The sound of quality

DN Staff

October 7, 2002

6 Min Read
The sound of quality

Automobile manufacturers have filled our televisions and magazines with advertisements for quiet cars. Indeed, noise, vibration, and harshness (NVH) performance is a major product differentiator in the automotive industry, with manufacturers actually designing sound to be a general attribute of the type and brand of the product.

In ArvinMeritor's tandem axle, the inter-axle driveline attitude is modified with an amboid gearset, permiting an above-centerline pinion in the trailing axle. The leading axle uses a conventional hypoid gearset.

No wonder: Some countries enforce noise-pollution requirements; and vibration can be not only annoying but also harmful to a car's structure. Plus, potential buyers often get their first impression of a car in a dealer showroom. Since they can't start the engine, they open and close the doors and windows, adjust the seats, and push the buttons. These first impressions can be crucial. MTS Systems, an NVH solutions and consulting company, says it once had an OEM client whose new-model launch was unsuccessful due to the "cheap" sound of the car locking system.

Today, OEMs regularly perform sound quality analysis on both their own and competitors' previous successful models to determine which components are causing the "pleasurable" sounds that can be reproduced or even enhanced in future models. Different regions have different conditions and subjective criteria for NVH as well. For example, the Japanese prefer a powertrain sound with more high-frequency content than Europeans. Automotive OEMs have created sets of matrices, which are up to one hundred different target measurements for different combinations of operating conditions in different regions. This is also why Ford, for example, has combined NVH teams with members representing the different markets in the U.S., Europe, and Asia so that the company can "tune" its vehicles to the different markets.

Here are three examples of how engineers use software to solve NVH problems:

Drivetrain vibration leads to a new axle design

Problem: Noise and vibration in heavy trucks with tandem axles and air suspensions. Component supplier ArvinMeritor studied the problem and said it was related to driveline excitation, specifically the inter-axle driveline universal joints (U-joints).

Analysis: Engineers modeled the entire drivetrain in ADAMS (Mechanical Dynamics). The analysis showed that at low gear/high torque, an inter-axle U-joint greatly exceeded the maximum angle of 4 to 6 degrees. When the driveline reached 2,250 rpm, the U-joint hit a torsional natural frequency, causing a resonance transmitted across the clutch.

Solution: ArvinMeritor modified the trailing axle to change the inter-axle driveline orientation, which reduced the U-joint angles. The new design uses an amboid ring and pinion gear set that provides above-centerline positioning of the drive pinion. "This maintains a high output at the leading axle and a high input at the trailing axle to keep the joint angles low," says Ragnar Ledesma, senior project engineer. This change also means that as the axles pitch, the leading and trailing inter-axle U-joint angles change in approximate equality, maintaining good U-joint "cancellation." Engineers simulated the new tandem axle design in ADAMS to confirm the U-joint angles, and found a reduction in maximum U-joint angle of 4 degrees when the driveline speed was near the resonance frequency. The new axle also provides U-joint angle cancellation as the suspension goes into jounce and rebound. The reduced angularity minimizes the vibration and damage to the drivetrain components, making for a smoother ride.

Acoustic simulations lighten the weight

Problem: Reduce vehicle weight while maintaining the acoustic damping required for users' acoustic comfort. Vehicle interiors supplier The Treves Group is working with vibro-acoustic software Rayon (Straco, recently acquired by ESI) to solve the problem. The French Research Ministry is partially funding the project.

Analysis: The bulkhead and floor panels are the main conductors of noise and vibration in the passenger compartment. For this reason, both of these elements have a spring/mass sound deadener added to the sheet metal body panel to cut the noise in the high frequency range (500 to 10,000 Hz). This spring/mass deadener is made from one layer of either injected polyurethane or felt and another layer of EPDM (Ethylene Propylene Diene Monomer, a synthetic rubber). For the floorboards, a layer of bitumen is added to the spring/mass deadener to reduce low-frequency vibrations (50 to 500 Hz). Treves wanted to show that the spring/mass sound deadener would also be effective for the low-frequency vibrations, making the heavy additional bitumen unnecessary. The company performed vibro-acoustic analysis with Rayon, simulating frequencies from 20 to 450 Hz.

Solution: "Through simulation, we have been able to show that OEMs could reduce both the weight and cost by 20 to 25% by removing the bitumen," says Maurice Fortez, Treves director of research and development. As a result of this research, the bitumen layer will be eliminated from the new vehicle development at Peugeot and Renault.

Software takes out the 'boom'

Problem: A "high speed boom" on the Ford Mondeo during acceleration that could cause passenger discomfort.

Analysis: Using CADA-X from LMS for both physical testing and analysis, engineers found that the boom occurred in second gear during wide open throttle vehicle acceleration. "The boom was caused primarily by the 4-cylinder inline powertrain excitation," explains Karl-Heinz Buerger, manager of vehicle NVH at Ford. "This is seen by the strong rise in the second order contribution above 4,600 rpm and its successive domination of the overall sound pressure level above 5,500 rpm," he says. The engineers checked back to the global mode shape of the Mondeo powertrain that was calculated in MSC/Nastran, and performed calculations to make this resonance visible to see where and how to make design changes. They found that the structural resonance of the global mode shape amplified the combustion and inertia forces in the engine. These amplified excitations induced higher forces, which transferred into the vehicle body via the powertrain mounts to cause the boom in the passenger compartment.

Solution: Engineers added ribbing in the structurally weak areas of the powertrain structure. This shifted the lateral bending frequency of the powertrain above the maximum second-order engine-excitation frequencies. Analysis with CADA-X showed that the structurally optimized powertrain reduced the amplification of excitation forces in the second order, and the overall sound pressure level.

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