A ground-up reimagining of the court-reporting stenomask using modern MEMS microphones, real-time DSP noise cancellation, and exponential-horn acoustic engineering. Open source. Buildable for $25.
↓ Full Design · BOM · Manufacturing Guide · License · GitHub →
The stenomask concept — sealing the mouth inside a mask to capture voice while blocking ambient noise — is sound. But every commercial implementation (Talk Tech, Martel, Stenomask Company) uses a $0.30 electret mic inside a padded plastic cup. The result: hollow, muffled, compressed audio that confuses speech recognition engines and costs $200-400. The designs haven't changed in 40 years.
Sealed plastic cup creates cavity resonance at 200-400 Hz. Audio sounds like talking into a jar.
$200-400 retail for a padded cup with a $0.30 electret capsule. Massive markup, no innovation.
Zero signal processing. No noise cancellation, no EQ, no optimization for ASR engines.
Holding a mask to your face for 6-8 hours causes wrist, hand, and jaw strain. No ergonomic alternative.
Mic capsule floats inside the cup. Position shifts between units. Inconsistent audio quality.
3.5mm analog output over long cables picks up EMI, ground loops, and signal loss.
The interior is not a cup — it's a truncated exponential horn (95×70mm mouth, 40×25mm throat, 55mm deep). This provides acoustic impedance matching between the vocal tract and mic capsule, with a natural high-pass characteristic that suppresses boominess. Non-parallel walls (8° splay) eliminate standing waves.
Two Knowles SPH0641LU4H-1 PDM MEMS mics, 20mm apart, hard-mounted to the PCB. PDM digital output means no analog noise, ever. Summing the pair gives +3 dB signal gain over noise. The mic position is physically fixed — no QC lottery.
A third MEMS mic on the exterior captures room noise. The RP2040 MCU runs 256-point FFT-based spectral subtraction: estimating noise spectrum during silence and subtracting it from the voice signal, band by band. No anti-phase artifacts — just clean speech.
Five biquad filters shaped for speech recognition: high-pass at 90 Hz, -4 dB cut at 250 Hz (boom), +4 dB boost at 3.5 kHz (consonants), +3 dB at 6.5 kHz (sibilants), -3 dB shelf at 10 kHz. A feed-forward compressor normalizes level so quiet and loud talkers both feed ASR consistently.
Magnetic quick-release headband made from carbon fiber tow. The mask snaps in with 4 neodymium magnets (1.5 kg holding force). Total system weight: 100g. No hand strain. Counterbalanced rear weight. Fits 50th-95th percentile heads.
A 6mm × 18mm port tuned to 280 Hz creates a controlled low-frequency escape path. This reduces internal chamber pressure without leaking voice, and doubles as a breath-pressure relief vent.
The complete engineering specification is in design-spec.md. Below are the key subsystems.
| Parameter | Value | Rationale |
|---|---|---|
| Geometry | Truncated exponential horn | Impedance matching, natural high-pass |
| Mouth opening | 95mm × 70mm (elliptical) | Covers mouth + lower nose |
| Throat | 40mm × 25mm | Mic PCB footprint |
| Depth | 55mm | Compact but sufficient horn length |
| Interior volume | ~110 cm³ | Small enough to isolate, large enough to breathe |
| Wall splay | 8° non-parallel | Eliminates standing waves / comb filtering |
| Wall treatment | 3mm polyester acoustic felt | Absorbs 2-6 kHz reflections |
| Tuned port | 6mm × 18mm (280 Hz Helmholtz) | Low-frequency pressure relief |
| Face seal | Medical silicone o-ring (6mm, 50 Shore A) | Airtight, comfortable, wipeable, durable |
| Mic | Type | SNR | Output | Cost | Role |
|---|---|---|---|---|---|
| Knowles SPH0641LU4H-1 | MEMS omnidirectional | 65 dBA | PDM digital | $2.85 | Voice capture (×2) |
| Knowles SPH0641VD4H-1 | MEMS omnidirectional | 65 dBA | PDM digital | $2.85 | Noise reference (×1) |
| Alternative: TDK T5838 | MEMS | 64 dBA | PDM digital | $2.50 | Drop-in replacement |
| Alternative: Infineon IM69D130 | MEMS | 69 dBA | I²S | $4.20 | Best SNR, higher cost |
Full BOM with supplier links: bill-of-materials.md
| Category | Key Components | Subtotal |
|---|---|---|
| Electronics | 3× Knowles SPH0641 mics, RP2040 MCU, AP2112K LDO, USB-C, PCB, passives | $13.92 |
| Mechanical / Shell | PETG filament, acoustic felt, silicone o-ring, acoustic mesh, hardware | $4.00 |
| Harness | Carbon fiber band, 8× N42 magnets, silicone pads, counterweight | $6.40 |
| Total (Wired) | $24.32 | |
| Optional: BLE Audio | nRF5340, 100mAh LiPo, charge IC, antenna, crystal | +$6.95 |
| ⚡ Solar + BLE Variant | ||
| Flexible CIGS solar film (25cm²) | Powerfoyle or equivalent thin-film, laminated to shell exterior | $4.50 |
| MPPT charger | TI BQ25570 energy harvesting IC | $2.80 |
| Supercapacitor buffer (2× 10F) | Eaton/Maxwell, handles BLE transmit bursts | $3.20 |
| LiPo battery (200mAh) | Standard 802030, fits in headband housing | $3.50 |
| BLE module (upgraded) | nRF5340 dual-core, LC3 codec, BLE 5.3 Audio | $8.00 |
| Total (Solar + BLE) | $46.32 | |
Flexible CIGS thin-film laminated to the mask shell + headband strap. Harvests from ambient office light (300-500 lux) and direct window light. Supercapacitor buffer handles BLE transmit bursts. Target: 8+ hour sessions without plug-in charging.
openstenomask.uf2 to the mounted drive. Device enumerates as "OpenStenoMask".Geometry descriptions for all printable parts are in stl-descriptions.md. These are precise enough to model in Fusion 360, FreeCAD, or OpenSCAD. The STL files will be published as community members model and validate them.
This design is published as prior art. By releasing the full technical specification, schematics, firmware, and mechanical designs publicly, we establish that these innovations were known and publicly disclosed as of August 9, 2026. This prevents any party from patenting the specific novel elements of this design.
Novel claims established as prior art:
This is not legal advice. For formal defensive publication, consult a patent attorney. The designs are additionally protected by the CERN-OHL-S v2 reciprocal license.
Everything in this project is open source. No commercial restrictions. No proprietary encumbrances. Build it, sell it, modify it, improve it.
Documentation
This page, design spec, BOM, assembly guides