Electrification Forces Fresh Perspectives on Vehicle NVH
Just because EVs are quieter doesn’t mean the job of NVH abatement is easier.
The quietness of EVs is, for many, a compelling selling point. But vehicles becoming inherently quieter overall hasn’t translated into making the job of noise abatement any easier – the job’s just different, asserted several NVH engineering experts at the Autotech conference in Detroit in early June. These NVH experts underlined a primary point of what’s now understood about the “new” quietness of EVs: the drastic reduction of powertrain-related noise, vibration and harshness (NVH) largely means vehicle occupants now can hear and feel other NVH sources.
“A little squeak becomes more prominent” in EVs, said Jian Pan, senior director of engineering at Auria Solutions. And although this seems self-evident, Pan added that EVs can open up new or unfamiliar frequencies to occupants – and some of those frequencies can be particularly displeasing. Certain frequencies emanating from EV drive motors, for one, can be distinctly annoying, Pan said, and are quite different from the “noise signatures” of combustion engines. Meanwhile, those relatively minor squeaks and squeals coming from seat-adjustment motors and power windows, in the absence of “masking” noise from an IC powertrain, now are prominent noise sources, he added.
Building on those points, Brent Dreher, CAE engineer – NVH & Durability at Faraday Future, pointed out that many current and coming EVs are built on “skateboard” chassis architectures that tend to use hollow extrusions as their primary structural elements, rather than conventional steel unibodies or body-on-frame architectures of most all internal-combustion vehicles. Hollow structural elements tend to transmit and augment noise more readily than cast steel structures, Dreher stressed.
Mirrors to gears
Structural challenges are just the beginning. Because of EVs’ comparative general quietness, conventional vehicle components are under fresh NVH scrutiny. Jeff Hodgkins, senior application NVH tech expert at VI-Grade, a Germany-based developer of software simulation tools, told conference attendees that side-view mirrors may become a flashpoint for NVH in EVs. Side-view mirrors have recently been the focus of aerodynamicists seeking to reduce vehicle wind resistance by replacing bulky conventional mirror assemblies with sleeker units housing cameras – but Hodgkins said the silence of EVs has made the wind rushing around side mirrors markedly more noticeable in the cabin. “It’s right around the driver’s ear,” he added, advocating for camera mirrors that would help reduce wind noise in this vital area.
A related solution could be increased use of specially laminated “acoustic” glass, many on the panel agreed. Already seeing increased use in IC-powered upscale vehicles or as part of premium-equipment packages, acoustic glass might see increased fitment in EVs to stifle road and wind noise. But some panelists said cautious application might be required; if the vehicle’s entire greenhouse isn’t comprised of acoustic glass, it can create undesirable, unbalanced sound zones within the cabin.
Faraday Future’s Dreher said EV quietness “has put a lot more pressure” on transmission parts and the suppliers who make them. “Before, you never heard the transmission,” he said, but in EVs, which for now generally have comparatively simple integral transmissions, gear-related noise is discernable in much the same fashion as certain drive-motor frequencies. He said polymer-material gear technology has advanced, but the generally higher amount of torque in EV traction motors, as well as the wider-rpm operating range, is more demanding on gear materials. Higher torque levels also increase the need for tighter tolerances, Dreher added, which typically adds cost.
Faraday Future has experimented with “noise-canceling” tires made by Pirelli, Dreher said. The tires, which have a polyurethane “sponge” inserted around the perimeter of the tire cavity, absorb vibration and certain sound frequencies, Pirelli claims, reducing “perceived noise by half.”
Lee Rodgers, principal engineer, Drive Systems Design, added that incumbent automakers with considerable vertical integration are likely to find NVH management more difficult if they now look to suppliers for major EV components such as traction motors, transmissions and even battery packs. The situation is likely to place more emphasis on whole-vehicle and component simulation models.
Sim is a win
VI-Grade’s Hodgkins said increasingly sophisticated simulation software can create sound-signature models that can allow suppliers to compare components head-to-head, but also for their contribution to whole-vehicle NVH. These simulations can be critical in determining if there’s justifiable customer value in choosing a more-expensive, NVH-optimized component, he said, adding that human determination of sound differences – at many levels of development and even by customers in clinics – can be difficult to quantify. Hodgkins said “acoustic amnesia” can render back-to-back comparisons ineffective, even if they occur in a short timeframe.
The best use of simulation models is to help inform design changes, added Auria’s Pan, saying that some OEMs are branding sound signatures that provide developers with guidance for tailoring NVH for an EV. Faraday Future’s Dreher perhaps best summarized the panel’s discussion by saying, “We’re not trying to kill the sound, we’re trying to design the sound.”