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Balancing noise, vibration and harshness

Balancing noise, vibration and harshness

Ignite Dodge Viper's V-10 and the vehicle responds with a deep, throaty roar. The noise satisfies and seems perfectly natural given the car's powerful visage--yet Chrysler engineers carefully craft the sound to meet driver expectations.

One of their tools includes an acoustic test dummy with specialized microphones for ears. The dummy, with associated data acquisition equipment, correlates frequency and sound. By identifying the sound qualities a Viper driver would expect and want, the engineers can isolate and tune the source, path, or amplifiers within the vehicle that make those frequencies stand out.

Buyers of the Chrysler Concorde expect a different sound quality, but the process of achieving it is much the same: Isolate those vehicle components whose frequency ranges please or displease, then dampen or enhance those parts accordingly.

More and more, noise, vibration, and harshness (NVH) play a critical role in satisfying today's car buyer. At Chrysler, a dedicated NVH laboratory at the Auburn Hills, MI Technology Center, lets individual platform teams work directly with the 75 engineers, technicians, and mechanics that make up the company's NVH specialty group. The lab houses several hemi-anechoic chambers, as well as 6- and 10-ft chassis dynamometers rolls with road surfaces molded from Chrysler's Chelsea Proving Grounds.

"Each platform team is the keeper of that product and its personality," explains Larry Achram, executive engineer for NVH. "Their job is to help establish the overall attributes of a vehicle and help manage those attributes through the development of all components. Our group works to integrate each component into subsystems and into the total vehicle to acheive the best balance between desired NVH and all other performance requirements."

The lab's power train chamber, for example, enables the whole vehicle to be used as a power train fixture. This, in turn, lets engineers analyze NVH with only the power train exciting the vehicle. With the full vehicle dyne cells, engineers can then add the drive train. In this way, they are able to isolate different components and subsystems to accentuate or dampen specific frequency ranges.

Software first. Designing for NVH begins with the particular image a plat-form team wants to project into a vehicle. Computer programs such as MacNeal-Schwendler Corporation's MSC/NASTRAN(R), as well as the pre- and post-processors that go with that package, guide the platform team through the many iterations that characterize concept development.

For example, NVH engineers at Chrysler can apply computational fluid dynamics (CFD) to a vehicle's intake manifold to analyze mass flow, energy distribution, or fluid velocity. Another in-house code uses the CFD output, along with design parameters such as runner length and port configuration, to create sound spectrum files. In this manner, the platform team can actually listen to the manifold before it is built.

Achieving a particular NVH personality, however, involves the manipulation of many different vehicle attributes--not all of them complementary. "If you worked only by gut feel," Achram says, "you would have a difficult time balancing ride and handling with NVH performance."

Chrysler's decision to install softer rear-suspension transverse-link bushings in this year's Concorde is a good example. Using MSC/NASTRAN(R), engineers modeled a variety of spring rates. Techniques included: changing the bushing's geometrical shape, substituting different rubber compositions, and adding voids or mechanical elements.

"Years ago we used to subjectively evaluate boxes and boxes of bushings," Achram says, "but it was not an ef-fective way of balancing the desired attributes. And while we will never replace the need for a "ride" in the car, iteration through computation allows NVH reduction without sacrificing handling or performance."

An ongoing battle. As cars become increasingly quiet due to excellent engineering, customers notice sound and vibrations that were previously masked by road noise. They describe these sounds with words like boom, rumble, warble, and whine. To find and isolate these sounds, NVH engineers look at three things: noise sources, paths by which the noise or vibration reaches the occupant, and system components that can amplify the sounds.

"We continually challenge ourselves to find ways to increase our competitive advantage and improve customer satisfaction," says Achram. "When we studied possible areas for improvement in the Concorde/LHS powertrain, NVH was at the targeted levels. However, our analysis and testing suggested that road noise and harshness enhancements would be desirable. As a result, the Concorde features a number of NVH refinements to the suspension and body.

Examples include resilient isolators to better separate sources from receivers, such as: softer rear-suspension transverse-link bushings; hydro-elastic rear- suspension trailing-arm bushings; retuned front-suspension lower-control arm bushings;and urethane engine cradle isolators.

In other problem areas, Chrysler engineers optimized structural design to minimize the effects of resonance. A stronger and stiffer cross member, for instance, resists deflection that passengers perceive as harshness and road noise, while improved dimensional accuracy of doors and door openings reduce wind, road, and traffic noise.

Adding damping layers to structural surfaces, or introducing tuned dampers at discrete points, eliminates unnecessary vibration. On the Concorde, Chrysler engineers spot-welded patch constrained layers (PCL) to the front wheelhouse, dash panel, and plenum areas. The PCL's visco-elastic material expands during paint oven baking, bonding the constraining patch to the body panel for effective damping.

Other methods of noise and vibration control used include: a reduction in excitation levels for certain components, and added acoustic material, such as molded, vs. die-cut, carpet-silencer pads to cover exposed side and corner areas.

"NVH is like a three-legged stool," Achram summarizes. "You need computational tools. You also need experimental tools to validate modeling approaches, as well as capture data easier to measure than to model. Third, you need some engineering judgment to verify that everything makes sense."


Acoustic design parallels component design

Armed with today's CAE tools, NVH engineers can apply acoustic analysis at the very outset of the design process. For example, Chrysler employs MacNeal-Schwendler Corp.'s MSC/NASTRAN(R) code to predict surface velocities of the engine block. The resultant FEA data then serves as input to acoustic analysis software packages like SYSNOISE, from Numerical Integration Technologies N.V., and COMET/Acoustics(R) from Automated Analysis Corp. Such systems model the engine block's radiated noise using finite-element, direct- boundary-element, and indirect-boundary-element methods. Engineers, therefore, can analyze complex acoustic and combined structural-acoustic behavior before hardware is even built.

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