Acoustic software streamlines truck cabin noise mitigation

During the past few years, the commercial truck industry has been on a bumpy road. With the global economic slowdown, orders are a fraction of what they were in 2008 and sales are uncertain. Moreover, manufacturers must make significant nose-to-tail redesigns for vehicles to meet tough new government fuel efficiency and emissions regulations. All this drains the limited resources of truck producers as they try to hang on in an evolving market while an economic recovery gains traction.

Process changes underway
In the face of these challenges, process changes are underway to bolster profitability and market share at Italian truck maker Iveco. Based in Turin, this Fiat subsidiary has five R&D centers and 28 factories in 16 countries around the world producing light, medium, and heavy-duty trucks as well as buses and vehicles for defense, firefighting, and other specialty applications.

A corporate-wide initiative focused on improving product design—including vehicle cost optimization and higher cabin interior quality standards—and on streamlining product development processes through the use of virtual prototyping and simulation. In light of this initiative, Iveco managers across all vehicle platforms and business units were directed to evaluate their operations and implement appropriate changes.

One area receiving close scrutiny was the prediction of truck cabin noise levels. Cabin acoustics is a critical element in vehicle development and a key selection criterion for truck fleet managers, especially in over-the-road trucks making long-haul trips of a week or more. In this premium vehicle segment, quietness reduces driver fatigue for increased safety, comfort, and productivity.

Diminishing sound levels is often difficult, however, because noise sources such as the engine, exhaust, air intake, and transmission all interact within short distances from the cabin—sometimes less than a meter away. As a result, acoustics must be exhaustively studied in minute detail to accurately predict excessive cabin noise early in development.

Smooth sounding interiors
The approach implemented at Iveco for truck acoustics is an adaptation of typical methods used throughout the automotive industry for passenger cars. The process begins with acoustic requirements provided by the marketing and quality group, basically a list of preferred sound levels taken at the driver’s ear at given vehicle speeds and loads. Engineers focus on reducing loudness, of course, but also focus on acoustic subtleties, such as sharpness, frequency, transitions between different gears and any sounds that drivers might consider unfamiliar or objectionable.

From these requirements, engineers compile a set of vehicle technical specifications that serve as global vibro-acoustic targets that are then cascaded down to secondary targets for major subsystems and components, such as the engine, transmission, exhaust, air intake, chassis, and suspension.

Next, engineers create finite element (FE) or boundary element method (BEM) models of the various components and subsystems to determine natural frequencies, bending moments, vibration amplitudes, and operational deflection shapes. Analysis results are then correlated with supplier data and tests of actual parts on predecessor vehicles and the simulation models are adjusted accordingly.

A BEM analysis of air-intake noise at driver’s ear (top) and sound emission on cabin envelope (bottom)

Hitting acoustic targets
Confident in the models’ accuracy, engineers can then iteratively modify the designs and run further simulations until acoustic targets are met for components and subsystems. These optimized models are combined into an overall global vehicle model, including a cavity model complete with various panels—the dashboard, doors, overhead area and floor—that vibrate to produce interior noise. Simulations are then performed on this global vehicle model to predict cabin noise and optimize the overall acoustic performance.

In this process, Iveco used LMS Virtual.Lab Acoustics software which has a range of tools aimed at identifying and reducing noise problems. Frequency response functions (FRF) are used to evaluate the behavior of the structure at various noise-producing vibration frequencies. Transfer path analysis (TPA) traces vibration paths back through the structure to determine noise sources. Noise transfer functions (NTF) represent stiffness properties in the structural transmission path.

In a full study of truck cabin sound levels, Iveco engineers first create a complete model (a) detailing cabin geometry and containing acoustic and structural FEM representations. From this, they generate an acoustic cavity model (b) from which cabin sound pressure levels are plotted (c). Likewise, the structural part of the model can be used to indicate deformation of the cabin envelope from coupled acoustic and structural loads (d).

Contribution analysis helps identify which interior panels are the greatest noise producers. This gives engineers insight into where to add damping materials, stiffen parts, insert ribs, modify component geometries, thicken walls or shift part locations to prevent vibrations from entering the cabin and also reduce material costs by determining which parts are not so useful in sound abatement.

Operational deflection shapes of the frame are also studied to understand how the full structure deforms at particular frequencies that may cause the most interior cabin noise. During this optimization process, engineers make adjustments throughout the structure, re-run the acoustic simulation, and see the result in color-coded sound-pressure maps.

With such tools, once the vehicle acoustic model is created it facilitates quick evaluation of alternative designs and engineers can explore acoustic modifications instead of building and reconfiguring a physical mock-up. Ten years ago, all acoustic work was done at Iveco through hardware testing. Now, simulation has cut the number of test cycles in half.

In the next five years, plans are to start trimming the number of test cycles down to two: One set at the beginning of design to adjust the simulation models and another near the end of development to verify the acoustic performance of the final product. This level of reliable acoustic prediction lowers the costs of testing and materials while continuing to improve the acoustic performance of the vehicles.