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The complexity of a typical passenger vehicle's NVH package laid bare. The potential to add mass is obvious. (Kolano & Saha Engineers)

Tackling NVH one dB per day

The vehicle chief engineer did not like what he was hearing on this recent evaluation ride.

He was reviewing a new transmission scheduled for production in his company’s iconic sporty car. But the marriage of transmission and platform was generating unacceptably high cabin-noise levels. The CE wasn’t pleased. His design and manufacturing engineers explained that the new gearbox featured a thin-wall aluminum case for reduced mass. However, the lightweight case was the noise transmitter.

The tooling investment had been made and the program clock was ticking. So a phone call went out to Pranab Saha.

“They called me in to help them devise a solution,” said Saha, an acoustics expert and board-certified noise control engineer with consultants Kolano & Saha in Waterford, MI. He said the client’s original design blanketed the lightweight transmission case with a foil-backed insulation material. Problem was, the blanket included multiple cut-outs so that linkages and wiring could pass through. The cut-outs were built-in noise paths.

Saha’s suggestion: Rather than place the insulation material on the transmission case, put it on the underside of the transmission tunnel. The engineers would revisit their CAD drawings and revised the parts. Their next step is to launch noise study to prove the new solution’s effectiveness.

“That’s one of many examples of us getting called in to solve a problem—where is the best place to install NVH countermeasures?” he noted.

The science of analyzing, attenuating and eliminating Noise, Vibration and Harshness has become an integral aspect of vehicle development. NVH engineering teams now push for “one dB [decibel] per day” reductions in cabin noise levels with the same verve that is devoted to vehicle mass reduction and greater fuel efficiency.

“The N&V [noise and vibration] discipline is unique because it’s driven by market competition, rather than by government legislation,” Saha explained. “The OEMs now brag about the superior interior sound quality and low noise levels of one model versus another. And this will certainly grow more important as technologies such as cylinder deactivation, hybrids and electric vehicles enter the market.”

Quieter and smoother are requisites for the new human-machine interfaces (HMI) being developed for automated vehicles, said Joe Emmi, vice president of global product strategy and head of the Anti-Vibration Systems group at Cooper-Standard. “Reduced cabin noise is particularly important for voice-recognition systems,” he noted.

OEMs and Tier 1s have invested billions in new NVH software, facilities, materials R&D and in the expertise of Saha and other acoustics specialists in the field. Makers of the absorptive materials that are vital to automotive NVH abatement also are riding a growth curve. The global market for their products is projected to reach $13.52 billion by 2021, at a compounded annual growth rate of 6% from 2016 to 2021, according to a 2017 study by Grand View Research.

Astute suppliers are branding their technologies to suit. The growing list includes Material Sciences Corp.’s “Quiet Steel” used in dash panels; Japan-based Technol Polymer Co. Ltd’s “Hushlloy” anti-squeak thermoplastics, and Sika Automotive’s “Sika Damp,” a co-extruded elastomer aimed at dampening body-panel resonance.

The industry’s commitment to NVH engineering has driven the market for dedicated software programs familiar to engineers: Brüel & Kjaer's PULSE; SCS’s V-Path for transfer path analysis; HEAD Acoustics’ Artemis; Genesis’ LEA; Altair Engineering’s HyperWorks suite, as well as NVH toolsets built into Dassault Systemes’ CATIA V6/3DS, Siemens PLM and many others.

SAE International has long served as a professional forum for NVH engineers, with its biennial Noise and Vibration Conference and Exhibition (held this month in Grand Rapids, MI) the premier technical event on the subject.

Playing the ‘what-if?’ game

N&V engineers live in a realm of Sources, Paths (structure-borne and airborne) and Receivers (the noise and vibration targets) where up-front problem identification and solving are critical to success. They focus on quantifying the relationship between objective parameters, such as precise sound and frequency measurements obtained through data-acquisition methods, with a degree of informed subjectivity.

End-customer input is part of the analysis process; to many of them enhancing sound quality is as important as reducing annoying noises and overall volume. Psycho-acoustics specialists agree. Loudness, sharpness and the articulation index (a measure of the intelligibility of hearing speech within a given noise environment) are all subjective parameters.

Experts interviewed for this article agree that lightweight aluminum body structures, in modal analysis, present different and often more challenging N&V characteristics than do steel structures. Keeping vibration in check requires greater stiffness in the body panels and additional damping materials such as LASD (liquid applied sprayable dampening) underbody mastics and cavity-filling foams.

In some applications, active noise control systems are employed (see Honda sidebar). Such solutions are sometimes dubbed “band aids” because they don’t address root causes that typically originate in a fundamental structure—an excessively resonant cylinder block, for example. And they add mass.

Observed Richard Yen, Senior VP Automotive at Altair Engineering: “A lot of companies do a good job in NVH but basically they’re applying a lot of testing at the end of the vehicle development cycle. They spend a lot of money trying to fix the problems.”

When working with aluminum, engineers often add more material to achieve the same amount of NVH reduction of a heavier steel structure. As a result, NVH specialists “see evidence of trying to ‘buy more space’ within the vehicle to make room for absorptive materials, instead of addressing the fundamental structure issues,” said Saha. “They rationalize that absorption materials don’t weigh as much.”

Dash mats have become a strategic bulwark in the battle to keep powertrain noise out of the passenger cabin. The mats cover both sides of the bulkhead formerly known as the firewall. They used to serve as a barrier and decoupler. Today, dash mates essentially play an absorptive role. Known as ‘dissipative’ materials, they require more cross-sectional space.

The growing number of OEMs who have embraced engine cylinder deactivation as a fuel-efficiency play is sparking innovations related to NVH created by the uneven firing when a 60° V6 switches to V4 and V3 operating mode, for example. Signature vibrations and acoustic patterns are created by these events, with powertrain excitation entering the cabin as low-frequency “boom,” depending on the leak paths.

Managing torque-converter clutch slip, adoption of active engine and transmission mounts (typically effective but costly) and active noise control using the vehicle’s audio system can be effective solutions for mitigating the unique NVH challenges of cylinder-deactivation engines. [SAE Technical Paper 2014-01-1675, published by Tula Technologies, offers more detail on this subject.] Composites offer vibration-absorbing benefits in some torque-reaction applications. For this reason Cooper-Standard is launching a new continuous-fiber thermoplastic engine mount on a European vehicle.

Full Vehicle Modeling

Modeling in the NVH arena is increasingly sophisticated. Core among them are SEA (Statistical Energy Analysis) packages including SEAM 3D, developed by Cambridge Collaborative Inc. to study structure-borne noise in submarines. It has since been adopted by the auto industry as an effective CAE design tool to develop NVH sound packages for use in lightweight vehicle design. Using SEA, engineers can evaluate the dynamic response of complex structures at mid and high frequencies and recover the NVH deficiency created by the lightweighting actions.

SEA “enables acoustics engineers to play the ‘what-if’ game,” said Pranab Saha. “For example, if mass is reduced in a subassembly, what then will be the noise level?”

General Motors recently began using a modeling tool developed in-house called DISPET that lets engineers optimize NVH packages across vehicle families. With primary inputs being vehicle body design and powertrain noise, they can specify the NVH materials needed. “Users only input the measured acoustic data for a given thickness and surface density,” says an engineer familiar with DISPET. “The system will tell you if it meets the target or not. And if you don’t know the target, DISPET will tell you if you’re 1 dB above or 1dB below (or not), at every frequency.”

Full-vehicle NVH simulation is viewed as a competitive advantage whose internal processes the OEMs guard closely. About five years ago, Altair Engineering took the lead on commercialization. Its Brüel & Kjaer NVH Simulator looks like the console of a cool computer video game, with a big flatscreen in front of you that puts the driver “on the road.”

“With the Full-Vehicle Simulator, we build a simulation model that uses testing data to drive a model of the vibration and acoustic environment in the vehicle,” explained Jiamin Guan, Altair’s Director of Vibration and Acoustic Solutions. “It takes data from vehicle operation under various conditions and allows you to listen to those as you would hear them in a real vehicle.”

Altair partners with noted instrumentation specialists Brüel & Kjaer to create the “real vehicle” experience. The model is built right at the beginning of a vehicle program, during benchmarking and setting specifications. Altair starts with an imagined target customer and how the vehicle fits into their usage. The model team then builds a competitive set, adds packaging and functional requirements, then brings in customers to listen to and experience the measured data.

The full-vehicle model “allows a broad spectrum of people to experience the noise and vibration levels we’re trying to target—and see if they agree with those targets,” Guan noted. “Based on the playback of the noise and vibration we hone in on specific areas. We see what people really prefer and object to and what “sound quality” means to them. Once we have the targets, this helps us and the client consider how we will achieve them.”

The tool, Guan claims, gives engineers the freedom to make changes and evaluate conceptual options under weight and cost constraints. “Without a full-vehicle model we wouldn’t be able to determine how aluminum, for example, factors into the vibration requirement versus a steel vehicle. These are constructs you can evaluate in the full-vehicle model, whether you can meet the targets or not.”

The subject of Vehicle Electrification seems to enter most conversations about NVH. Is it making acoustics and N&V experts’ jobs easier or more difficult? “We’re getting calls to help decide on sound-package materials—acoustics and seals—for new EV programs,” said Pranab Saha. “The supply base is moving on this trend although we haven’t seen dramatic changes yet.” He and other experts noted that although EVs’ overall sound level is low, other noises previously masked by the combustion engine are now more audible.

“NVH engineers are still working out the sound packages to address the new frequency ranges of future electrified vehicles,” he explained.

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