Acoustic Target Setting Supports BMW’s Premium Brand Image

Construction Recognizing that acoustics define the character of an automobile, BMW is driving business forward with acoustic target setting and hybrid simulation to engineer the sound of its vehicles. Using state-of-the art technology and processes, they combine test data on existing components with virtual models of new parts to accurately represent entire vehicles and set acoustic targets up-front in development. In this way, BMW is revolutionizing vehicle development by predicting and tuning passenger compartment sound early in the conceptual stage, even before detailed design of the car is started. Their strategy pushes the envelope in automotive development and fortifies a brand image of high-quality premium vehicles. 

Clearly, BMW is doing all the right things to develop and build some of the finest vehicles for a discriminating clientele that demands nothing less than the best. The company sells far fewer cars than the automotive industry giants, but BMW is one of the most profitable. It is posting significant gains when the overall market is struggling and most competitors are barely getting by.

The reason for success: higher price margins for premium automobiles with strong brand identity. BMW is one of the few manufacturers focusing exclusively on the premium segment of the international automobile market. Only a select number of carmakers can match its per-vehicle profit or the brand value behind the BMW name.

The foundation of customer appeal in this high-end segment of the market is built upon decades of meticulous design and engineering to refine vehicle performance, styling and acoustics. This means not merely eliminating squeaks and rattles and suppressing overall noise levels, but tuning the sound of the automobile to reflect the high quality and distinction of a BMW.

“Apart from the look of a vehicle, its acoustic behavior is the aspect most directly observable to the user,” says Dr. Peter Zeller, Director of Acoustics and Vibration at BMW. “The subjective impression of the car is shaped by its sound character. Drivers expect quietness at cruising and a particular acoustic feedback when they accelerate - the silky-smooth tonality in a luxury sedan and a rugged roar in a sports car. Acoustic design is therefore a critical task not simply for reasons of comfort, but also because it is a key means of conveying brand identity.”

BMW looks at many types of sounds in establishing the acoustic targets of a vehicle. Disturbing noises to be minimized include gearbox noise, load reversal, auxiliaries, road noise, resonance effects, mechanical squeaks/rattles, idle noise and pass-by noise. Other sounds tuned during concept design include communications and audio acoustics, actuation noise from windows and other devices, and driving noise comprised of engine sounds and wind/rolling noise during acceleration and cruising. Tuning sound and eliminating noise represent a top priority in the vehicle development at BMW.

“Engine noise tone and amplitude in relation to the level of wind/rolling noise is a major factor in defining the character of the vehicle,” explains Dr. Zeller. “The goal is to set the acoustic target so that engine noise becomes the dominant sound during acceleration while masking by wind/rolling noise at constant-speed.”

According to Dr. Zeller, a quiet comfortable sedan typically has low wind/rolling noise and a low level of engine noise. A vehicle that glides along very quietly at constant-speed travel, but whose engine roars as soon as the accelerator pedal is pressed is perceived as more sporty. On the other hand, a vehicle where high wind/rolling noise largely conceals the engine noise even at full load is lacking in character and is therefore not desirable.

Getting a vehicle to sound just right is of course no simple task. Otherwise, every car would sound like a BMW. Acoustic engineering requires considerable background, skill, technology - and traditionally time. But compressed time-to-market in the automotive industry forces automakers to squeeze product development cycles as much as possible.

BMW’s resources are being further stretched by a drive to expand its product lines according to a corporate growth strategy where the company reinvents its entire range of vehicles. During the last three years, BMW has added Mini and Rolls-Royce to its vehicle portfolio. In 2003, the company introduced more new models than any other year in its 87-year history.

Tuning the sound of such a variety of vehicle models in the face of limited time and resources, Dr. Zeller says, “BMW is pioneering the use of hybrid simulation to set targets in a “V” approach to vehicle development. The process starts with full-vehicle performance targets that are cascaded down to requirements for subsystems (drivetrain, chassis, suspension, etc.), and finally to components (bushings, mounts, struts, etc.). Hardware is then designed, built, and assembled into a prototype vehicle in the upward part of the “V” where physical testing usually leads to several redesign cycles to iron out problems.”

In this “V” approach, most car companies use simulation like FEA (Finite-Element Analysis) to help speed the process after CAD has defined the geometry of subsystems, assemblies and parts. By that time, important design decisions have been made and take considerable time and expense to reconfigure. BMW is eliminating this problem with function-driven design that aims to accurately establish functional performance requirements through acoustic target setting, much earlier in the process before the detailed design has started.

“This eliminates the repetitive build-test-redesign cycles later in development by performing analysis earlier with more system-level full-vehicle simulation during the conceptual target setting stage,” says Dr. Paul Venhovens, Head of Methods Development in the BMW Acoustics and Vibration Group. “We put function first, before CAD is used to define geometry.”

By performing up-front engineering, the company can more accurately establish performance targets, and thus make better strategic decisions about vehicle behavior. In this way, BMW saves time and expense later, in the upward part of the “V”, as fewer prototype testing cycles are required.

The difficulty companies have in performing full-vehicle simulation in the early conceptual stages of development is that modeling the various parts, assemblies, and subsystems - and how they interact - is extremely difficult. The complexity of the problem does not enable to tackle it properly during the time made available. And this is where hybrid simulation comes in.

Dr. Venhovens explains that a new vehicle is mostly made up of existing parts. “In general, some existing components such as assemblies, engines and drive trains are carried over from predecessor models,” he says. “Hybrid simulation combines test data from existing predecessor hardware with virtual models of the new parts. The resulting hybrid model of the full vehicle can then be exercised to accurately predict the behavior of the proposed vehicle.”

Part of the advantage of exercising the hybrid simulation model early in development, according to Dr. Venhovens, is that if engineers do not like what they see in terms of functional performance (or hear, in the case of acoustics), they can modify portions of the model to change the predicted sound. This allows engineers to “tune” the sound of the car early in the development to meet the acoustic targets.

“This approach shifts part of the role of testing from that of strictly a problem-solving tool at the end of product development to that of a critical analytical tool at the beginning of the cycle and a valuable benchmarking tool at the end of development, when data is captured for use in designing future vehicles,” Dr. Venhovens explains. “Hybrid simulation gives us the best of both worlds – physical testing and virtual simulation.”








For its vehicle acoustic testing, BMW uses multichannel testing systems from LMS that acquire data from hundreds of sensors and also analyze the data and provide results in easy-interpretable graphic format, such as time, frequency, order and rpm domain processing as well as specialized plots, such as waterfalls and Campbell diagrams. Front-end data acquisition and signal conditioning is provided by LMS SCADAS III systems. The LMS testing solutions are especially useful at BMW for modal analysis and Transfer Path Analysis (TPA) in testing full vehicles as well as the body-in-white (the vehicle frame and chassis only).








Dr. Luc Cremers, Senior Engineer at BMW’s Structural Dynamics Group, is in charge of pilot programs to investigate advanced technologies that BMW is considering for future vehicle development projects. According to Dr. Cremers, one of the pilot programs focuses on evaluating ways to use LMS Test.Lab. This testing software suite has a workflow-based interface for easier setup and greater productivity in testing and data interpretation. Advancements in LMS Test.Lab incorporate the feedback of LMS’s yearlong experience with LMS CADA-X.

To complete the hybrid model of the vehicle, test results for existing components are combined with virtual models of new components. The resulting hybrid model is then used as a basis for studies to predict sound levels inside the vehicle passenger compartment.

A major advantage of predicting noise in this manner is that engineers can go back to the hybrid model and change any of the parameters to modify the sound, essentially shaping the sound to the exact profile needed for the particular vehicle. Guided by the TPA, engineers can change component characteristics along the major transfer paths and immediately see the effect of these modifications. This allows engineers to perform “what-if” studies to see how modifying the stiffness of a bushing or engine mount, for example, affects the sound heard in the passenger compartment.

As part of its hybrid simulation efforts, BMW is currently engaged in another pilot program, which investigates the use of LMS Virtual.Lab. This software suite provides an integrated infrastructure that enables to automatically and conveniently handle and combine functional performance variables and other data from design, test and simulation. The technology links together CAE packages that otherwise would run separately and produce isolated results. In this respect, LMS Virtual.Lab readily facilitates hybrid simulation and offers the potential of significant improvements in terms of efficiency.

The software provides advanced coupling methods for combining test models of existing components with virtual models of new components. FRF Based Substructuring, for example, attaches parts according to measured data on the free-free suspended components. Coupling can also be accomplished based on resonant vibration modes of components, or through FE-FE coupling where test-based modal models are automatically transformed into dynamically equivalent Finite-Element (FE) models.

The key to success of hybrid simulation, according to Dr. Zeller, is the close interaction of physical testing and virtual simulation. Appropriately, BMW’s state-of-the-art test facilities are located at the center of the company’s honeycomb-shaped glass-and-steel Research and Innovation Center, which is located close to the company’s headquarters in Munich. Around the periphery of this campus are the many design offices, all within a few minutes walk from the test facilities.

“The central location of the test beds greatly facilitates collaboration with design engineers and underscores the importance of these empirical methods to the continuing work in vehicle development at BMW,” Dr. Zeller explains. “Through hybrid simulation, testing is a powerful tool in setting acoustic targets in the early conceptual stage of vehicle development. At the end of the cycle, testing validates that targets have been met and captures valuable benchmarking data that will help design future vehicles. In this respect, testing has been expanded to include a broader range of effort and is key part of our product development strategy.”