Engineering at the New NVH Frontier
Electrification brings new benchmarks, tools, and challenges to the ongoing battle with noise, vibration and harshness.
The complex science of analyzing and abating noise, vibration, and harshness has entered a “new frontier” as the industry transitions to electrified vehicles, experts in the NVH field tell SAE Media. New design and engineering challenges at the component, system, and full-vehicle levels continue to emerge as EV offerings expand beyond the initial wave of predominantly premium-spec products. Engineers note that benchmarking activity and the introduction of new analysis and testing tools related to NVH mitigation are at “crazy” levels.
“Our interest in acoustically improved vehicles always is going to accelerate and the NVH technology must always meet customer expectations,” observed Pranab Saha, whose company Kolana & Saha Engineers specializes in acoustics, noise and vibration analysis and testing. He noted that some of the latest EV designs show progress in attacking both NVH sources and their propagation paths. There is less reliance on “patch” materials — urethane foams deployed in body cavities and mastics baked into underbody areas that add mass, cost and complexity. But Saha and other experts do not believe those so-called “Band-Aid” sound dampers will be eliminated entirely.
“Occupants should not hear anything going on underneath them in the battery pack and electric drive system, while outside the vehicle noise remains,” Saha asserted. “At some point, those noises are going to become a quality issue.”
Customer expectations of refinement in EVs versus the incumbent IC-engine vehicles are still evolving. In ICE vehicles, about 50% of NVH issues are related to the powertrain. The other half are generated mainly by road and wind noise. In EVs, the noise sources are more balanced; the electric drive unit (EDU) noise is important but road noise at low- to mid- speeds and wind noise at high speeds “tend to dominate,” explained Kiran Govindswamy, senior VP - Drivetrain, e-Mobility and Vehicle, at FEV.
“One way to look at this is, noise coming from the traction motor or geartrain is only important if the customer hears it in the vehicle,” he said. “Sufficient road and wind noise can, to an extent, mask the noise coming from the EDU. This might be beneficial, but you don’t want it [road and wind noise] to be so bad that the customer feels it’s not a refined vehicle at all.”
Govindswamy noted that higher-end EVs that are more refined typically “do a very good job” at reducing the road and wind noise entering the cabin—but sometimes at the expense of exposing the higher-frequency whine of the high-rpm electric machine and geartrain. Power inverters also exhibit high-frequency noise in the range of 10,000 Hz.
Three main sources of EDU noise include electromagnetic noise from the motor itself that radiates off the unit’s housing; geartrain noise, and overall mechanical noise from bearings, fluids and rotating systems. Defining the noise sources, their transmission paths, and establishing acceptable levels, is almost a clinical process for NVH engineers. FEV, for example, employs simulation-based processes that develop EDUs to meet customer acoustic targets.
“We use a combination of what we call ‘multibody systems simulation’ [a dynamic system-level analysis of how interconnected multiple moving parts interact with each other] and finite-element analysis, which consider the electromagnetic forces as well as the geartrain forces, then predicts what the noise levels are.” FEV is continuously improving the fidelity and accuracy of its predictions to ensure it can meet customer targets. “When we come into the test cell with the first prototype, we’re not too far off from where we need to be,” Govindswamy said.
Simulation is evolving and continues to improve, but it’s still a work in progress in terms of correlation, experts assert. “We rely a lot on simulation, but I think many new companies [automotive start-ups] rely on it too much,” Saha opined. “They’re not verifying the simulated results until very late in development, or their simulation predictions may not be totally correct. We’ve seen examples of predictions being adjusted based on what the measurements were telling. I can’t understand how they get to their target.”
He noted that while the established OEMs who are moving into EVs also are using predictive sim, “they have a much stronger measurement background” and verify their predictions much sooner in the development process.
Lightweighting and simulation
The alarming mass of lithium-ion battery packs in electric trucks, large utilities and high-performance EVs has led body engineers to offset that weight with lightweighting material solutions that, in some cases, have caused NVH issues at the cabin level.
“In the interest of lightweighting we’re seeing steel and other metals being replaced by polymeric materials, and barrier decouplers being replaced by dissipative systems,” noted Saha. “There are vibracoustic issues associated with lighter weight body panels. Passengers want a quieter cabin, while the engineering team is focused on mass efficiency. This is bringing new and different types of lightweight damping materials and other technologies into the picture.”
Reducing the mass of all the additional NVH treatments and acoustic packages is a related focus. “This comes back to the importance of reducing noise at the source,” said Govindswamy. “A lot of simulation work is going into that, and it’s a challenge. But it’s also teaching engineers to use lighter weight acoustical materials more efficiently.”
As with all areas of vehicle development, simulation is increasingly integral to NVH mitigation. “I see it as a pyramid with the sign-off at the top tip and an immense amount of work that goes on underneath it to get to the sign-off point,” observed Dave Bogema, senior director of product management at VI-grade, maker of simulation tools for NVH and vehicle dynamics work. NVH analysis begins at the desktop where engineers using the latest, increasingly capable analysis software rapidly work through various ideas and weed out those that are unfeasible (see sidebar). The next step is subjective evaluation, where NVH really must be experienced if decisions are to be made with confidence.
“You can get a certain level of answers from the desktop,” Bogema said. “But desktop simulation doesn’t have the full benefit of immersion and reality that experiencing virtual prototypes on physical simulators delivers.”
Predicting acoustic noise and vibration generated by electric machines is challenging due to the variety of physical domains and complex interactions among these domains. Electromagnetics, thermodynamics, and vibro-acoustic analyses are required in a coupled fashion for a precise prediction of acoustic noise generated by electric machines. According to Ansys Solutions, there are four essential elements for accurately predicting acoustic noise: need for a high-fidelity simulation solution to account for all the physics involved; a platform to couple all the different physics elements; ability to parameterize and optimize machine design parameters for each of the physics involved, and high-performance computing capability to accelerate the simulation.
There’s also vehicle dynamics, in many ways inseparable from NVH including how they influence the virtual-prototype evaluation process and the occupant’s perception of vehicle refinement. Physical simulation tools are expanding to meet the many facets of EV refinement.
“When you drive a vehicle, you feel everything at the same time,” VI-grade’s Bogema said. “If you’re doing ride comfort, which is a multi-sensory motion, vibration, and sound experience, putting NVH and vehicle dynamics together delivers the repeatability and control that are vital to overall fidelity.” His company recently launched its new Compact Full-Spectrum Simulator (FSS) at the 2023 SAE Noise and Vibration Conference. Capable of simulations from 0.5Hz to 20kHz, the Compact FSS delivers both primary and secondary driving motion, vibration, and sound, simultaneously. While not a traditional hexapod-type simulator design, the FSS’s four legs enable it to accurately replicate the motion of the road as generated when driving over potholes, cobblestones, or any other surface. Its vibration shakers produce high-frequency vibration through the steering wheel and the seat, in addition to the motion from the road.
VI-grade also offers a dedicated Compact NVH driving simulator that allows engineers to dial in exactly what the car is going to feel like for NVH. Its seat structure allows an array of virtual seat designs to be tried, where the FSS requires a physical seat change. The dedicated NVH simulator “can instantly change the seat parameters and feel the difference between seats,” Bogema noted. “They’re designed for different use cases,” he said—the FSS is very much aimed at ride comfort; the NVH version is for dialing in refinement.”
Engineers also are realizing they can do sound design at the desktop, interactively, and come up with different soundscapes for the car. Vibration plays into that. “Do you really need the whole motion of the car in the sound design? Maybe not all the time,” Bogema said.
Bespoke NVH components
Raising the thresholds of vehicle refinement is not exclusive to EVs. Experts say EVs and hybrids are driving new designs and NVH-reduction approaches across product segments including those still dominated by IC-engine vehicles. They note, however, that cost pressures can make sophisticated designs a tough sell unless their NVH performance is exceptional.
An example of a new NVH-focused component is a new tunable bracket for air suspension air-supply units (ASU) recently launched by Vibracoustic, a leading global automotive NVH solutions supplier. The simple yet sophisticated new ASU, shown in the accompanying image, features a plastic base (previously steel) for improved damping, and bellow-style rubber bushings and bump stops. The bushings’ special material properties give the required stiffness levels while also damping both radial and axial excitations generated by the compressor.
The new mounting bracket is lighter and more robust than traditional brackets, according to Vibracoustic, and enables NVH tuning in all directions—something not possible with coil springs typically used in such applications, the company claims. Additionally, the bellow-type bushing design allows for high displacement at low strain levels, replicating the performance of springs. The rubber material also helps maintain resonance peaks within a reasonable range, because the rubber compound demonstrates a dynamic stiffness peak that is up to 60% lower than of a coil spring, company engineers claim.
The key to NVH science and engineering in EVs going forward, according to Govindswamy at FEV, is striking a balance: meeting customer refinement expectations without degrading vehicle range. “If you can secure the electric motor’s efficiency and accept a slightly higher noise but improve the traditional vehicle to minimize the effect to the customer—that’s the challenge we’re working on. And it comes down to systems optimization.”
NVH software simulation solutions
Among dozens of suppliers offering NVH simulation software tools are these popular products used by SAE readers:
Part of Hexagon AB (see below), Actron is an FE-based tool for modeling acoustic behavior of mechanical systems and components. It is widely used to solve NVH problems in the mid-frequency (400-1500Hz) range that are critical for EV design. An ‘open’ tool, it leverages existing FE and CFD sim models and is used to study EV transmission and ancillary (pumps, compressors) noise sources. GM engineers used Actran to optimize noise-damping treatments in the Cadillac Lyric EV.
The HyperWorks simulation suite contains a broad range of solutions for NVH model build, assembly, diagnostics, analysis and optimization. The suite’s customizable NVH Director automates the tasks involved in NVH analysis by integrating the entire process of meshing, assembly, loadcase setup, and post-processing, to reduce full-vehicle NVH simulation time.
Rapid NVH workflow for the concept design stage compares noise levels for different e-motor designs/topologies to predict the noise over a full speed sweep. It can identify the cause of motor noise early on and help make relevant design changes in trade-off with other motor performance targets, allowing the NVH, thermal, and electromagnetic behavior to be investigated at the same time. The NVH workflow is integrated into Ansys Motor-CAD to provide designers with various representations of force, displacement, and acoustic power. 2D and 3D models can then be generated in Ansys Maxwell and vibroacoustic analysis performed in Ansys Mechanical.
The X-FEM NVH is a 4-channel acquisition module suitable for use with all common acoustic sensor output types such as voltage, charge and ICP/IEPE. Each channel can be individually configured and raw signals are stored in a compressed file at high resolution and in time-synchronized formats. An optional NVH TOOLBOX for AVL IndiCom or AVL Concerto includes the most common NVH analysis tools and online NVH macros.
Brüel & Kjær
Desktop NVH, SimSound, Source Path Contribution, and VSound allow engineers to design and evaluate interior and exterior vehicle sounds for vehicles of any configuration. Insight+ enables the total experience of NVH data – creating sounds directly from CAE models that NVH engineers can listen to and experience. The CAE model data can be combined with test data to create an immersive, realistic environment.
The widely used, iconic SIMULIA suite offers a comprehensive simulation toolset for NVH work.
Among the NVH-focused tools is Sound Quality Measurement, which addresses the need to empirically evaluate how sound produced by different kinds of machines is perceived by the human ear. The tool helps engineers determine how the sound is perceived, tune the sound, and make it appealing to the customer.
The company’s Romax Spectrum enables electro-mechanical powertrain NVH simulation and offers complete, parametric whole-system modelling of the powertrain including gear and bearing contact surfaces. The Romax suite offers interfaces to third-party CAE tools, including Actran for acoustics: Adams for multibody sim; JMAG and Maxwell for electromagnetic simulation Nastran for FE modeling, and VI-grade for vehicle NVH and sound quality.
Siemens Digital Industries
Siemens’ Simcenter suite continues to add NVH analysis tools that benefit from the digital twin approach to accurately predicting vehicle interior and exterior NVH performance.