Reducing the effects of noise, vibration, and harshness (NVH) is a top priority for electric vehicle (EV) manufacturers. Indeed, all critical EV systems, including electric motors and regenerative brakes, produce various forms of NVH as functional byproducts. Moreover, noise created by axles, tires, and wind resistance is often more noticeable in EVs due to the absence of internal combustion engines (ICE) and their conventional exhaust systems.
This article reviews the primary sources of NVH in EVs and highlights how simulation tools and various mitigation techniques help engineers ensure a quiet, smooth driving experience. It also explores evolving NVH-dampening technologies, including three-dimensional acoustic barriers and synthesized in-cabin sounds.
Tracking NVH emissions in EVs
The high-pitched whine emitted by many EV motors is caused by airgap field harmonics (variations in electromagnetic fields) and inverter switching voltage inputs, which control motor speed and torque primarily through pulse width modulation (PWM). These functions generate force waves and vibrations at various resonant frequencies.
Inherent imperfections in torque and stator load profiles can further intensify vibrations, potentially affecting electric drive unit (EDU) components (Figure 1) such as power electronics and gearboxes, and extending to the entire powertrain, including drivetrain components. Notably, these components — such as axles, transmissions, and bearings — produce varying levels of mechanical noise and vibrations.
EV regenerative braking systems also create NVH during the conversion of kinetic energy into electrical energy — and when switching between regenerative and mechanical modes. Similarly, power electronics systems such as on-board chargers (OBCs), battery management systems (BMS), power delivery modules (PDMs), and dc-to-dc converters generate high-frequency noises, whether electromagnetic or otherwise.
Additional NVH sources include EV thermal management systems (TMS) and heating, ventilation, and air conditioning (HVAC) units, encompassing fans, pumps, heat exchangers, and circulating cooling liquids. Lastly, tires and suspension systems produce noticeable noise and vibrations, as does wind resistance caused by side mirrors and cargo racks.
Holistically simulating system-level NVH
Mitigating EV NVH starts at the design level. Engineers use digital simulators to meticulously analyze electromagnetics, thermodynamics, and vibro-acoustics for key systems such as electric motors. Although many conventional simulation methods rely on finite element (FE) analysis within generalist multi-physics packages, this paradigm is often time-consuming and compute-intensive.
In contrast, application-specific packages efficiently construct comprehensive drivetrain models by merging lightweight analytical methods with numerical simulations. This integrated approach helps engineers proactively correct and mitigate electro-mechanical NVH issues in electric motors and ancillary systems, encompassing everything from the powertrain — such as inverters, gearboxes, axles, and transmissions — to additional drivetrain components.
For example, holistically simulating EV motor airgap field harmonics and PWM inverter switching voltage facilitates precise adjustments of key motor design parameters (Figure 2) while guiding vehicle-wide NVH mitigation techniques. These span resonance damping materials, active noise cancellation within powertrains, and adaptive control algorithms for inverters. Integrated NVH simulations also help optimize gearbox tooth profiles, tune axle mass dampers, and calibrate vibration-absorbing mounts for transmissions.
Additionally, integrated simulations improve regenerative braking and onboard charging systems functions, significantly reducing unwanted noise and vibrations across the cabin, undercarriage, and wheel assemblies. Applying a similar comprehensive simulation strategy to TMS and HVAC units effectively streamlines NVH management — with targeted insulation materials, isolators, and advanced control algorithms mitigating noise and vibrations.
Holistic NVH simulations also play a crucial role in synchronizing speed acceleration feedback (SAF) systems, bolstering driver perception of noiseless acceleration with synthesized sound cues. Other artificial cues and background noises, such as pedestrian warnings, cabin sounds, and dynamic road noise, require similar optimization. Lastly, integrated simulations precisely calibrate the volume and direction of acoustic vehicle alerting systems (AVAS), advanced driver-assistance systems (ADAS), and infotainment systems.
Minimizing NVH with new materials and aerodynamic designs
Whether in EV or ICE vehicles, mechanical and electric systems invariably generate various forms of NVH. EVs, however, eliminate engine revving, noisy mufflers, and loud idling. For example, the Nissan Leaf (Figure 3) is up to 20 dB quieter than the average ICE vehicle at speeds between 0 mph and 12.43 mph. As speeds increase to 21.75 mph and beyond, this difference narrows to less than 5 dB.
Certain sounds, such as those produced by tires, wind, and suspension, are often more noticeable in EVs. Moreover, the high-pitch humming or the whirring of electric motors is typically amplified at lower speeds. Although even the most sophisticated simulations and mitigation techniques fail to fully eliminate all forms of NVH, many automotive companies continue to explore new materials and aerodynamic designs to significantly minimize their effects.
Dow, for instance, has developed a polyurethane-based foam that creates a physical, three-dimensional acoustic barrier. Known as Betafoam, the material is particularly effective in isolating electric motor frequencies ranging from 500 to 10,000 Hz. Similarly, non-pneumatic tires can potentially minimize one of the most persistent sources of EV cabin noise. Featuring a solid material design, these airless tires could eventually provide smoother, more consistent contact with road surfaces, reducing vibrations and noise. Lastly, replacing bulky external mirrors with smaller cameras and sensors decreases aerodynamic drag.
Summary
NVH prevention and mitigation is a crucial design priority for EV manufacturers. Although many electronic systems can produce various forms of NVH as functional byproducts, EVs eliminate common sources such as engine revving, noisy mufflers, and loud idling. Noise created by wind resistance and tires is still noticeable, as are EV-specific NVH emissions produced by electric motors and regenerative braking. Mitigating NVH should start at the design level, with digital simulators holistically analyzing electromagnetics, thermodynamics, and vibro-acoustics across all EV components and systems.
References
- Integrated Approach to NVH Analysis in Electric Vehicle Drivetrains, IET
- System-level Approach to NVH Models, E-Mobility Engineering
- What is Automotive NVH?, Ansys
- E-motor NVH and Active Sound Design for EV, Ansys
- Electric Vehicles and NVH – Less is More?, Molygraph
- Simulation Can Help Resolve NVH Issues in EVs, Automotive World
- How to Analyze Noise, Vibration and Harshness in Electric Powertrains (e-NVH) Using Simulation, Gamma Technologies
- Engineering at the New NVH Frontier, SAE
- EVs Drive NVH Material Innovations, SAE Mobility Engineering
- Recent Progress in Battery Electric Vehicle Noise, Vibration, and Harshness, NCBI
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