368-1 (last edited August 7, 2025).
- European Commission, Low Voltage Directive (LVD) page.
- Kaiser et al., Concept and proof for an all-silicon MEMS micro speaker utilizing air chambers, Microsystems and Nanoengineering 5, 43, 2019; Kaiser et al., The push-pull principle, Microsystems and Nanoengineering 8, 125, 2022; Ehrig et al., Electrostatic Push-Pull MEMS Audio Transducer for In-Ear Applications, Forum Acusticum 2020.
- Review of Recent Development of MEMS Speakers, Micromachines 12(10), 1257, 2021 and Wittek, Melnikov, Kaiser and Zimmermann, Review of MEMS Transducers for Audio Applications, arXiv 2025 (revised September 14, 2026).
- Data watch: no independent lab in this session published a tabulated THD percentage or a moving-mass figure (in milligrams) for a current full-size electrostatic headphone, so the distortion and mass comparisons with dynamic and planar drivers rely on vendor claims, research prototypes and MEMS devices. Most recent available data for independent full-size measurements: Stereophile and InnerFidelity, 2011 and 2017. The Zhou and Zettl graphene data (2013) and the UL technical brief (archived 2014) are also older than three years. The ES1 and ES2 limits come from DigiKey and UL explanatory documents, not from the 823-page standard text, which is paywalled; check the standard itself before any certification work. The 9.7 and 15.7 ratios compare voltages only and do not determine an energy class, because touch-current limits also apply. IEC 62368-1 reaches its stability date in 2026, so a revision or confirmation may change these figures, and Element expects the fourth edition to become mandatory in 2026 or 2027.
Last updated: October 27, 2026. We update this roundup quarterly, and the next refresh is expected when the IEC publishes its stability-date decision on IEC 62368-1 Edition 4.0.