'New Technologies and Approaches to NVH' Symposium

Engineering Integrity Society

3 November 2005

Courtyard by Marriott Hotel
Coventry, UK

ABSTRACTS

An integrated STSF/Beamforming solution for efficient broad-band noise source location
Kevin Bernard Ginn & Jørgen Hald, Brüel & Kjær Sound & Vibration Measurement SVM A/S

Near-field Acoustical Holography (NAH) can perform high-resolution sound source location on the source surface, even at low frequencies. But a measurement grid with less than half wavelength spacing is required, and the measurement area must cover the main radiating regions to avoid spatial windowing effects. These requirements make the method impractical at higher frequencies, typically above 3-5 kHz. At those higher frequencies, however, Beamforming (BF) can provide good resolution with typically only 40-90 measurement points. For this, an irregular array is used to perform a directional resolution of the source distribution based on measurements at intermediate measurement distances. Various array designs exist that can provide good suppression of ghost images up to frequencies, where the average element spacing is much larger than half a wavelength. A combination of NAH and BF therefore seems attractive, but it is not practical to have to use two different arrays to perform the two types of measurement. The desired solution is to take two measurements with the same array: One for NAH at a short measurement distance and one for BF at a somewhat longer distance. The present paper describes some array designs that provide good performance with both of these two measurements, and the NAH processing based on irregular array measurements is explained. NAH typically provides calibrated sound intensity maps, while BF provides un-scaled maps. The paper described a method to perform sound intensity scaling of the BF maps based on an assumption of uncorrelated omni-directional point sources. The combined NAH/BF method and the sound intensity scaling are verified through a set of measurements on various loudspeaker configurations.

RT60: Its Use as an Optimiser in Automotive Cabins
A Bennetts, Bay Systems Ltd & R Morris-Kirby, Pelzer

The perceived need by automobile manufacturers to develop a brand DNA that extends well past the styling of vehicles has meant that in the last ten years the acoustic signature of a vehicle has become important. What is the correct acoustic signature is one problem the other, which we address here, is how to achieve that desired signature. Every part of the automobiles cabin will reflect and absorb sound energy and hence will shape the overall sound pressure level and its spectral content. Because the power-train, road and wind noise enter the cabin by multiple transmission paths there is no quick fix to improving the noise signature. The other major complication is that the occupants wish to hear the radio/sound system and converse with one another. The first of these requirements require that the sound system is tuned to overcome the effects of the acoustic trim and this is possible but the second requires that the occupants overcome the effects of the acoustic trim themselves. This is tedious to do if it means extended elevation of voice levels or straining to catch every word. The a possible solution is therefore to optimise the acoustic trim in such a way that speech is lightly affected but power-train, road and wind noise is aggressively attenuated. To effectively achieve this the localised sound fields must be known and appropriate acoustic trim fitted to absorb/reflect/attenuate as appropriate. The use of local and global RT60 as a figure of merit within the cabin has enabled the acoustic trim to be installed cost effectively with the minimum iteration.

10 to 1: The Benefit of Squeak&Rattle Prevention
Hans-Jürgen Damms, ZINS Ziegler-Instruments GmbH

50 to 100 Euro per vehicle is the current warranty cost of customer complaints in automotive industry. For some models and some manufacturers Squeak&Rattles are becoming a nightmare and the largest part of overall warranty cost.

Gluing felts when hardware is ready is not only unprofessional but is also reducing profits dramatically keeping in mind that the cost of gluing a felt having a production volume of 1000 vehicles per day is equal to 500.000,--€uro.

A vehicle typically has around 1000 contact points that could squeak or rattle. More than 40 detail measurements are required in order to make a decision about the risk of the considered contact point. These detail measurements include specific answers about materials, surface structure and coatings or paints of the contact partners, detail measurements about fixing methods, types of relative motion, result of relative motion, tolerance chains as well as environmental influences. It is impossible for most of engineers to keep 40000 detail measurements in mind. Therefore a systematics is necessary in order to make prevention reliable and efficient.

SES is the Systematics for the Elimination of Squeaks&Rattles.

SES computes the risk of each contact point objectively based on the detail measurements shown above. The algorithm behind it is based on more than 5 years of experience of a major German OEM of high class vehicles. SES shows not only the landscape of problems, but also supports in creating solutions in materials, grains, paintings, fixing systems, tolerances and environmental questions. Simulating these solutions in the system creates a range of change requests and activities in order to have S&R free hardware in the production line.

The efficiency of SES was proven to be higher than 90%.

Examples of several successful solutions will be given.

Advances in Non-Contact NVH Measurement: Full Tri-axis Scanning of 3D Vibration by Laser Doppler Vibrometry
Roger Traynor - Lambda Photometrics

Laser Doppler Vibrometry techniques have seen significant advances in recent years, with measurements now being taken quickly and effectively from structures previously considered ‘difficult' or ‘impossible' to measure. Single point, dual differential, rotational and inplane vibrometers are now routine tools for development and test engineers to tackle vibration issues.

By exploiting advances in system configuration, plus computer hardware and processing power, it is now possible to scan structures using laser beams from three separate scanning sensor heads and obtain full tri-axis vibration information. This includes velocity displacement and acceleration at the resonant frequencies, plus full FRF and other modal parameters.

The presentation will include a basic description of the PSV-400-3D tri-axis scanning vibrometer system and method of operation, with colour maps and animated examples from full vehicles, brake assemblies and other structures.

On the modelling approach of NVH issues – the driveline “clonk” case study
S. Theodossiades and H. Rahnejat, Wolfson School of Mechanical & Manufacturing Engineering, Loughborough University, Loughborough, UK

In recent years, NVH issues have become a major priority in the vehicle development cycle, since they are strongly connected with customer satisfaction. Perhaps one of the main concerns is driveline clonk, which is an acute metallic noise, radiated from the drivetrain, under impacting conditions in the various lash zones. A detailed virtual prototype that has been developed by a combination of multibody dynamics, finite element method, contact mechanics and boundary element technique, introduces a holistic approach in NVH modelling and is used to identify the root causes of clonk. The virtual prototype - which has been verified by an experimental rig - is used to examine the response of the drivetrain system under a variety of driving conditions, eliminating the cost of similar exercises either by vehicle testing or in the laboratory. The same approach is now extended to the study of other NVH concerns, such as gear rattle, driveline thump and piston slapping noise.

Comparison of Aeroacoustic Data from MIRA's Full Scale Wind Tunnel and an Equivalent Aeroacoustic Facility
Mark Burnett - MIRA

Interior wind noise predictions based on exterior pressure measurements made in MIRA's Full Scale Wind Tunnel (FSWT) are presented. The predictions are based on the structural excitation and radiation from the vehicle's surfaces. <?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />

The external flow data has been used as the power input to a Statistical Energy Analysis (SEA) model of the relevant parts of the vehicle. The pressure loading on the panels is represented in the model as a wave propagating parallel to the surface of the vehicle at some fraction of the
free-stream velocity. The rationale in using this means of representing the pressure loading is discussed.

The FSWT is shown to give accurate data up to approximately 1kHz in the a-pillar/side glass region despite being a noisy tunnel. The technique assumes that the interior wind noise is purely due to the propagating wave, not flow radiated noise. Tones from aerials, grills etc are not included in the solution and so the FSWT is not suitable for benchmarking. It is, however, highly relevant for the assessment of relative changes in the body's surface geometry such as door mirror and a-pillar investigations.

Wave based modelling of disk brake vibration
Dennis Boennen and Stephen Walsh, Department of Aeronautical and Automotive Engineering, Loughborough University

In this presentation two wave based models of automotive disk brake vibration will be discussed. The first model approximates the brake disk as a simple beam structure with cyclo-symmetric boundary conditions. Analytical results are compared with vibration measurements made upon idealised test structures. Since the beam model is a one-dimensional approach and, therefore, the modelling of the inner boundary conditions of the brake disk is impossible, a second model based upon Kirchhoff’s plate theory is also presented. The mode shapes and natural frequencies of a static brake disk with different inner boundary conditions are calculated and compared to vibration measurements of a brake disk made upon a static test rig. Finally, the advantages and disadvantages of both models will be discussed.

New approaches enabling NVH analysis to lead design in body development
Herman Van der Auweraer, Director Research and Technology Development
Stijn Donders, Rabah Hadjit, Jos Jans, LMS International

With the current tendency to bring more and very innovative body variants to market while shortening the overall time-to-market, body development is on the critical path in the development process. This calls for a CAE-driven approach that makes it possible to evaluate and optimize all critical functional attributes like weight, crashworthiness, durability and NVH early in the process.

This paper discusses new technologies to support this approach. Morphing the FE mesh of a predecessor model enables the analyst to formulate design recommendations in the concept phase. Advanced contribution analysis allows quick detection of weak points. Novel model reduction techniques shorten calculation time, enable more modifications to be evaluated and make real optimization studies feasible. A first approach is based on a modal projection of the modifications and is used to carry out sensitivity analyses and assess small modifications. The second one is based on wave-based substructuring and allows implementing large modifications in a very efficient way.
A complete process, based on the combination of these approaches, is presented and illustrated using industrial examples. They prove to shorten the total development cycle and even eliminate one series of physical prototypes, thus turning the vision of "analysis led design" into reality.

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