E-scooter NVH engineering: Noise/Vibration/Harshness as the fifth cross-cutting infrastructure axis — UN R51 (motor-vehicle noise) + UN R138 (AVAS quiet road transport) + UN R41 (motorcycle noise) + EU Regulation 540/2014 + FMVSS 141 (49 CFR 571.141 minimum sound for hybrid/electric) + ISO 362-1:2015 vehicle drive-by noise + ISO 2631-1:1997+Amd 1:2010 whole-body vibration + ISO 2631-5:2018 multi-shock + ISO 5349-1/-2:2001 hand-arm vibration (cross-ref) + ISO 11819-1:2023 SPB + ISO 11819-2:2017 CPX road-pavement noise + IEC 60068-2-6:2007 sinusoidal vibration + IEC 60068-2-64:2019 broadband random vibration + MIL-STD-810H:2019 Method 514.8 + ISO 16750-3:2023 automotive mechanical loads + ISO 8608:2016 road surface PSD + ISO 1680:2013 rotating electrical machines airborne noise + ISO 532-1:2017 Zwicker loudness + IEC 61672-1:2013 sound level meters + ISO 13473-1 mean profile depth + SAE J2889 + SAE J3043 + NHTSA NPRM 2009 + EU Reg 540/2014 AVAS mandate (M/N from 2019/2021) + Japan MLIT Article 43-3 + China GB/T 41788-2022
In the engineering guide series we covered the lithium-ion battery + BMS + thermal runaway intro, the braking system, the motor and controller, the suspension, the tires, lighting and visibility, the frame and fork, the display and HMI, the SMPS CC/CV charger, the connector and wiring harness, ingress protection, bearings with ISO 281 L10, the stem and folding mechanism, the deck, the handgrip + lever + throttle, the wheel as an assembly, fastener engineering as the joining-axis, thermal management as the heat-dissipation cross-cutting axis, EMC/EMI as the interference-mitigation cross-cutting axis, and cybersecurity as the interconnect-trust cross-cutting axis. These 21 engineering axes described individual subsystems, means of joining, heat dissipation, coexistence of electromagnetic fields, and establishment of trust — but none of them described what the e-scooter emits into the mechanical and acoustic environments: vibrational energy that becomes sound (accessible to hearing) and vibration (accessible to the body through soles, palms, and seat).
A modern e-scooter is an acoustic-vibrational source with two opposing risks. First, it is too quiet for pedestrians and reduces detectability for vision-impaired people (the Silent EV problem), which forced regulators to mandate AVAS (Acoustic Vehicle Alerting System) for M (passenger) and N (commercial) vehicle categories in the EU (Reg 540/2014, from 1 July 2019 for new models and 1 July 2021 for all new production vehicles) and in the US (FMVSS 141, 49 CFR 571.141, fully effective on 1 September 2020). Second, it transmits vibration into the body through the deck, handlebars, and (where present) saddle — partially covered by ISO 5349-1/-2 for the hands in the article on handgrip + throttle, but not covered for whole-body vibration through the feet (ISO 2631-1) or multi-shock from pavement irregularities (ISO 2631-5). A third category is acoustic defects that signal a problem: motor PWM whine, freewheel-pawl rattle, brake-pad squeal, bearing hum — each of these is a diagnostic signal that an NVH engineer decodes into a mechanical fault.
This is the twenty-second engineering-axis deep-dive in the guide series, and the fifth cross-cutting infrastructure axis (parallel to fastener engineering as joining, thermal management as heat-dissipation, EMC/EMI as interference-mitigation, and cybersecurity as interconnect-trust). NVH describes the means by which motion energy is converted to sound and vibration, which is present in every previous engineering axis: a BLDC motor with an 8-kHz PWM creates magnetostriction vibration that radiates as aerodynamic whine; bearings with BPFO/BPFI defect frequencies radiate broadband noise; a tire tread pumps air through grooves and radiates tire-pavement noise; brakes under thermal judder conditions generate high-frequency squeal; the freewheel pawl in coast mode rattles a pulse train in the 200-2000 Hz audible range. The NVH engineering task is to quantify each source, engineer mitigation (spread-spectrum PWM, skewed stator slots, three-point isolators, tuned-mass dampers, in-tire foam), verify durability on a shaker table to MIL-STD-810H / IEC 60068-2-64 / ISO 16750-3 standards, and prove compliance with AVAS regulations (where applicable).
A note on the PLEV (Personal Light Electric Vehicle) context: the e-scooter is not in scope of UN R51 (motor vehicles ≥4 wheels), not in scope of UN R41 (motorcycles), not in scope of EU Reg 540/2014 (covers M and N categories), and not in scope of FMVSS 141 (hybrid/electric passenger cars and light trucks). PMD acoustic and vibrational behaviour is a regulatory grey zone: the governing standards for motor-vehicle drive-by noise (ISO 362-1) are methodologically applicable but not mandatory. EN 17128:2020 (the PMD umbrella) contains no acoustic-emission or whole-body-vibration limits. The industry baseline is therefore voluntary compliance with ISO 2631-1 (for comfort-level assessment), ISO 1680 (for motor airborne-noise rating), and independent AVAS solutions for market differentiation (for example, the Segway-Ninebot KickScooter Max G2 emits an audible chime at low speeds).
1. Why NVH is a separate cross-cutting axis
NVH is not just “loud/quiet”. It is a system in which every element has quantified engineering specifications:
| Element of the NVH system | What it describes | Governing standard |
|---|---|---|
| Acoustic emission | Sound pressure level L_p (dB SPL) at the pedestrian point + sound power level L_W (dB PWL) as a source-invariant of surface radiation | ISO 362-1:2015 vehicle drive-by, ISO 1680:2013 rotating electrical machines, IEC 61672-1:2013 sound level meters |
| Hand-arm vibration | Frequency-weighted r.m.s. acceleration a_hv (m/s²) on the palm in orthogonal X/Y/Z with the Wh filter, ISO 5349-1 Annex A | ISO 5349-1:2001 + ISO 5349-2:2001 + EU Directive 2002/44/EC (cross-ref to the handgrip article) |
| Whole-body vibration | Frequency-weighted r.m.s. acceleration a_w (m/s²) on the body under the seat or feet with the Wk filter (vertical) and Wd filter (horizontal), ISO 2631-1 Tables 4-5 | ISO 2631-1:1997 + Amd 1:2010 general, ISO 2631-5:2018 multi-shock via VDV |
| Tire-pavement noise | Pass-by sound level L_veh (dB A) at 7.5 m from the trajectory using the SPB or CPX method | ISO 11819-1:2023 SPB + ISO 11819-2:2017 CPX + ISO 13473-1 mean profile depth |
| Road excitation PSD | Geometric mean displacement PSD G_d(n_0) (m²/m⁻¹) at the reference spatial frequency n_0 = 0.1 cycle/m for classes A-H | ISO 8608:2016 mechanical vibration road surface profile classification |
| AVAS sound | Minimum sound pressure level as a function of speed 0-30 km/h + frequency content + pitch shift | UN R138 EU/Japan, FMVSS 141 US (49 CFR 571.141), GB/T 41788-2022 China |
| Vibration durability | Endurance test profile PSD or sinusoidal sweep + dwell at resonance + post-test functional check | IEC 60068-2-6:2007 sinusoidal, IEC 60068-2-64:2019 broadband random, MIL-STD-810H:2019 Method 514.8, ISO 16750-3:2023 |
| Psychoacoustic loudness | Zwicker loudness N (sone) and loudness level L_N (phon) — closer to human perception than linear A-weighting | ISO 532-1:2017 Zwicker method, ISO 532-2:2017 Moore-Glasberg |
| Sound level meter class | Type tolerance (Class 1 ±0.7 dB at 1 kHz, Class 2 ±1.0 dB at 1 kHz) + frequency weighting A/C/Z | IEC 61672-1:2013 sound level meters, IEC 61672-3:2013 periodic tests |
| Reverberation control | Sound power to sound pressure conversion via the room constant R + Sabine reverberation T60 | ISO 3744:2010 sound power survey method (engineering grade), ISO 354:2003 absorption measurement |
NVH does not reduce to any of the previous engineering axes: hand-arm vibration is already covered in the handgrip engineering article, but whole-body vibration, acoustic emission, tire-pavement noise, and vibration durability tests had not received a deep dive. This is what this article fixes.
2. Standards matrix
| # | Standard | Scope | Key point |
|---|---|---|---|
| 1 | UN R51 | Noise from passenger vehicles ≥4 wheels on pass-by | ISO 362-1 methodology; e-scooter out of scope |
| 2 | UN R138 | AVAS quiet road transport | Min L_p as a function of 10/20/30 km/h; pitch shift ±0.8 % per km/h |
| 3 | UN R41 | Motorcycle noise, L-category | Drive-by ≥77 dB(A) acceleration |
| 4 | EU Reg 540/2014 | Sound level of motor vehicles | M/N category from 1.7.2019 new type-approvals + 1.7.2021 all new production |
| 5 | FMVSS 141 | Min sound HEV/EV USA | 49 CFR 571.141; fully effective 1.9.2020 |
| 6 | ISO 362-1:2015 | Drive-by ASEP urban | 50 km/h constant + WOT acceleration; 7.5 m from trajectory |
| 7 | ISO 2631-1:1997 + Amd 1:2010 | Whole-body vibration general | a_w r.m.s. with Wk/Wd weighting; health/comfort/perception |
| 8 | ISO 2631-5:2018 | Multiple shock WBV | VDV (vibration dose value) in m/s^1.75; non-Gaussian shocks |
| 9 | ISO 5349-1/2:2001 | Hand-arm vibration | A(8) daily exposure; DEAV 2.5 m/s² / DELV 5.0 m/s² (EU Dir 2002/44/EC) |
| 10 | ISO 11819-1:2023 | SPB tire-pavement noise | Statistical pass-by; 7.5 m mic; 100 vehicle samples per category |
| 11 | ISO 11819-2:2017 | CPX tire-pavement noise | Close-proximity trailer mic; pavement characterisation |
| 12 | IEC 60068-2-6:2007 | Sinusoidal vibration | Resonance search 10-2000 Hz; dwell time per axis |
| 13 | IEC 60068-2-64:2019 | Broadband random vibration | PSD profile m/s²/Hz; total Grms; duration per axis |
| 14 | MIL-STD-810H:2019 | DOD environmental | Method 514.8 Procedure I-V; ground vehicle PSD categories |
| 15 | ISO 16750-3:2023 | Automotive mechanical | Sinusoidal + random + shock for category-vehicle-mount |
| 16 | ISO 8608:2016 | Road surface PSD classification | A (smooth highway) … H (very rough); geometric mean G_d(n_0) |
| 17 | ISO 1680:2013 | Rotating electrical machines noise | L_WA declared value per ISO 3744/3745 |
| 18 | ISO 532-1:2017 | Zwicker loudness | Stationary signal; 1/3-octave critical bands |
| 19 | IEC 61672-1:2013 | Sound level meters | Class 1 (lab) and Class 2 (field) tolerance limits |
| 20 | SAE J2889 / J3043 | EV AVAS recommended practice | Sound character, pitch-shift slope, switch-off speed |
Not all entries are simultaneously applicable to the e-scooter as a regulated product, but all are methodologically applicable as an engineering reference.
3. Three-level energy transport chain: air — structure — body
NVH recognises three parallel paths along which vibrational energy travels from source to receiver:
- Acoustic (airborne) path: the source radiates oscillations into the air as sound waves (longitudinal pressure waves), which propagate to the receiver (pedestrian, rider). Intensity decreases as
1/r²in free-field (inverse-square law). Mitigation: acoustic enclosure, acoustic absorption materials (mineral wool, open-cell foam). - Structure-borne path: the source transmits vibration through mechanical contact (mount, joint, weld) into the structure (frame, deck), which re-radiates sound as vibroacoustic noise. Intensity is a function of coupling stiffness and modal density of the structure. Mitigation: isolators (rubber mounts, PUR pads, Sorbothane), constrained-layer damping (CLD).
- Tactile (body-borne) path: vibration is transmitted through the structure → contact point (palm, foot, seat) → human body. Specific health hazards: HAVS, WBV-induced lumbar disc disease, motion sickness. Mitigation: suspension (passive damping), tuned-mass damper at critical modes, ergonomic-grip damping.
The boundary frequency at which the dominant path crosses over from structure-borne to airborne is roughly 500-1000 Hz for a typical e-scooter case. Below that — structure-borne (the body hears with feet/palms). Above that — airborne (the ear hears). This is critical for testing: an accelerometer on the frame captures structure; a microphone at 1 m from the motor captures acoustic.
4. Noise sources: 7-row noise-source matrix
| # | Source | Typical frequency range | Mechanism |
|---|---|---|---|
| 1 | Motor PWM whine | f_PWM (8 / 12 / 16 / 20 kHz) + harmonics | Stator magnetostriction at f_PWM + amplitude modulation by rotor mech. frequency; Maxwell radial force on stator teeth |
| 2 | Motor electromagnetic | order-6f_e (for 3-phase) + slot-pole modulation | Slot-pole interaction; tooth-tip Maxwell force harmonics |
| 3 | Tire-pavement rolling noise | 500 Hz - 2 kHz dominant peak | Air pumping in grooves + tread block stick-slip + radial vibration of tread |
| 4 | Bearing noise | BPFO, BPFI, BSF, FTF + sidebands | Defect rolling pass-frequencies; envelope spectrum reveals the fault |
| 5 | Brake squeal | 1 - 16 kHz narrow-band | Disc-pad mode coupling; thermal-induced friction-coefficient hysteresis; DTV |
| 6 | Freewheel pawl ratchet | 200 - 2 kHz pulse train | Pawl-tooth engagement during coast; POE points per rev |
| 7 | AVAS speaker | 100 - 5 kHz synthesised | Intentional emission per UN R138 / FMVSS 141 / GB/T 41788 |
Each of these components is a distinct diagnostic signature. A rider at 25 km/h without AVAS radiates predominantly motor whine (8 kHz fundamental + harmonics) + tire-roll (500 Hz - 2 kHz broadband) + freewheel pawl during coast. A bearing defect appears as a new narrow-band peak at BPFO ((N_b/2) × f_r × (1 - (d/D)×cos(α)), where N_b is the number of rolling elements, f_r is shaft rotation frequency, d/D are the element/race diameters, α is the contact angle).
5. AVAS: Acoustic Vehicle Alerting System — 4-row regulatory matrix
The Silent EV problem was first documented in the National Federation of the Blind (NFB) USA petition, 2008 after hybrid-vehicle incidents involving blind pedestrians. NHTSA published an NPRM (Notice of Proposed Rule Making) in 2009; the final rule FMVSS 141 followed in 2018 (49 CFR 571.141). The EU implemented UN R138 (Quiet Road Transport Vehicles) in parallel, incorporated into Regulation (EU) 540/2014.
| # | Jurisdiction | Regulation | Min L_p requirement | Applicability date |
|---|---|---|---|---|
| 1 | EU + UN-ECE | UN R138 + EU Reg 540/2014 | 56 dB(A) @ 10 km/h, 50 dB(A) constant 20 km/h, pause-on @ ≥20 km/h; pitch shift ≥0.8 %/km/h | 1.7.2019 new type-approvals + 1.7.2021 all new M/N production |
| 2 | USA | FMVSS 141 (49 CFR 571.141) | 47 dB(A) @ 10 km/h, 51 dB(A) @ 20 km/h, 56 dB(A) @ 30 km/h, max 75 dB(A); switch-off ≤30 km/h | 1.9.2019 50 % production, 1.9.2020 100 % |
| 3 | Japan | MLIT Article 43-3 Safety Standards Road Vehicles | Per UN R138 (Japan harmonised via WP.29) | 1.10.2018 new models + 1.10.2020 all production |
| 4 | China | GB/T 41788-2022 | Per UN R138 baseline + China-specific harmonic content | 2022-08 publication; voluntary → mandatory GB/T 28382 successor |
The e-scooter — as a PMD — is not regulated by these provisions in any jurisdiction. But voluntary AVAS solutions exist (Segway-Ninebot KickScooter Max G2 chime; Lime fleet acoustic alert beep at ride-start). The industry trend is to add a soft chime as a differentiation feature, especially for shared-mobility units in road traffic.
6. Tire-pavement noise: ISO 11819-1 SPB and ISO 11819-2 CPX
Rolling noise is a mixture of air pumping in tread grooves (air is pushed out at each contact-patch passage) and tread block vibration with frequency = v / λ, where λ is the tread block length. For a typical e-scooter 8.5″ tire with 5-mm blocks at 25 km/h this gives 25/(3.6×0.005) = 1389 Hz dominant frequency — squarely within the human ear’s most sensitive range (1-4 kHz).
ISO 11819-1 SPB (Statistical Pass-By): 100 vehicle samples per category passing a reference microphone at 7.5 m from the trajectory, 1.2 m height; the result is L_veh,75 km/h normalised to 75 km/h for category 1 (cars). For e-scooters the methodology is applicable at lower speeds (25-30 km/h).
ISO 11819-2 CPX (Close-Proximity): a trailer with 4 microphones at 0.2 m from the tire-contact patch + reference tires (SRTT for P1, AAV4 for P2); the result is L_CPX for pavement classification.
ISO 13473-1: macrotexture mean profile depth (MPD) in mm via ASTM E2157 / EN ISO 13473-1 measurement; MPD < 0.5 mm → smooth (more rolling noise); MPD > 1.2 mm → rough (more dropout + dB content).
Inverse trade-off: rough-textured pavement reduces aerodynamic swoosh but increases vibration-induced contact noise. A drainage open-graded asphalt (PA — porous asphalt) reduces SPB noise by 3-5 dB versus traditional DA (dense asphalt) thanks to air absorption in the porous matrix.
7. Vibration sources: 6-row vibration-source matrix
| # | Source | Typical frequency range | Mechanism |
|---|---|---|---|
| 1 | Motor rotor unbalance | 1×f_r (first-order rpm) | Rotor mass-eccentricity → centrifugal force F = m × e × ω² |
| 2 | Road surface excitation | 0.5 Hz - 30 Hz at 25 km/h | ISO 8608 spatial PSD × v (speed) → temporal PSD in Hz |
| 3 | Suspension transmissibility | 1 - 5 Hz body bounce + 8-12 Hz wheel hop | T(f) = √[1 + (2ζf/f_n)²] / √[(1-(f/f_n)²)² + (2ζf/f_n)²] (Rao, Mechanical Vibrations) |
| 4 | Frame/fork bending modes | 15 - 80 Hz first bending; 80-300 Hz torsional | Modal analysis; FEM eigenvalue extraction; experimental modal hammer |
| 5 | Bearing defect | BPFO / BPFI / BSF / FTF + 2× harmonics + sidebands | Envelope spectrum after band-pass filter + Hilbert demodulation |
| 6 | Tire harmonic | 1× tire-rotation harmonic; 2× 2-belt; non-uniformity | Imbalance + radial-runout + RFV (radial force variation) |
A drop from an 80-mm curb at 15 km/h creates a shock with broadband PSD content 0-100 Hz — within the scope of ISO 2631-5 multi-shock VDV. Continuous riding over cobblestones (ISO 8608 class E-F) is in scope of ISO 2631-1 stationary WBV with a_w ≈ 1.5-2.5 m/s² (feet vertical Wk), which is classified as “Likely uncomfortable” (ISO 2631-1 Table B.1).
8. Whole-body vibration: ISO 2631-1 + ISO 2631-5
ISO 2631-1:1997 + Amd 1:2010 defines the method for assessing human exposure to mechanical vibration through frequency-weighted r.m.s. acceleration:
$$a_w = \sqrt{ \int_0^\infty (W(f))^2 G_{aa}(f) df }$$
where W(f) is the frequency weighting filter (Wk for vertical seat/floor, Wd for horizontal, Wf for motion sickness 0.1-0.5 Hz); G_{aa}(f) is the acceleration PSD.
Daily exposure A(8):
$$A(8) = a_w \sqrt{T_{exp} / 8}$$
where T_{exp} is daily exposure time in hours.
ISO 2631-1 Table B.1 health-comfort scale:
a_w r.m.s. (m/s²) | Perceptual reaction |
|---|---|
| < 0.315 | Not uncomfortable |
| 0.315 - 0.63 | A little uncomfortable |
| 0.5 - 1.0 | Fairly uncomfortable |
| 0.8 - 1.6 | Uncomfortable |
| 1.25 - 2.5 | Very uncomfortable |
| > 2.0 | Extremely uncomfortable |
For an e-scooter the feet receive vertical vibration through the deck — the Wk filter applies. A typical 25 km/h ride on ISO 8608 class C pavement gives a_w ≈ 0.8-1.4 m/s² at the feet, classified as “Fairly uncomfortable” to “Uncomfortable”.
ISO 2631-5:2018 extends the methodology to non-Gaussian multi-shock content (cobblestones, expansion joints, curbs) via the VDV (Vibration Dose Value):
$$VDV = \left[ \int_0^T a_w^4(t) dt \right]^{1/4} \text{ [m/s}^{1.75}\text{]}$$
VDV is more sensitive to peak shocks vs r.m.s. (4th-power factor vs 2nd-power factor). The daily VDV exposure value (EAV) in the UK Workplace Vibration Regulations 2005 is 9.1 m/s^1.75; the daily VDV exposure limit value (ELV) is 21 m/s^1.75. For a recreational e-scooter user (1-2 hours/day) the VDV is unlikely to reach the regulatory threshold, but for commercial fleet riders (food-delivery, 6-8 hours/day) it is a real occupational hazard with the risk of lumbar disc problems.
9. Vibration durability test: 4-row matrix
E-scooter components must withstand the vibration spectrum over their service life. Standardised vibration testing is the methodology for accelerated life testing:
| # | Standard | Profile | Type test |
|---|---|---|---|
| 1 | IEC 60068-2-6:2007 | Sinusoidal 10-2000 Hz logarithmic sweep or dwell at resonance | Resonance search + dwell endurance |
| 2 | IEC 60068-2-64:2019 | Broadband random PSD; Grms 1-30 g rms total per axis | Service-life simulation |
| 3 | MIL-STD-810H:2019 Method 514.8 | Procedure I (general vibration) - V (helicopter vibration); ground mobile + manpack profiles | DOD environmental qualification |
| 4 | ISO 16750-3:2023 | Automotive mounting category I-IV; random + sinusoidal; environmental T sweep | Automotive component qualification |
IEC 60068-2-64 ground-mobile profile for an e-scooter frame (mounted on a vehicle):
- PSD 5 Hz @ 0.01 g²/Hz roll-off
- PSD 20 Hz peak @ 0.04 g²/Hz
- PSD 200 Hz @ 0.02 g²/Hz roll-off
- PSD 2000 Hz @ 0.001 g²/Hz cut-off
- Total Grms ≈ 4-6 g rms
- Duration 1-3 h per axis (X/Y/Z)
MIL-STD-810H Method 514.8 Procedure I (Loose-Cargo) for an accessory-bag e-scooter (commute-rider in transit):
- PSD per ground-mobile Category 24 base profile
- Random vibration 5-500 Hz
- Total Grms ≈ 4 g rms
- Duration 32 min per axis
ISO 16750-3:2023 Category III (Vehicle: rigid mounted at body):
- Random PSD 10-1000 Hz
- Total Grms ≈ 2.5 g rms
- Duration 8-24 h per axis
-
- Thermal cycling -40 / +85 °C synchronously
Post-test pass criterion: no functional degradation and no visible structural failure.
10. ISO 8608: road surface classification
ISO 8608:2016 classifies road surfaces by the geometric mean displacement PSD G_d(n_0) at reference spatial frequency n_0 = 0.1 cycle/m:
| Class | G_d(n_0) [10⁻⁶ m³] | Description |
|---|---|---|
| A | 16 (8 - 32) | Smooth highway, new asphalt |
| B | 64 (32 - 128) | Highway, well-maintained |
| C | 256 (128 - 512) | Urban roads, moderate wear |
| D | 1024 (512 - 2048) | Rough urban, expansion joints |
| E | 4096 (2048 - 8192) | Damaged urban, potholes |
| F | 16384 (8192 - 32768) | Cobblestones, granite block paving |
| G | 65536 (32768 - 131072) | Unsealed gravel, dirt roads |
| H | 262144 (>131072) | Severely degraded, off-road |
Spatial PSD converts to temporal PSD via the speed v:
$$G_d^{time}(f) = G_d^{spatial}(n) / v, \quad f = v \cdot n$$
where f is temporal frequency (Hz), n is spatial frequency (cycle/m), v is speed (m/s).
Riding an e-scooter at 25 km/h (6.94 m/s) on ISO 8608 class C delivers peak excitation in the 0-100 Hz range — precisely the band of frame bending modes (15-80 Hz) and rider WBV (4-8 Hz peak sensitivity of ISO 2631-1 Wk).
11. Mitigation: 6-row matrix
| # | Technology | Principle | Expected reduction |
|---|---|---|---|
| 1 | Skewed stator slots | Pole-slot harmonic cancellation through axial skew | 6-12 dB HF motor whine |
| 2 | Spread-spectrum PWM | Random / hopping PWM frequency 8-16 kHz | 8-15 dB peak f_PWM tone |
| 3 | Elastomeric isolator | Resonance shift via elastic mount + damping | 15-25 dB structure-borne above 30 Hz |
| 4 | Tuned mass damper (TMD) | Antiresonance at f_problematic | 10-20 dB at the specific mode |
| 5 | Constrained-layer damping (CLD) | Visco-elastic material between panel and constraining layer | 10-30 dB structure-borne broadband |
| 6 | Acoustic enclosure | Mass-spring-mass with absorption lining | 20-40 dB airborne above 200 Hz |
Practical e-scooter applications:
- Motor: skewed stator slots (Δ ½-1 slot pitch) + spread-spectrum PWM (Microchip dsPIC33CK, TI TMS320F28004x with SVPWM dithering). PWM-tone audibility reduction — 8-15 dB.
- Frame: 3-point elastomeric mount for the controller box; TMD on the frame headstock for the bending mode (typically first mode 40-60 Hz).
- Deck: CLD sandwich (Sorbothane 30A duroshore 1-2 mm between two aluminium panels).
- Brake: anti-squeal shim of graphite-impregnated NBR rubber on the pad back-side; cross-drilled disc for thermal stability (anti-judder).
- Tire: in-tire foam ring (Continental ContiSilent, Pirelli Noise Canceling System) reducing 4-7 dB broadband 500 Hz - 2 kHz.
12. Real incidents and regulatory drivers
| # | Event / Standard | Date | What happened |
|---|---|---|---|
| 1 | National Federation of the Blind petition | 2008 | NFB petition to NHTSA on small-vehicle vs hybrid silent-mode pedestrian incidents → NHTSA NPRM 2009 |
| 2 | Pedestrian Safety Enhancement Act of 2010 | Public Law 111-373 | US Congress mandates NHTSA to develop HEV/EV minimum sound standard |
| 3 | FMVSS 141 NPRM | 2013-01-09 | Notice of Proposed Rule Making, 78 FR 2797 |
| 4 | FMVSS 141 Final Rule | 2016-12-14 | 81 FR 90416; initial compliance date 2018-09-01 |
| 5 | UN R138.00 | 2017-04-12 | UN Geneva adopts AVAS minimum sound requirements |
| 6 | EU Reg 540/2014 AVAS Annex VIII | 2014-04-16 + 2019-07-01 | AVAS mandatory for new M/N production from 2019-07-01 (new type-approvals), 2021-07-01 (all production) |
| 7 | FMVSS 141 Phase-in completion | 2020-09-01 | 100 % HEV/EV production compliance USA |
| 8 | GB/T 41788-2022 publication | 2022-08-09 | China voluntary AVAS standard published |
The e-scooter remains outside the scope of all these regulations, but industry practice is to opt-in an audible chime for shared mobility (Bird, Lime, Tier, Voi mostly add a chime at ride-start; the Segway-Ninebot Max G2 emits a continuous chime ≤10 km/h). This is a voluntary safety baseline forming from market expectations and potential local-municipal mandates (NYC Open Streets pilot 2024-2025, Paris bylaws 2023).
13. DIY NVH check (8 steps)
Performed on a parked scooter (static tests 1-3) and during riding (dynamic tests 4-8). Minimum kit: a smartphone with a sufficiently-calibrated sound-meter app (NIOSH SLM, Decibel X) + a free-falling vibration recorder (Vibration Analyzer app using the phone IMU at 100-400 Hz cutoff).
- Idle motor noise. Lift the wheel off the ground, hold throttle 25-30 % open. Listen for a tonal peak — this is f_PWM (typically 8 kHz). If you hear it as a “whining” tone rather than broadband swoosh — spread-spectrum PWM is not active or motor laminations are loose.
- Coast freewheel test. Accelerate to 15 km/h, release throttle, listen to coast. A sharp “rat-tat-tat” above 1 kHz means the pawl spring is over-tensioned or POE points are too sparse.
- Static deck tap test. Strike the centre of the deck with a 1-2 kg bicycle-mallet; listen to the ring-down. Long resonance (>0.5 s) = low damping; bright tone = over-stiff deck; muted dull thud = well-damped.
- Constant-speed run, 25 km/h. On smooth asphalt, hold 25 km/h for 60 s; obvious tonal peaks in the SLM app point to potential motor whine, gear mesh, or bearing defect.
- Bearing isolation listen. During coast (no motor torque) on smooth pavement; a chirp/grind/howl from the axle signals bearing wear (BPFO).
- Brake squeal trigger. Light front brake @ 15 km/h; high-frequency squeal 2-5 kHz indicates disc-pad mode coupling.
- Cobblestone WBV walk. Walk-roll over cobblestones (ISO 8608 class E-F), recording 3-axis phone IMU. PSD peak in 4-12 Hz is body-bounce WBV; > 2 m/s² crosses the uncomfortable threshold.
- Pothole shock test. Drop a 50-80 mm curb @ 10 km/h; phone IMU peak indicates shock magnitude. > 5 g_peak puts frame/fork at risk over time.
14. DIY remediation (6 steps)
- Lubricate the freewheel pawl. Open the hub, apply light oil (10W sewing-machine oil) at the pawl-spring contact — 30 % reduction in pawl rattle.
- Tighten/replace deck-handlebar bolts. Re-torque per spec; structural rattle most often comes from a loose stem-collar (cross-ref fastener-and-bolted-joint-engineering.md).
- Add a visco-elastic damping pad under the deck. 2 mm Sorbothane 30A or 1-2 mm automotive Dynamat Extreme; expected reduction 5-10 dB structure-borne.
- Replace squealing brake pads. Bed in new pads + anti-squeal compound (Permatex Disc Brake Quiet) between pad and caliper piston.
- Bearing replacement. If BPFO is confirmed (envelope spectrum peak) — replace the bearing per the bearing-engineering-iso-281-l10-life.md procedure.
- In-tire foam ring (where compatible). Continental ContiSilent or an aftermarket polyurethane ring for tubeless 8.5″+ tires; expected reduction 4-7 dB broadband rolling noise.
15. Industry shift: silent EV → AVAS adoption
2008 NFB petition → 2010 PSEA → 2016 FMVSS 141 Final Rule → 2018 phase-in start → 2020 100 % compliance USA. In parallel, EU UN R138 → 2019 mandatory AVAS for new production. By 2025 practically all regulated HEV/EV in the two largest markets carry AVAS systems.
For PMD/e-scooter we have voluntary adoption: Segway-Ninebot KickScooter Max G2 (2024) — the first mass-market e-scooter with a continuous AVAS chime. Lime fleet (2023-2024) — ride-start chime. Bird (2024) — speaker-based audible alert. This is proactive self-regulation, ahead of the expected future regulatory push (US Conference of Mayors 2024 review; EU Mobility Strategy 2030 includes PMD-AVAS discussion).
The industry NVH trend for e-scooters 2018→2026:
- 2018-2020 generation: no NVH mitigation; motor whine 65-72 dB @ 1 m; cogging torque audible; freewheel rattle loud.
- 2021-2023 generation: skewed-stator motors deploy; spread-spectrum PWM on higher-end controllers (Segway G30/G65, Apollo Pro line); motor whine 55-62 dB.
- 2024-2026 generation: standard skew + spread-PWM on mid-tier; CLD-deck panels on flagships; AVAS chime optional add-on; motor whine 48-58 dB.
Δ 2018 → 2026: 17-20 dB reduction in motor airborne noise, which corresponds to a ×8-10 suppression of acoustic power at the same mechanical output.
16. Recap (10 points)
- NVH is the fifth cross-cutting infrastructure axis after joining (DT), heat-dissipation (DV), interference-mitigation (DX), and interconnect-trust (DZ). It describes the conversion of motion energy into sound and vibration.
- Three energy-transport paths: airborne (air → ear), structure-borne (mount → frame → re-radiated sound), tactile (frame → contact → body).
- Acoustic side: motor PWM whine + tire-pavement roll + brake squeal + freewheel pawl ratchet + bearing noise — each with its own signature decoded by spectrogram or envelope analysis.
- Vibration side: motor unbalance + road excitation (ISO 8608 class A-H) + suspension transmissibility + frame modes + bearing defect + tire harmonics — each with its own frequency and modal contribution.
- AVAS is mandatory for M/N categories in the EU (UN R138 + Reg 540/2014) and HEV/EV in the US (FMVSS 141), but PMD is out of scope — voluntary adoption in industry.
- Whole-body vibration through the deck is in scope of ISO 2631-1 (stationary WBV) and ISO 2631-5 (multi-shock VDV); recreational riding does not threaten regulatory limits, while commercial fleet riders face a potential occupational hazard.
- Vibration durability tests for component qualification — IEC 60068-2-6 (sinusoidal sweep) + IEC 60068-2-64 (broadband random PSD) + MIL-STD-810H Method 514.8 (DOD) + ISO 16750-3 (automotive).
- Tire-pavement noise — ISO 11819-1 SPB (vehicle category) + ISO 11819-2 CPX (pavement); dominant peak usually 500 Hz - 2 kHz; mitigation via in-tire foam (4-7 dB) and open-graded pavement (3-5 dB).
- Mitigation toolbox: stator skew + spread-PWM (8-15 dB motor whine) + elastomeric isolator (15-25 dB structure-borne) + CLD (10-30 dB broadband) + TMD (10-20 dB specific mode) + acoustic enclosure (20-40 dB airborne).
- Industry trend 2018→2026: Δ 17-20 dB reduction in motor airborne noise; AVAS adoption voluntary baseline 2024+; PMD regulatory framework taking shape (US Conference of Mayors 2024, EU Mobility Strategy 2030).
Cross-references in earlier engineering articles touching NVH:
- fastener-and-bolted-joint-engineering.md — preload loss causes rattle (structural)
- bearing-engineering-iso-281-l10-life.md — defect frequencies BPFO/BPFI/BSF/FTF
- brake-system-engineering.md — brake squeal mode coupling
- motor-and-controller-engineering.md — BLDC PWM-frequency selection
- suspension-engineering.md — transmissibility T(f) and damping ratio
- tire-engineering-rolling-resistance-grip-standards.md — tread block geometry
- handgrip-lever-and-throttle-engineering.md — HAVS ISO 5349 detailed §
Source standards (English-language, 0 Russian):
- UN R51 (UNECE Reg No. 51 — motor vehicle noise); UN R138 (Quiet Road Transport Vehicles AVAS); UN R41 (motorcycle noise) — unece.org
- EU Regulation (EU) 540/2014 (sound level of motor vehicles) — eur-lex.europa.eu
- FMVSS 141 (49 CFR 571.141, Minimum sound for hybrid/electric) — nhtsa.gov + ecfr.gov
- ISO 362-1:2015 (vehicle drive-by noise constant speed); ISO 362-2:2009 (category L); ISO 362-3:2016 (indoor) — iso.org
- ISO 2631-1:1997 + Amd 1:2010 (whole-body vibration general); ISO 2631-5:2018 (multi-shock VDV) — iso.org
- ISO 5349-1:2001 + ISO 5349-2:2001 (hand-arm vibration) — iso.org
- ISO 11819-1:2023 SPB; ISO 11819-2:2017 CPX — iso.org
- ISO 13473-1 (mean profile depth pavement texture) — iso.org
- IEC 60068-2-6:2007 (sinusoidal vibration); IEC 60068-2-64:2019 (broadband random) — webstore.iec.ch
- MIL-STD-810H:2019 (DOD environmental test) Method 514.8 — dla.mil
- ISO 16750-3:2023 (automotive mechanical) — iso.org
- ISO 8608:2016 (road surface PSD classification) — iso.org
- ISO 1680:2013 (rotating electrical machines noise); ISO 3744:2010 (sound power survey) — iso.org
- ISO 532-1:2017 Zwicker loudness; ISO 532-2:2017 Moore-Glasberg — iso.org
- IEC 61672-1:2013 (sound level meters Class 1/2); IEC 61672-3:2013 (periodic tests) — webstore.iec.ch
- SAE J2889 (EV pedestrian alert sound character); SAE J3043 (AVAS recommended practice) — sae.org
- Pedestrian Safety Enhancement Act of 2010 (PL 111-373) — congress.gov
- China GB/T 41788-2022 (AVAS for electric vehicles) — std.samr.gov.cn
- Japan MLIT Article 43-3 Safety Standards for Road Vehicles — mlit.go.jp
- NHTSA NPRM 78 FR 2797 (2013-01-09); FMVSS 141 Final Rule 81 FR 90416 (2016-12-14) — federalregister.gov
- Rao, S.S. “Mechanical Vibrations” (Pearson, 6th ed.) — transmissibility derivation
- Beranek & Vér “Noise and Vibration Control Engineering” (Wiley, 2nd ed.) — acoustic enclosure / sound power
- Norton & Karczub “Fundamentals of Noise and Vibration Analysis for Engineers” (Cambridge UP, 2003) — modal analysis methodology
NVH is the 22nd engineering axis and the fifth cross-cutting infrastructure axis, completing the five-instance set of continuous infrastructural layers: joining (DT) + heat-dissipation (DV) + interference-mitigation (DX) + interconnect-trust (DZ) + acoustic-vibration-emission (EB).