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Getting the buzz on quieter vehicles

   
 
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As automotive manufacturers compete to produce ever-quieter vehicles, an improved way of analysing structural noise promises an important breakthrough in reducing the time of prototype refinement. In the DIANA project, a specialist company focused on noise and vibration analysis and worked with a technical school, a motor industry research centre and several vehicle manufacturers to transform a textbook procedure into a practical engineering tool.
The result is so successful that it is rapidly becoming a standard procedure during the vehicle refinement process. The software developed by the project partners has matured into an established commercial product and has been the foundation for further technologies that address the complex world of vibroacoustic modelling.

When this project started in 1992, automotive engineers had no practical way of modelling structure-borne noise inside a vehicle. Five years later, 'transfer path analysis' has become a standard tool used by many of Europe's vehicle manufacturers. This new method quickly pinpoints the locations on the vehicle where an optimal design change should be made. This not only helps engineers build quieter cars - it reduces by several weeks the time it takes to refine the prototype.
The engine, driveline and suspension system all generate vibrations that are transmitted through the structure of a vehicle. These vibrations eventually excite the large body panels, such as the floor pan, which then radiate noise into the interior. LMS International, a Belgian company specialising in noise and vibration analysis, knew that it is not difficult to set up a mathematical model to describe the experimental data. To be of use, however, the measurement procedure needed further refinement to make it applicable to an industrial environment, and some of the mathematics needed to be strengthened.
Realising the potential value of a practical way to analyse structure-borne noise, LMS initiated the DIANA project to overcome these difficulties. As partners the company invited the Fachhochschule Bielefeld in Germany, the Motor Industry Research Association (MIRA) in the UK, and vehicle manufacturers including Renault and Fiat. Ford Germany sponsored the project by providing test vehicles.

How vibration is transmitted

A typical vehicle has a relatively small number of connection points through which vibrations from the sources are transmitted to the body structure. Most of these connections are mounts: rubber components designed to isolate vibration. Some of these mounts are relatively soft, others, such as the suspension bushings, can be quite stiff in comparison to the body stiffness. Transfer path analysis is used to answer the following questions:
Which inputs contribute to the problem of a particular noise at a specific speed or frequency?
Does a particular contribution appear because of high injected force levels, or because it is being transmitted too efficiently?
How do the above questions relate to the overall design of the vehicle? (Factors include body stiffness at connection points, mount stiffness, source and body resonance.)

An existing theoretical background called transfer path analysis (TPA), provided the partners with a mathematical framework. TPA describes the total interior noise as a vector sum of individual contributions from a given set of force inputs entering the body over a known set of bridges - typically the engine, exhaust and suspension mounts. The method requires two pieces of information: knowledge of the operating forces at the body side of the mount, and a measurement of the vibro-acoustic transfer functions between that point and the target receiver. A ranking of the transfer paths then becomes possible.

Transfer functions and forces

To measure the vibro-acoustic transfer functions the vehicle is set up in the test lab. Known forces are injected into the structure at each attachment point in turn using either a shaker or impulse hammer. The acoustic response is then measured by a microphone placed near the driver's ear. It is then quite straightforward to calculate the transfer function for each attachment point. An alternative transfer function measurement procedure was also developed. This used the principal of reciprocity, whereby a sound source is placed inside the cabin instead of a microphone and all attachment points are set up in parallel with vibration pick-ups. In this way, all the transfer functions are measured simultaneously.
Operating forces are more difficult to measure because force transducers would have to be physically built into the vehicle's structure. This is not feasible because force transducers are very bulky and cannot be placed in the confined areas around the mounts. This was the limiting factor for the technique until the DIANA project solved the problem.
One approach is to measure the vibration amplitude of the mount, which can be done in a moving vehicle using small acceleration sensors. It is then possible to calculate the forces that were needed to cause the acceleration using a knowledge of the mount stiffness. There was a problem that had to be overcome - mounts are very non-linear devices which meant that special techniques had to be developed to enable the stiffness value to be measured in the laboratory.
The Fachhochschule Bielefeld and MIRA developed the laboratory equipment necessary to measure the dynamic stiffness of rubber mounts. They investigated the influence of different parameters, such as temperature and static preload on the stiffness values. Both parameters are important as they vary to a large extent, depending on the driving conditions.
The Bielefeld test rig allows mounts to be tested in many ways not previously possible in the laboratory, such as with multi-axial preloads and measurements of rotational stiffness.
To measure the operation vibration and noise data under different engine and road speeds, a vehicle is first fitted with accelerometers attached to each mount. Data are then measured during a drive on a test track, or in the laboratory using, a chassis dynamometer. Combining the vibration data with the laboratory measurements of mount stiffness gives the forces acting on each mount.
The partners also developed another approach to force determination which could avoid having to know the mount characteristics. Instead, forces can be back-calculated from measured operating vibration data and local body dynamic stiffness at each input connection. Local body stiffness is quite easily measured in the laboratory. In practice the researchers found that both methods of force measurement were important to the success of the project.

Refinements and adaptations

Vibrations originating in the engine and transmission system are all coherent, that is, they are all related to each other. On the other hand, vibrations caused by the contact between the wheels and the road transmitted through the suspensions are random. This makes the analysis of road noise transfer paths more difficult.
The basic TPA approach is not directly applicable in this situation, so the team adapted a supplementary technique called 'Principal Component Analysis' (PCA) to enable road noise analysis. PCA breaks down non-coherent vibrations into coherent sets of vibrations and assigns the latter to 'virtual' sources that can be treated in a similar way to coherent sources.
Companies can apply both TPA and PCA at several different levels. The simplest is to treat large components, such as the engine, as self-contained units. An engineer planning to re-use an existing engine design in a new vehicle can measure the engine's vibration characteristics in an existing vehicle and assume that it will behave identically in its new environment. Combined with calculated transfer functions for the new body, the resulting noise level can then be predicted. Similarly, an alternator supplier can model and predict the vibration levels that are transmitted to and from the engine for different alternator configurations. A design optimisation cycle can then be applied.

Everyday applications

The success of the DIANA project lay in its adaptation of a well-known textbook mathematical analysis to the rigorous demands of an industrial application; solving the technical details of force identification and incoherent excitation; solving the practical issues of stationary and mobile measurements; and making sure that the massive amounts of data that would be generated during the testing phase could be easily handled, visualised and interpreted.
LMS has used the knowledge gained during the project to develop and launch commercial software packages for TPA and PCA. Interest in both the software and in LMS's consulting activities in this area has been high, with orders from many European car manufacturers. The original consortium members are now using the techniques to help them to develop better and quieter vehicles.

 

Project Title:  
Development and integration of an advanced unified approach to structure borne noise analysis

Programmes:
Industrial and Materials Technologies (BRITE-EURAM/CRAFT/SMT)

Contract Reference: BE-4436

Cordis DatabaseFor more information on this project,
go to the CORDIS Database Record

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