Automotive Acoustics Conference 2019

Regulatory pressures on internal combustion engines (ICEs), combined with technological improvements in electric powertrains and batteries lead to an increase of demand for electric vehicles (EVs). Both, the traditional carmakers as well as new entrants without ICE legacies are developing and putting on the market new EV models.

Worldwide sales of pure battery EVs (BEV-excluding hybrids) grew by approximately 45% in 2016. Since, BEVs become mass-market product, it is necessary that automotive acoustic engineers apprehend and evaluate related technology trends and their influences on vehicle NVH. The BEV is a radically different vehicle and it needs an updated NVH approach, which targets all relevant NVH comfort issues in a specific electric environment. As demand rises, BEV technology and design will continue to evolve, and strategic challenges will follow. Established OEMs and their traditional suppliers will need to rethink their approaches to preserve their revenue and profitability.

The ZE driving implies “silence”. Consequently, some NVH performances are to be increased for BEVs. The lack of IC engine noise masking exacerbates: the noises of different systems & devices, wind noise, rolling noise and tire hiss, etc. the corresponding NVH requirements are to be strengthened, comparing to the IC engine vehicles. The completely new, but crucial NVH challenge for BEV is to control e-Powertrain whining. Disturbing noises like squeak rattle and buzz, particularly in low noise BEV context are prohibited, too.

Compared with the mechanical and combustion noise of low-frequency firing orders of an internal combustion engine, the electric drive assembly noise of electric vehicles is mainly the high-frequency whining noise generated by electromagnetic forces and transmission gear meshing. Although the radiated sound power is far less than that of an internal combustion engine, the high frequency noise of the electric motor and the transmission is much annoying. Noise below 2000–2500 Hz generated by an electric drive assembly in a vehicle passenger compartment is mainly contributed by structural transfer paths. Due to the high torque of the electric drive motor of electric vehicles, it is a challenge to achieve better mount isolation performance at the same time when the anti-torque performance of the electric drive assembly mounting system is realized. In this paper, by comparing the vibration characteristics of electric vehicles and traditional ICE vehicles, comparing different mount layout schemes, and analyzing the main factors affecting the vibration isolation rate of the electric drive assembly mounting system, the general design principles of the electric drive assembly mounting system are summarized to achieve balanced attribute performance.

By 2025, Battery electric vehicles (BEV) and plug-in hybrid electric vehicles (PHEV) will probably account for more than 20% of the total production of vehicles. With the development of specific vehicle platforms for electric cars, NVH strategies and technologies may strongly change and with them the challenges for NVH engineers.

The integration of the battery pack system in the vehicle is steering the focus towards an “integrated” design of the floor system. Consequently, the design of NVH floor treatment, body in white and battery pack configuration has to be engineered simultaneously to enhance performance and weight reduction. From this, the need for CAE tools which are able to support the integrated design is recognized, even more so in the early stages of development.

The aim of this paper is to analyze how existing and well-established CAE methodologies, such as Statistical Energy Analysis (SEA) and Finite Elements Methods (FEM), can support the NVH design of BEV floor systems. This includes how they can be exploited for the optimization of an integrated NVH package, including not only the carpet and damping pads, but also battery sealing and treatments between the battery and body.

In the first part of the paper, the test setup for the floor insulation assessment of the Jaguar I-Pace, selected as vehicle test case, will be illustrated. Transmission loss measurements in a coupled room and their correlation with FEM simulations are presented thereafter. Simulation will be then applied to explore the possibility of optimizing the positioning of the abovementioned sealing and its impact at vehicle level. Last, the impact of sealing and battery gap treatments on the structure borne noise performance of the floor system are discussed.

One of the challenges in the development of hybrid and electric vehicles is the interior and exterior noise refinement: for interior noise, the absence of masking by the internal combustion engine during EV operation may be critical due to the appearance of noises that the customer is not used to hear (e-compressor, cooling pumps, electro-hydraulic actuators etc.). On the other hand, the absence of ICE noise may be an opportunity for designing artificial sounds that could be seen as an added value for improving customer’s driving experience, perhaps whilst also providing some additional masking of unwanted noise.

For the above reasons, brand requirements and a deep understanding of customers’ expectations, may be the most relevant key factors for interior noise development. In addition to this, the new homologation standards for exterior noise leave some design space for a “brand customization” of AVAS.

Although these sounds are not directly connected with the mechanics of the powertrain, nevertheless their link to vehicle’s CAN bus parameters (speed, “gas pedal” position etc.) is crucial for a satisfying driving experience.

In this work, an approach has been developed for designing artificial interior noise and AVAS sound for a sporty version of the 500e, where both interior and exterior sounds have been prototyped in an NVH Simulator, allowing nearly all sound prototyping and test work to be accomplished quickly, easily, and efficiently.

The entire process, from the on-track assessment of a “sporty sound” key features, passing through the virtual sound’s assessment inside a listening room, to the final delivery of the demo vehicle for jury testing and type approval measurements, is here described.

This approach has then been extended to include the use of panel transducers (PT) to deliver the AVAS sound. PT have the advantage that they are much smaller to package and can even deliver lower frequencies than the small loudspeakers typically used for standalone AVAS sounders. A method for using PT to deliver AVAS is also presented.

Numerical analysis plays a significant role in the development of passenger comfort in modern vehicles. Customer perception of quality and comfort is largely influenced by the vibro-acoustic properties of a vehicle. Numerical simulation models of sufficient quality for predicting these characteristics are highly complicated and large numbers of eigenvalues are necessary to enable accurate analysis through the required frequency-range. It follows that extensive computational resources are required and calculation times are long.

This paper describes the generation and use of so-called FRF-Substructures, a modern technique in model reduction for frequency-response analysis in Nastran. Using a practical example of a Mercedes-Benz S-Class model it is shown how FRF-Substructures can significantly increase the efficiency of frequency-response calculations. This enables the evaluation of multiple model variations and complex load-cases in a reasonable time. Further, the advantages of FRF-Substructures are shown by comparison with the traditional superelement approach. Finally, a brief outlook on further uses of the FRF-Substructure technique are addressed.

The chemistry of polyurethane flexible foam enables its use in a wide range of sound absorption, sound transmission loss and vibration damping automotive applications. A challenge for developing new acoustic materials is to have a reliable, reproducible and representative small-scale measurement technique to obtain acoustic properties of polyurethane foam. In this paper we present a new measurement technique, based on an impedance tube, where the four pole parameters, or transfer matrices, can be obtained for these materials. A library of transfer matrices for various polyurethane foam materials is obtained and absorption, reflection and transmission coefficients calculated. The materials may also be combined in various layered assemblies. Furthermore, the meaning of the transfer matrix is explored with respect to previous studies on polyurethane foam microstructures.

To develop optimum vehicle designs, engineers must balance vehicle attributes to meet various product requirements, relating to specific market demands. It is commonplace to see trade-off relationships between noise and other important attributes. Balancing decisions are best made when detailed information is available through appropriate simulation and testing. This paper describes three case studies exemplifying such trade-off situations, and the technical approaches taken to decide the best attribute balance for each vehicle product.

Demand is increasing for outstanding fuel economy in gasoline engines especially in Hybrid Electric Vehicles (HEVs). An enabler for good efficiency is rapid combustion, but this can erode the refined sound quality that customers expect from gasoline engines. A simulation approach was used to improve gasoline combustion noise early in a project. Mitigation measures were optimized for drive-cycle fuel efficiency, combustion noise and engine mass.

Car makers must provide cruising comfort through low road noise (more prominent with quieter electric propulsion) and simultaneously deliver competitive driving dynamics. A vehicle structure’s dynamic response strongly influences both ride comfort and road noise, and is significantly impacted by the integration of a heavy battery pack. Using a multi attribute optimization approach, an optimum balance was found for a Battery Electric Vehicle (BEV) to control road noise whilst giving good vehicle structural response and reduced mass.

Electric cars deliver strong and responsive vehicle performance and good cruising comfort, but their dynamic driving experience and brand appeal is marred by the lack of informative acoustic feedback from the power unit. An augmented power unit sound system was calibrated to deliver an exciting and authentic acceleration feel during spirited driving, while retaining acoustic comfort during cruising and minimizing environmental noise pollution.

During the development of an Electric Power Steering system (EPS), it is critical that NVH is incorporated early in the development to mitigate noises that the end customer will find objectionable (roughness, whine … etc.) before start of production. This includes the determination of critical noise rating drivers and propagation of noise to the driver’s ear through several vehicle tests. However, early in development, vehicles are not easily available, making it difficult to identify significant noise concerns. Because of this, it is critical that predictive methods are used. This can be done with a proper vehicle acoustic model being provided either by the OEM or by Transfer Path Analysis (TPA) conducted on a representative vehicle.

TPA has been used heavily throughout the automotive industry as a method for understanding the propagation of noises from the noise source to the driver’s ear for several different automotive systems. However, conducting a TPA on an EPS at the vehicle level presents a challenge in visualizing and interpreting the data since a continuous speed sweep is not possible to cover the entire operating speed range of the EPS. Typically, several lock-to-lock iterations at different operating speeds need to be conducted to cover all operating conditions and results in several 2-dimensional plots to present and interpret. The approach of using 3-Dimensional Transfer Path Analysis improves the presentation and interpretation of Transfer Path Analysis for EPS Systems in the following ways:

Once a verified model is complete, it can be demonstrated that with force measurements from the ZF Wheel to Wheel Rig, where forces are measured directly, the in-vehicle noise performance of an EPS system can be predicted.

This approach significantly reduces the dependency on a vehicle by making it possible to identify noise risks from the simplicity of a bench test.

The different connections between the powerplant and vehicle chassis/body should be developed carefully in order to minimise the transfer of unwanted vibrations. The engine mounts are well known to have significant contribution in the total transfer of powertrain noises in a broad frequency band, ranging from low-frequency booming noise to high-frequency gear noises. Also the driveshafts can often be an important contributor.

In order to control the NVH performance during the development of those components, clear component KPI’s and targets are needed. A suitable KPI should fit in a consistent breakdown of the performance, e.g. based on a (simple) model of the considered contribution path. It is shown that an output/input side transmissibility can be a suitable KPI for driveshafts and engine mounting brackets.

In this work, the use of a transmissibility KPI is illustrated on two case studies: one for engine booming noise and one for gear noise. It is shown how the proposed KPI can fit in a robust target cascading process that can be used throughout the different project phases.

Since the final host vehicle is often not available during the component development, it is important that the selected KPI’s can be evaluated on a test bench (or from an isolated component model). The correlation between on-vehicle and on-bench evaluation is reported, including an investigation of the trade-off between evaluation accuracy and bench complexity.

Over the past decades, automotive companies have sought lightweight and performant noise, vibration and harshness (NVH) solutions to comply with stringent regulations for CO₂ emissions and noise pollution. Combining lightweight design with improved NVH solutions is often a challenging task: low mass and high stiffness materials are generally characterized by poor NVH behavior and low noise and vibration levels often require heavy and bulky additions, especially to be effective in the low frequency regime. To face this challenge, recently, locally resonant metamaterials (LRMs) have come to the fore. These materials combine in one solution lightweight design and superior noise and vibration attenuation performance, beating the mass law in tunable frequency ranges, referred to as stop bands. The LRM concept is used in this work to tackle a low frequency structure borne road noise problem in a commercial vehicle. A LRM solution is applied on the rear shock towers of the vehicle, with the goal of attenuating the vibrational energy entering into the vehicle body through the suspension assembly, which is excited by the interaction of the tire with the road while driving. This results in a reduction of the noise in the interior compartment around 190 Hz. In order to benchmark the performance of the LRM concept, a vehicle is chosen which is sold with a tuned vibration absorber installed on each of the rear shock towers as NVH solution. Each of the tuned vibration absorbers (TVAs) adds 1.46 kg of mass to the vehicle. The LRM concept is designed to reduce the mass of the current solution by 48% and to have similar NVH performance. The LRM concept is realized through additive manufacturing and it is added as patches on the rear shock towers to replace the TVAs. Both numerical and experimental results in lab and on a smooth road profile validate the performance of the LRM concept proposed.

The latest Pass-By (PB) noise regulation ECE R51.03 requires OEMs to achieve ever more challenging limits. In particular, the target of 68 dB(A) in 2024 set by the ECE R51.03 Phase III appears as a critical challenge, considering that such limit has to be achieved with the updated ISO 362-1:2015 measurement method. With this new measurement method, the contributions to PB noise given by tires and powertrain are more balanced compared to the past, when powertrain was the dominant source. This fact makes the achievement of the above mentioned demanding levels inherently more difficult.

As a matter of fact, for the achievement of PB noise limits OEMs are now compelled to define quantitative strategies for the reduction of powertrain and tire noise separately. This must start from an accurate assessment of the relative share that these two sources have in the PB noise level, then pass to the definition of corresponding reduction targets and eventually design concrete countermeasures to achieve them. In particular, in this last step, vehicles are to be fitted with some optimized exterior acoustic package, which requires a thorough insight into the noise radiation from both powertrain and tires.

This article is subdivided into two parts. In the first part, the mentioned process is elaborated in some detail, with special emphasis on important practical aspects, e.g. how to define sensible noise reduction targets for tire and powertrain noise. In the second part of the article, an application example is shown, in which the process is put in place and the package of a B-segment European vehicle is optimized by means of different indoor and outdoor testing procedures, allowing the achievement of the 68 dB(A) PB noise level.

Evaluating powertrain sounds in a virtual vehicle environment is a key technology enabler for improving NVH development efficiency. NVH Simulator models constructed utilizing a source-path-contribution approach allow for assessment of individual subsystem contributions during simulated drives, all in the absence of prototype vehicles. Application examples are presented to show how the tool is utilized to facilitate powertrain NVH hardware/target compatibility decisions in the early stages of program development.

Recent evolutions in terms of regulations for pass-by noise force Automotive OEM’s to build more silent cars than before. To be able to decrease the emitted noise level while meeting other constraints such as weight, space or sound quality, the OEM’s and their suppliers have to come up with innovative solutions to reduce the different noise sources and control the noise propagation around the vehicle. In particular, powertrain and tire noise sources are dominant contributors to the exterior sound emitted by vehicles and efficient methods to understand, analyze and reduce their contribution need to be developed. Exterior acoustic simulations are required to find optimum acoustic treatment location and physical properties while offering the possibility to improve and predict final pass-by noise performance before the first complete vehicle prototype is built.

This paper presents a framework for the efficient simulation, including parametric analysis, of wheel arch acoustic treatments. A finite element scheme is used to solve the acoustic propagation of different sources around a car tire while a locally-reacting admittance boundary condition is used to model the wheel arch acoustic treatment.

The parametric analysis focuses on the investigations of acoustic treatment materials, location and geometries, on the tire contribution to the total pass-by noise level. A model reduction strategy based on a condensation of the car surface and exterior acoustic problem on the tire and wheel arch surface is presented as a mean to a drastic speed-up of the computational process, therefore opening the door to a parametric analysis process.

Comparisons between the different designs as well as the computational performance of the proposed approach are presented. Numerical simulations are conducted with the Actran commercial acoustic simulation framework.

Due to intensified development of electric vehicles, more and more focus is put on noise and vibration from the electric drive unit. One characteristic sound of electric vehicles is the tonal noise from the e-machine. This paper first shows how this tonal noise can be simulated and validated by measurement. It has been observed in vehicle measurements that identically designed electric axles can have a very high deviation in noise, so the question remains on how accurate the simulation results can predict this behavior. Using DoE methods and robust neural network (RNN) modelling, it is outlined how much manufacturing tolerances and material properties can affect the NVH behavior. This study concludes that even small deviations from the nominal geometries and material properties can have a significant effect on NVH. By applying the proposed method significant tolerances can be identified and revised in the early design stages of an electric motor.

To identify the sound quality of fuel injection systems as a frontloading measure, a system test bench has been developed that simulates the structure and airborne noise emission. The setup of the system test bench includes an assembly of the acoustical relevant engine parts and an encapsulated drive for the high-pressure pump. A specific control unit enables a high variability for the operation strategy of the injection system.

The focus of this study is on the development of a methodology to analyze and optimize the sound quality of gasoline direct injectors in critical operation points by suitable active means. For this, the injection parameters in engine idle have been identified and subsequently transferred to the system test bench. Additionally, the injection parameters have been optimized acoustically and the impacts on the key functions regarding injection rating and shot-to-shot scattering of the injected fuel mass are discussed.

This paper presents several Noise, Vibration, Harshness (NVH) post-processing techniques used on a dynamic acquisition system to troubleshoot acoustic noise and vibration issues due to electromagnetic forces in e-powertrains. Standard NVH post processing techniques based on Order Tracking (OT), Operation Deflection Shapes (ODS) and Experimental Modal Analysis (EMA) are presented and as well as their application to discriminate the frequencies of magnetic forces responsible for NVH issues. However, they do not filter the different wave-numbers of vibration waves. Some special NVH post processing techniques are introduced to go deeper in the analysis and quickly capture electromagnetically-excited NVH root cause. They are based either on “spatiogram” or on flux sensor waveform post processing. “Spatiogram” relies on the Discrete Fournier Transformation of accelerations measured on the machine. It aims at introducing a space information besides the standard time and frequency information. It can be seen as an extension of the spectrograms done at variable speed as it efficiently discriminates wavenumbers of magnetic forces responsible for NVH issues. Experimental measurements run on a Permanent Magnet Synchronous Motor (PMSM) are interpreted and confirmed by simulation results. Similarly flux sensor waveforms can be post-processed to quantify geometrical and magnetic asymmetries, to estimate radial magnetic forces per tooth and derive Frequency Response Functions (FRF) of stator teeth under radial electromagnetic excitations.

This paper presents tools and methods that can be used to track the noise quality of auxiliary equipment in the development process without extensive vehicle measurements. Its aim is to provide a guideline for the evaluation of sound quality on component test benches and to sensitize engineers to the challenges of the individual steps. Besides the identification of relevant noise components and the results of test bench evaluations, the focus of this paper is on the determination of characteristic values. The selection of specially tuned psychoacoustic analyses and a suitable combination of characteristic values are demonstrated using examples of tonal noise components.

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