Noise, Vibration & Harshness Challenges in Vehicle Lightweighting

By on September 3, 2018 in PROCESSES & FABRICATING

With consumers and government regulators demanding greater fuel economy and fewer CO2 emissions, reducing vehicle weight remains a global megatrend and a key goal in new vehicle development for manufacturers and component suppliers. As the vehicle body structure represents a significant percentage of total vehicle mass, much of today’s engineering efforts focus on lightweighting the Body-in-White and closure panels.

To achieve this, engineers are using non-traditional materials such as aluminum, magnesium and carbon fiber reinforced plastics. Lighter-gauge sheet metal is also in play. However, these materials create unique challenges, among which is a significant potential for degradation in vehicle noise, vibration and harshness (NVH) performance. In simple terms, the vehicle interior can get louder, and the overall automobile may lose body structural stiffness, affecting occupant comfort and vehicle dynamic handling.

Sika, with its wide range of products and extensive technical capabilities has been helping automotive engineers for decades with weight reduction through the design and implementation of NVH solutions. As weight is removed, Sika products help the overall vehicle maintain the same or offer enhanced NVH performance to address vehicle interior noise issues. Typical approaches include blocking and absorbing sound, reinforcing body structure and body panels, and finally, damping body panels.

Each technology has its own mechanism for enhancing the vehicle, with the real benefit being realized when they are used in the right applications and combinations. These products are further optimized through full-vehicle quantification testing. It has been found that usually the best results are yielded from a combined array of approaches.

BASIC VEHICLE NVH CONCEPT

Vehicle interior comfort is dictated by both sound and vibration. While humans can perceive sound from 20 Hz to 20 kHz, typically the dominant noise in vehicle interior is from 30 Hz to 8.5 kHz. The nature of the interior noise can be further divided into structure-borne noise and airborne-noise elements. For the former, the main frequency of interest is typically below 500 Hz. Above that, frequency is dominated by airborne noise. The region from 500 to 1000 Hz is the transition where the vehicle sensitivity to structure-borne noise reduces and airborne noise increases. (See Figure 1.)

Fig. 1 – Vehicle interior noise can be further divided into structure-borne noise and airborne- noise mechanisms

Vehicle NVH characteristics can be understood by the source-path-receiver relationship. The main sources of vehicle noise are powertrain, wind, tires and road feedback. The paths are the body panels, body frame and windows. The receivers are the vehicle occupant’s ears. (See Figure 2.)

General NVH treatments attack the paths and can be classified as dissipative or non-dissipative. The former works by absorbing sound energy while the latter reflects sound energy. Examples of dissipative NVH treatments include dampers for structure-borne noise and absorbers for airborne noise. Non-dissipative treatments include reinforcers for structure-borne noise and baffles for airborne noise.

Absorbers convert incoming sound pressure to heat via deformation on cells or fibers of the material. Typical treatments are compliant materials with open-cell structures. The effectiveness of an absorber can be determined by measuring its sound absorption coefficient across a defined frequency spectrum.

Barriers or baffles reflect incoming sound-pressure waves back toward the direction of the source. Products with high surface weights make good barriers. A variety of measurement standards exists to determine the material NVH performance including sound transmission loss, noise reduction and insertion loss.

Dampers convert vibration energy into heat energy, which is dissipated to the surrounding environment. Damping loss factor is the main metric measured across a range of temperature conditions. Reinforcers change structural stiffness and redirect energy back toward the source or to an alternate path. Finite element analysis (FEA), transfer path analysis and modal analysis are the typical tools in understanding the effectiveness of reinforcers.

In the scenario where the body structure uses lighter-gauge sheet metal, it could potentially have a negative effect on local and global stiffness of the body, reducing lower frequency NVH performance. The lightweighting of body panels and structure can also degrade airborne-noise NVH performance. As the body panel becomes lighter, its ability to block noise decreases.

Fig. 2 – Vehicle NVH
characteristics are understood by the source-path-receiver relationship


BODY CAVITY SEALING

The use of acoustic cavity fillers to prevent the propagation of airborne noise, water and dust into the interior spaces of vehicle structures has been in practice for many years. Although various technologies are available, one dominant technology from Sika is the pre-formed baffle technology. It consists of thermoplastic-based systems that incorporate a heat-reactive thermoplastic sealer applied to a nylon or steel carrier and attached to the body structure. There are also heat-reactive rubber-based sealer systems that incorporate a carrier, push pin or pressure-sensitive adhesive layer for attachment. (See Figure 3.)

Fig. 3 – Sika provides a range of body-cavity sealing products including SikaBaffle® and SikaSeal®

Thermoplastic baffle designs range in complexity from simple extrusions to a highly engineered two-shot injection molded part (or three-shot injection molded part that combines structural reinforcement into one part). Nylon for the carrier is shot into the tool, and the sealer is overmolded onto the carrier. Rubber-based designs typically include a co-extrusion of heat-reactive sealer with a pressure-sensitive adhesive or an extrusion of heat-reactive sealer using a push pin applied for attachment to the BIW. The parts are applied during assembly of the sheet-metal panels. The sealer material expands with exposure to heat that occurs in the paint shop bake ovens, forming a complete seal of the cavity cross sections in which they are applied.

Sika provides a range of body-cavity sealing products, including SikaBaffle® and SikaSeal®. Typical application areas are A-pillar, B-pillar roof rail, rear pillar, C-pillar, center tunnel and rear shelf. The pre-formed part provides both sound blocking with the carrier and sound absorption with the expanded foam to reduce airborne noise.

Another type of cavity-filler technology is a two-component chemically reactive expandable polyurethane foam. The bulk applied system includes a polyol component and a methylene diphenyl diisocyanate (MDI) component. The two are kept in storage tanks in the assembly plant and pumped to an application station as needed. The foam systems are applied after paint and bake cycles. An exothermic reaction occurs as the material is applied to each cavity section, which causes the material to gel and expand.

Fig. 4 – Material NVH performance of different acoustic baffles and sealing products.


Another type of cavity-filler technology is a two-component chemically reactive expandable polyurethane foam. The bulk applied system includes a polyol component and a methylene diphenyl diisocyanate (MDI) component.

Because of the potential health issues associated with these formulations, “low MDI” formulations were developed and are the norm in North America today. Bulk foams generally provide a higher expansion rate, but it is necessary to control the flow of the product into the body structure to avoid overfilling, which can interfere with assembly processes further down the line.

Pre-formed baffles are typically 40 to 50 percent lighter than bulk foam systems when comparing specific applications because a pre-formed part is more precise in sealing a cross-section. Comparatively, the two-component foam systems require a greater volume of material to be applied.

In Sika-conducted sound insertion loss evaluations, which measure the effectiveness of acoustic baffle in blocking noise, the bulk foam formulation tested performs better than the pre-formed baffles from 1000 Hz to 1600 Hz and then drops off significantly at higher frequencies, while pre-formed baffles block more noise in mid-frequency and high-frequency bands. It is likely that the greater mass and thickness of the foam, due to the volume of material required to seal a specific section, provide higher performance in this frequency range, while the foam’s greater stiffness has a negative impact on acoustic performance at higher frequencies.

In full vehicle testing where both structure-borne and airborne noise inputs are impacting the body structure, the heavier and thicker bulk foam application, based on the volume of material that is required to seal the section, demonstrates higher acoustic performance at low frequencies, while the less-rigid expanded baffle parts perform better at mid and high frequencies.

BODY STRUCTURAL REINFORCEMENT

For interior noise contribution, structure-borne noise is highly dependent on vehicle body structure. Thus, the vehicle can be very sensitive to any reduction in sheet-metal thickness throughout. One of the current basic material technologies for strengthening body structure is hydroformed high-strength steel reinforcements. The technology, however, requires access in the structure to weld the piece in place, which negatively affects vehicle-assembly cycle time in addition to design freedom.

As an alternative to metal reinforcement, Sika offers a range of non-metallic structural inserts based on heat-reactive epoxy structural expandable foam, combined with a steel or structural plastic carrier. They are lightweight and high performance, and can serve as a structural member to enhance the bending resistance of vehicle body sections. The solution is a three-dimensional design.

In the vehicle-assembly process, the reinforcer parts are put together in the BIW using clips or spot welds. During the E-coat process, an engineered gap with the sheet metal allows E-coat fluid to flow through. In the E-coat oven, the heat-reactive expandable portion swells, cures and bonds the entire part to the metal. The product is designed to minimize vehicle-assembly time.

Fig. 5 – Alternative to metal reinforcement,
SikaReforcer® is a non-metallic, lightweight structural insert.

Fig. 6 – Finite element analysis (FEA) is used to optimize structural-reinforcement designs

The reinforcer can significantly increase bending properties of the body section, resulting in increased stiffness, reduced displacements and more resonant frequency of the section over standard thin-walled construction. In essence, SikaReinforcer® helps the vehicle structure carry more load by preventing section movements in node areas by distributing force over a greater surface.

SikaReinforcers are not only lighter than steel parts, but they are more efficient because the steel parts have to be added in small sections with weld points in areas with limited fixation points and design potential. SikaReinforcersare injection-molded to complex geometric shapes, as needed, directly mimicking the filled section, and they provide direct connection across a total surface where traditional steel solutions cannot.

To properly engineer reinforcer-application solutions, extensive engineering knowledge and the use of computer-aided modeling (CAE), such as finite element analysis, are employed. Sika engineers, drawing on this in-depth experience, have further optimized typical designs to consistently achieve best performance/mass ratio. For NVH applications, SikaReinforcer parts can be as light as 40 grams to 150 grams each. Within vehicles, reinforcers can be used efficiently in floor cross sections, as well as B-pillar, roof frame, door sills, door strikers, door frames and door openings. (See Figure 6.)

ROOF PANEL STIFFENING

“Dachverstärkung Dämpfung,” or DVD, is German and translates to roof (dach), stiffening (verstärkung) and damping (Dämpfung). Sika Automotive has been actively developing and promoting a systematic roof-panel stiffening and damping-technology approach with systems currently in use at OEMs. The basis of the technology uses a lightweight fiberboard lamination, adding stiffness to the roof panel, allowing a thinner gauge of steel for the roof, thus reducing weight and improving fuel economy. It performs by stabilizing the car roof and increasing its buckling resistance, thereby reducing roof-panel thickness. It may also lower the number and dimension of roof beams. Collectively, the roof-panel subsystem yields a lower weight. Some vehicles have realized savings of 2 kilograms or more. (See Figure 7.)

A potential issue when using thinner panel is sheet-metal read-through. Sika supplies an adhesive specially formulated to counter the condition. Sikaflex®-ULM is an ultra-low modulus one-component polyurethane adhesive developed for joint sealing and bonding applications. It is designed to induce low stress to the mated substrates and is ideally suited for applications where read-through can occur. Assembly of the system is performed in trim shop operations using a robot for efficiency.

Fig. 7 – Roof panel stiffener (DVD) enables lighter roof systems with no loss in stiffness

In empirical vehicle testing, noise-level improvements of up to 40 percent (3dB) at the dominant frequencies of 80 Hz to 500 Hz are realized with the addition of pad and adhesive on the roof panel. The additional stiffness of the DVD altered the structure-borne vibration characteristic of the panel to yield the NVH improvement. (See Figure 8.)

Fig. 8 – A roof panel stiffener (DVD) improves vehicle interior performance by reducing overall sound


BODY-PANEL DAMPING

Additional body-panel damping will likely be needed when panel-gauge thickness has been reduced to achieve lower weight. Subsequently, it can cause higher structure-borne vibration. Sika provides a broad range of SikaDamp® materials, including free layer (extensional), constrained layer (multi-layered sandwiched) damping technology. These materials are commonly used on the floor pan, wheelhouse, doors, roof, front of dash and any other body panel subject to high levels of vibration. The type of damping strategy implemented in each of these areas varies with vehicle platform. Among the factors that contribute to these decisions are cost, mass, performance and compatibility with in-plant manufacturing and assembly processes. Each of these material strategies has relative strengths and weaknesses.

Fig. 9 – Asphalt melt sheets are still widely used to reduce panel vibration in vehicles

In general, dampers are polymeric materials applied to body panels to reduce vibration. The kinetic energy is transferred to heat energy during a dissipation process. Viscoelastic damping performance depends on both temperature and frequency. Maximum damping performance occurs at TG (glass transition temperature) and is proportional to activation energy of viscoelastic material.

EXTENSIONAL DAMPERS (ASPHALT MELT SHEETS)

Asphalt melt sheets are still widely used because they typically have the lowest material cost. Applied in the sealer deck after electrodeposition operations, the material adheres to the body structure during the paint bake process. These materials often exhibit very high flow, which allows the material to flow into irregular surface areas such as stiffened floor-pan structures. Conversely, this high-flow characteristic can restrict the use of these materials to only horizontal surfaces. (See Figure 9.)

These asphaltic materials generally exhibit moderate levels of damping performance relative to the other available materials. In typical vehicle applications, these materials are put on over the majority of the floor surface, at a surface weight ranging from 2.4 to 3.6 kg per square meter. A common standard asphaltic pad can achieve a damping loss factor of 0.15 to 0.3, depending on the choice of material and mass per unit area.

CONSTRAINED LAYER DAMPERS (MULTI-LAYERED SANDWICHED SYSTEM)

Constrained layer dampers are generally higher performing damping systems than asphalt melt sheets. Also referred to as a multi-layered sandwiched system, the material consists of a layer of self-tacky viscoelastic mastic with an aluminum, steel or woven synthetic fiber constraining layer. The treatments are applied to body structures manually or with a vacuum assist tool to pressurize the patch. (See Figure 10.)

Some materials can be applied at any point during vehicle assembly in either the body, paint or trim shop operations, while others can only be applied in the trim shop. Because of the aggressive adhesion characteristics of these materials, they are well-suited for vertical and inverted applications. Typical uses of these types of materials are for door slam and roof dampers. In some cases, they are also used for floor pan and wheelhouse damping treatments.

Typically, the constrained layer materials exhibit twice the damping performance of the free layer asphalt system. The corresponding cost of these materials is higher than that of the asphalt materials. From this, it is apparent that the constrained layer approach is used to reduce the weight of the damping system to half that of the asphalt sheets, while keeping the performance and cost of the application about the same. Sandwich materials can even push damping loss factors well more than 0.5.

Fig. 10 – Asphalt melt sheets are still widely used to reduce panel vibration in vehicles


SPRAY-ON LIQUID APPLIED SOUND DAMPERS (LASD)

Due to the material handling and application issues associated with the asphalt sheets and constrained layer dampers, OEMs have been selectively converting to sprayable damping materials. The main advantage of the sprayable systems is cost savings associated with robotically spray-applying the damper. This application versatility optimizes the material and placement only where needed. No tooling or part design is necessary, as is often the case with asphalt and constrained layer damper parts.

Fig. 11 – Ultra-lightweight constrained layer material system can provide up to 60 percent weight savings over other damping technologies


SikaReinforcer® helps the vehicle structure carry more load by preventing section movements in node areas by distributing force over a greater surface.

Typically, the products are spray extruded in the paint shop onto the electrocoated floor pan and oven cured. The material cost of the sprayable technologies may be marginally higher than asphalt melt sheets. With a sprayable damper, you have the benefits and limitations of the bulk system. Sprayable dampers require significant capital expenditure for the pumping and robotic equipment. Assembly-plant floor space is required for the equipment, and space on the assembly line is needed to perform the actual application to the vehicle.

Modern pad-based systems, by contrast, can be tailored to each use case—such as covering a broad range of frequencies or temperatures or addressing specific problems by using a variety of materials, instead of a single material, as in the case of LASD. At the same time, single-layer damping pads cost much less than LASD coatings while offering equal or better acoustic performance.

Various NVH studies performed for customers by Sika have shown that applying the right combination of standard die-cut sound-damping sheets to structure-borne noise hotspots results in more than a 30-percent weight reduction compared to LASD systems without any loss in acoustic comfort.

INNOVATIONS IN DAMPING

Sika is continually innovating panel-damper technology. The latest lightweighting technology is a range of sandwich systems, first developed for the BMW i3 electric vehicle as the car body is made from an epoxy carbon fiber composite. To extend the vehicle’s driving range, the sound-damping package had to lose as much weight as possible without sacrificing acoustic comfort. Sika was able to reduce the installed weight of the original sound-damping package by nearly 60 percent while retaining 80-percent acoustic efficiency. It was achieved by developing a special bitumen-foam system characterized by ultra-low density and minimal rigidity with excellent acoustic performance when bonded to an aluminum layer. (See Figure 11.)

Another recent lightweighting innovation, designed for deeply shaped surfaces, combines expanded bitumen with a special, ultra-hard thermoplastic outer layer. The technology is thermal-moldable, bonds tightly to the surface and weighs 40 percent less than conventional damping products. All told, the foamed polymer bitumen with the right combination of adhesives and high-modulus coverings reduces total weight of the damping package by 40 to 60 percent.

To enable the desire by OEMs to add more automation in the vehicle assembly, but also address the generally poorer performance of LASD, Sika developed an automated process to apply damping pads for serial production. The technology not only avoids the high material costs of LASD, but also reduces labor costs by applying the pads automatically. The robotic system eliminates the need to install materials overhead in ergonomically unfriendly positions.

It can achieve 20 to 40 percent less weight than LASD by placing the right material in the right location. Sika believes the system has up to 50-percent lower material costs than LASD, but results in the same acoustic comfort for vehicle occupants. It will have lower investment costs than automated LASD application systems.

VEHICLE NVH DEVELOPMENT CAPABILITIES

As mentioned earlier, in order to develop innovative and effective NVH solutions for vehicles, extensive technical knowledge and testing capabilities are needed. The competence is needed not only in measuring material NVH characteristics such as damping loss factor, sound absorption and sound insertion loss, but also in measuring effectiveness of these NVH treatments in vehicle subsystems and the vehicle as a whole. Sika, with the recent acquisition of Faist ChemTec, has now globalized its NVH development capabilities with testing facilities in the U.S. and Germany.

For airborne-noise development, many methods exist to evaluate the noise-blocking performance of cavity-filler materials. Component level noise-reduction tests can be performed using cavity sections cut from a BIW or generic cavity sections that represent typical pillars, posts and sills of vehicle bodies. Standards such as the SAE J2846 test method can determine “insertion loss” of the material in application. (See Figure 12.)

SIKA’S PHILOSOPHY IS as heavy as necessary, as light as possible!

 

Fig. 12 – Test facility layout for measuring material insertion loss performance

For structure-borne damping applications, before a single sound-damping pad is installed, most car bodies undergo a vibration analysis to help identify the lowest-cost and lightest weight solution. At Sika, all these analyses take place at its two global acoustics centers. A material testing standard such as the Oberst beam method is used to determine the material’s damping-loss factor.

The results of the characterization analysis are then plugged in to develop custom solutions to meet the OEM’s unique needs. Acoustic “hot spots” are identified using a laser vibrometer and targeted with suitable deadening products. Large areas are covered with low-cost, sound-damping sheets. Higher-weight pads are employed as needed to block sound transmission. Sika’s philosophy is “as heavy as necessary, as light as possible!” (See Figure 13.)

To evaluate the full impact of these NVH treatments on vehicles, Sika has been utilizing the hemi-anechoic NVH chassis dynamometer cells under full vehicle operating conditions. The chambers are equipped to simulate real-world road conditions. (See Figure 14.)

Fig. 13 – Acoustic hot spots are identified using a laser vibrometer and targeted with suitable damping products

 

Fig. 14 – For full evaluation of NVH treatments in vehicles, Sika has hemi-anechoic NVH chassis dynamometer cells


PARTNERSHIP IN TACKLING VEHICLE LIGHTWEIGHTING & NVH ISSUES

It is exemplified throughout automotive history that the best challenges are solved with partnerships and collaboration between automotive OEMs and suppliers. Such is the case in tackling both lightweighting and NVH. Sika has proved itself to be a valuable and effective partner with a comprehensive arsenal of innovative products, technical development tools, material application expertise and full NVH evaluation capabilities across the globe.

 

By Denis Souvay, Phil Weber, Jose Bautista, Michael Fasse and Martin Hörauf

Denis Souvay is a global product marketing manager with global business development responsibility for body reinforcements at Sika Automotive AG. With more than 24 years of experience, he was instrumental in introducing the SikaReinforcer® solution to the Mercedes S-Class sedan.

Phil Weber is vice president of product engineering for Sika Corporation, responsible for both the Industrial and Automotive Business units. In his more than 20 years of experience with Sika, Mr. Weber has served in various technical capacities, including NVH development.

Jose Bautista is a product marketing manager with global business develop-ment responsibility for body-cavity baffles and panel-damping technologies at Sika Automotive AG. With Sika since 2016, he has been instrumental in successfully bringing Sika’s latest acoustic product innovations to the automotive industry.

Michael Fasse is the senior noise vibration hardness project engi-neer responsible for acoustical test develop-ment, validation and optimization of acoustic baffles, dampers, and other automotive and industry products. He is the author of many technical papers on the areas of automotive NVH testing.

Martin Hörauf is the head of business development automotive Europe for Faist ChemTec, a Sika company. With more than 25 years of product development, project management and automotive sales experience, he now directs automotive OEM sales for Faist’s German customers.

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