EVs Drive NVH Material Innovations
Skateboard platforms, high-frequency motor noise push lower dB thresholds and new countermeasure solutions.
The definition of a ‘quiet’ vehicle, as determined by the occupants’ ears, is changing radically, as electric motors replace the internal-combustion engine. And electric vehicles are changing the technologies used to reduce, and eliminate, NVH – noise, vibration and harshness. Removing the piston engine, with its exhaust, intake and valvetrain/injector noises, unmasks a plethora of cabin sounds that occupants never heard before.
“ICEs generate at least 20 dB more noise at the cabin level,” noted Vahid Mortazavian, Ph.D, global CAE leader at Ascend Performance Materials, in a presentation at the 2022 SPE Automotive conference. Tire and road noise become more prevalent in an EV. Even the heater fan is louder without engine noise to mask it. But EVs generate their own noises, including high-frequency buzz from the electric motor. These are among the new challenges facing development engineers and NVH specialists.
“Reducing and eliminating cabin noise in electric vehicles is now a significant focus for the OEMs,” noted Arnold Braun, a veteran polyurethane systems engineer at Dow. Another challenge for the NVH analysis and materials communities is the speed in which EVs are being designed. “Everybody’s in a race and have reduced their design cycles,” added his colleague Selamawit Belli, Dow’s automotive acoustics strategic manager, in an interview with SAE Media.
Braun and Belli noted the dramatic change in vehicle architectures, with the “skateboard” layout for EVs emerging as preeminent, presents new challenges and opportunities for NVH engineers, as well as for their company’s widely-used Betafoam product line. In an EV, with the flat floorplan and battery pack underneath, “we’re now looking at body structures that have stiffer and stronger rocker systems than those in ICE vehicles,” Braun explained. “We face the question of how to apply our NVH treatments into those areas so that the strength is retained along with the acoustic performance.”
The water-blown polyurethane-based Betafoam is a family of two-component acoustic foam technologies featuring a >4000% expansion rate. When robotically injected into vehicle body cavities, the foam (both open- and closed-cell types) creates an effective, three-dimensional acoustic seal, according to Braun. Betafoam solutions have been industry workhorses in reducing tire/road noise – a longstanding cabin noise source for all vehicles – through the wheelhouse. It’s also being employed on recent EV platforms to block electric motor frequencies (a new EV acoustics bogey) in an impressive dynamic range from 500 Hz to 10,000 Hz.
In their quest for cabin quiet, OEMs now are pushing suppliers for solutions with vibration dampening as well as sound-absorption qualities. “A lot of the new EV structures the OEMs are showing us have a closed plenum in the cowl section of the body. And we see growing interest in cast parts, not the welded sheetmetal fabrications used today,” Braun noted. “There may not be areas where you can apply acoustic foams in a castings environment, but they will need more resonance damping due to that type of construction.”
He admits that because dampening is not a Betafoam top attribute, Dow chemists are working on innovative solutions that retain the incumbent material’s ultimate flexibility for filling a wide variety of cavity geometries. OEMs deem this important as it obviates the need for re-design and tooling after sheetmetal changes. Asking an OEM to move an [access] hole is costly and time consuming. In the assembly plant, we’re shooting our foam anywhere from five to eight locations, per body side. Moving holes around is usually not feasible.”
Dow R&D also is well into development of new liquid-applied coatings to address the high-frequency modes (from 800 Hz to 3000 Hz) excited by electric propulsion, noted Dow associate research scientist Dr. Ian Robertson, in a presentation at the 2022 SPE conference [see SAE Technical Papers 2021-01-1123 and 2017-01-1877]. The company’s current Acousticryl line is a water-borne, sprayable copolymer emulsion designed for acoustic dampening applications. Dow claims Acousticryl outperforms competiive NVH treatments including bake-on bitumen pads, epoxies or PVC.
Dow’s NVH team receives body structure CAD data from the customer two to three years in advance of vehicle launch. Braun works with the automaker’s CAD engineers to study cavities, with an eye toward how the foam material would flow. “When there are areas of concern, we take that CAD data and 3D-print the parts long before release of any sheetmetal. We don’t do any acoustic testing on them. 3D printing has been immensely helpful. The point is to simply understand how the material flows through that space. There are areas within vehicles, both ICE and EV, that have particular areas needing to be treated.”
Dow uses a proprietary engineering methodology called ‘Acoustimize’ to evaluate and optimize the NVH performance of a vehicle as the driver’s ear experiences it. “Acoustimize gives us the DNA of the vehicle body,” Braun said. “It tells us how sound travels from front-to-back, back-to-front, side-to-side and corner-to-corner.
“We measure and introduce noise throughout certain areas in a microphoned vehicle that typically has about 32 microphones fitted,” he noted. “We have six to eight locations where we introduce noise.”
The Acoustimize process has enabled Dow engineers to speed development of robust sound packages for OEMs and tier suppliers by identifying manufacturing complexities, such as metal fit, sealer skips and pass-throughs which are not detectable by other methods. It also helps pinpoint noise communication between different segments of the vehicle.
The company claims that Acoustimize studies have demonstrated a noise reduction improvement of five to 20 dB in applications using Betafoam, compared to competitive NVH-attenuation treatments and designs.
Road/tire and wind noise remain an arch enemy of vehicle development teams, according to NVH engineers. As tire industry investigates a new generation of non-pneumatic tire tech aimed at EV applications, new challenges and opportunities will emerge. Passerby noise, initially for European vehicle applications, also is a focus of NVH material innovations for the wheelhouse area.
Wind noise is the omnipresent tricky foe and while incremental noise-reduction improvements are being made in this area--vehicle designers would most like to eliminate exterior mirrors, replacing them with cameras--OEMs are focusing on the noise sources (road/tire, structural vibrations, e-motor frequencies) on the “lower end” of the body.
As to the future of NVH analysis and abatement strategies, OEMs increasingly want to see prediction modeling of foam behavior in cavities, Braun reported.
“They want us to provide flexibility and to keep pace with their faster and faster development cycles,” he said. “More virtual testing solutions, to supplement and in some cases replace physical tests, also are on the way. OEMs are asking for virtual data more than ever.”
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Acoustical Materials recently earned a Gold Medal from the Independent Book Publishers Assoc. (IBPA) in its annual Benjamin Franklin Award program, Professional Reference category. The book is available through the SAE website .