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35 EUROPEAN AUTOMOTIVE DESIGN February 2004

NOISE, VIBRATION & HARSHNESS

oise pollution is now considered to be a majorthreat to healthy lifestyles and a drain oneconomies. In Europe alone it is estimated to

cost some €10-30 billion a year. Research in Germanyreveals that inner city traffic reaches 81dB(A) in theday, falling to 72dB(A) at night, and this is expected toincrease by a further 5dB(A).

This is despite the fact that some OEMs designtheir vehicles to 71dB(A) pass-by noise (the legalrequirement being 74dB(A)) and that cars are 10times quieter than they were 20 years ago, accordingto Rieter’s head of acoustics, Maurizio Mantovani.

The EU is setting new standards for 2010 that willresult in a 19dB(A) drop in traffic noise, while theWorld Health Organisation has an ambitious 29dB(A)reduction target for traffic noise.

Reducing tyre noise is one major concern.Thereal problem is that no one really understands howthe noise is made. Matthew Harrison*, senior lecturerat Cranfield University’s school of engineering, says:“We don’t really know what the noise sourcemechanism is, but there are two possibilities: airpumping, physically displacing air from the tread asthe contact patch slaps down on the road surface, andthe other is vibration and that must radiate noise. If

NVH [noise, vibration and harshness] needs a leapforward then it’s understanding of the noise sourcecharacteristics of the tyre.

“The EC has been discussing a directive thatwould add another noise test just for tyres.The vehiclewould have to coast by at certain speeds witheverything switched off and the tyres alone wouldhave to meet a specific number.They haven’t decidedto go down that route yet, but have issued documentsshowing they’re discussing it.”

While the tyre-road interface is a major source ofvehicle noise, improving the overall quality of a car’sNVH for the benefit of its occupants also means lessnoise is radiated externally.

The growing use of interior modules has helpedreduce NVH, but this has not been without its ownchallenges. Ove Arup NVH specialist James Hargreavessays:“OEMs need to specify subsystem NVH targetsvery carefully, as there can be potential problems atthis level. Each module might be all right on its own,but problems can arise when they’re assembledtogether. It’s imperative to specify and maintain tightbuild and quality standards.”

One problem is that many suppliers do not havethe in-house expertise to ensure that NVH issues donot arise, says Jason March, Ricardo’s seniormanager—powertrain analysis and NVH. Even so,March believes the devolvement of design,

N

Silence,please!

Noise is problem for engineers, both in terms of addressing pass-by targets and eliminating the irritating

squeaks and rattles that consumers are demanding as cars continue to improve generally. Ian Adcock looks

at some of the issues involved and the solutions being devised to deal with them

*Matthew Harrison is author of “Vehicle

Refinement” published by Butterworth Heinemann

36 EUROPEAN AUTOMOTIVE DESIGN February 2004

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NOISE, VIBRATION & HARSHNESS

development and production through the supplychain helps when addressing NVH.“Overall it’s anadvantage as there’s a broader base of expertiseworking on the problems and everyone knows whattheir responsibilities are and have to deliver.”

However, drastically reducing noise levels meansmore aural intrusions, squeaks and rattles that wereonce buried beneath a blanket of NVH becomeapparent. Colin Peachey, technical manager of LotusEngineering’s NVH group, says:“As build quality andsealing improves, this helps NVH performance.Thisbecomes more apparent as problems are eliminatedand you start to hear NVH you hadn’t experiencedbefore.The nature of what you’re trying to fix alsochanges, so we’re looking at a wider range of issuestoday than in the past because the ancillary noises arebecoming more significant in the customers’experience so that as vehicle quality improves, thecustomer’s expectations also increase.”

There are three broad ranges of NVH: lowfrequency up to 200Hz, mid frequency from 200 to500Hz, and high frequency above 500Hz. It is the midand high frequency noises that are creating thebiggest challenge, says Trelleborg European presidentJohn Badrinas.“The low frequency problems of a fewyears ago have gone.The challenges we are faced withtoday are very high frequency and things that onlyappear after the car is assembled. By reducing interiornoise we can now detect ones that were oncehidden.”

The answer, says Mantovani at Rieter, is robustdesign and better quality of assembly, which is helpingto reduce the squeaks, rattles and infiltrations thatbecome more evident when the overall interior noiseis low.What makes a real difference, though, is to beinvolved early, at the start of the design phase.

This is a point that both Hargreaves and Harrisonstrongly agree with.“In the past NVH was a movingtarget and very often it wasn’t tackled until thevehicle reached prototype stage, by which time it wastoo late to solve the more difficult problems,” saysHargreaves.

“Now at least there are NVH engineers,” saysHarrison,“but the suppliers and the OEMs need to askthemselves what the NVH implications will be forevery step change in the development process.”

While a whole raft of development tools areavailable to the NVH engineer, it is still very difficult topredict precisely what the NVH problems will beahead of that first run on the test track.The wide

spectrum of sounds alone makes NVH difficult tocompute and predict; throw in marginalmanufacturing and assembly variability and thecomplexity of the software and computing powerneeded becomes an enormous challenge.

Despite some very powerful tools available thereare areas of NVH that still cannot be analysed andpredicted with any great degree of confidence.Accuracy is particularly problematic above 300Hz; thenumber of modes within the vehicle is too great andit becomes very hard to predict all the vibration fieldswithin the vehicle because there are so many.Andbecause the wavelengths are small, in mathematicalterms it becomes more difficult.

Statistical energy analysis (SEA) is employed forhigh frequency measuring, but according toHargreaves that still requires a lot of model validationand data feedback, whereas finite element analysis(FEA) is predictive in the true meaning of the word.

Nevertheless progress is being made ondeveloping tools that will assist the NVH engineers intheir search for increasingly refined vehicles and tominimise the use of what Harrison describes as“palliative measures” to control NVH.

Trelleborg technical director Manfred Hoffmansays his company has developed predictive tools up to800Hz for its components and to analyse phenomenasuch as chain whine and combustion noises beyond800Hz.“I don’t share the view that NVH is anafterthought and there’s a lack of developmentsoftware. For the past decade Trelleborg has beenbenchmarking all new cars and making measurementsabout the body, engine and mounts trying tounderstand the whole NVH performance andcorrelating that to basic parameters we use in ourmathematical models.”

Rieter’s Mantovani is confident in many usefulcomputational tools at hand.At Rieter, NVH expertsmanage, thanks to internally developed software likeVector and Gold, the full simulation chain, startingfrom the deep knowledge of acoustic materialsbehaviour, through their modelling withincomponents, up to the simulation of full vehicles bothin the low frequency with FEM-BEM methods and inthe high frequency with SEA and ray-tracing. But hedoes temper his enthusiasm:“The capability of thosetools is constantly being improved.We still need to dowork on the mid frequencies from 200 to 400Hzwhere it’s too high for FEA and too low for SEAmethods.We need an answer to that as it’s very

38 EUROPEAN AUTOMOTIVE DESIGN February 2004

38

important for tyre and road noise.“The trouble is there’s a wide sound bandwidth

that has to be dealt with and an infinite number ofmanufacturing tolerances.This is why SEA works inthe high frequencies and also takes into account thespread of assembly processes.The difference betweenvehicles in the low frequency range is becomingsmaller thanks to better manufacturing.

“The problem with the mid range is that there arevehicle systems that behave in a deterministic waylike beams, spaceframes, crash beams and thencomponents that behave in a statistical way like doorpanels etc.This makes it very difficult to predict.”

If these are just some of the challenges, whatsolutions are pending? Some of the most talked aboutare various ‘active’ systems.Yet despite theirdemonstrable effectiveness this is still top shelftechnology that has yet to be seriously adopted bymanufacturers. Saunders at Lotus admits cost is onesignificant drawback as well as the sheer physical sizeof active engine mounts and the energy needed todrive them, although he remains optimistic that theywill have their place:“We’ve always believed anti-noiseis an ideal solution for a family car that has the leastamount of money to spend on features but has themost need for active noise control.

“Engine noise cancellation is aimed at secondorder firing frequency from 900rpm/30Hz to6000rpm/200Hz.A signal is taken from enginemanagement control that tracks the engine speed andprovides cancellation within the passengercompartment.You do that by adding premiumspeakers if possible, otherwise use the car’s own.

“Road noise is a broadband problem between 40-50Hz through to 250Hz, looking at bands that involvecavity modes within the passenger compartment from80 to 200Hz and internal tyre resonance circa 250Hz.

“We get the reference for the road noise by usingaccelerometers on the wheel hubs and body—up to

12 on the demonstrators but on production cars fourto six would be enough.

“The sound synthesis concept has changedslightly over the years.The focus now is to make achip that has all three systems on board to bring costsdown and allows the OEM to decide whichcombination of technologies it wants to employ.

“We think synthesis could be a cheaper optionthan tuning inlet ducts to get good cabin noise fromthe engine.You can tune it electronically and couldadd sounds in at the end of the production line, andhave sound options depending on your preference ordriving mode from sporty sounding engine note to aquieter one while cruising.”

While others such as Trelleborg are also workingon active engine mounts there seems a consensus thatsuch systems are still a long way in the future, not justfrom a technical standpoint but also an engineeringideology. Hargreaves says:“Maybe it’s to do withquality that a cheaper brand can achieve the samerefinement levels as a more expensive marque butwithout the engineering finesse.”

The other drawback with these technologies isthe fog of patents covering them that has yet to becleared, says Harrison.“Not only that, but they needpower constantly and nowadays manufacturers arelooking at more efficient fuel saving technologies.”However, he still sees ANC as a potential for tuningmore efficient exhaust and inlet systems that mightotherwise radiate excessive noise.

A step towards active engine mounts is theswitchable hydraulic and electric mounts developedby Trelleborg and fitted to top-line diesels such as thenew Audi A8. Hoffman says:“We have concentrated ondeveloping cost-effective solutions. Closed-loopsystems require a lot of sensors. Over the past twoyears we’ve concentrated on open-loop systems thatdon’t need sensors and only take the rpm signal andmake forces typically only in the second order and

Rieter’s

underfloor

module cuts

noise by

1dB(A) as well

as drag by up

to 10 per cent

39

39 EUROPEAN AUTOMOTIVE DESIGN February 2004

NVH

apply a 180deg wave force on the right-hand mountto counteract these forces.You only need an actuatorand simple signals from the ECM to provide a cost-effective solution for diesels.

Trelleborg is also working on alternative materialsto natural rubber for use in conventional enginemounts. Hoffman says:“We are working on compositesprings that would work in ranges 20 per cent higherthan natural rubber’s 400-600Hz range, andmicrocellular polyurethane foams.”

Rieter’s Mantovani also sees opportunities inexploiting structural stiffness in the powertrain, inusing gaskets to induce damping in the structure andin encapsulating the powertrain at weak points toabsorb noise rather than just reflect it back.With theirItalian colleagues, Rieter engineers are also examiningthe potential for composite sumps in trucks, and saythe results in terms of cost and performance lookpromising. Introduction of such a product to carswould have to be carefully evaluated, but Rieterbelieves the high volumes would justify theinvestment. One potential Rieter development is itsunderfloor module which encapsulates the vehicle’sentire underside, not only helping to reduce drag by 8to 10 per cent but, says Mantovani, reducing pass-bynoise by 1dB(A).

Meanwhile Ricardo believes that internalisingengine ancillaries such us water pumps and front-endchain and belt drives will go a significant way toreducing NVH.“We’ve also got a long way to go withengine mapping and injection sequencing, especiallyfor diesel engines,” says Ricardo NVH specialist GarethStrong.“The downside is you have a huge number ofvariables.We’re seeing the emergence of knock sensortype technology in diesel engines to give us a closed-loop capability and that will improve matters further.”

The challenges facing NVH engineers are going toget more rather than less complex as time goes by, butit’s also clear that as a discipline within overall vehicledevelopment it is now recognised, perhaps belatedly,as a key attribute in the development of a successfulproduct range.

Active materials

Intelligent materials and memory alloys could be the keyto even more refined vehicles, says Dr-Ing Thilo Bein ofDarmstadt’s Technical University.

Originally developed for aerospace applications, multi-function materials such as piezo ceramics introduced intoload paths of the basic structure sense the structuralvibration and counter it by setting up opposing forces.

Bein and his colleagues are also working on newmaterials like carbon fibre reinforced plastic with piezoceramics for body-in-white applications or sheet metalswith 0.2mm PCP that is designed to interrupt structuralsound paths.

Currently the automotive part of the programme is“between the laboratory and prototype stage”, says Bien.“The major hurdles are cost and miniaturising thecomponents. We also need about 10W of AC power todrive the systems.”

Bien predicts it will be a decade before the firstcommercial application of these technologies appearseither in high-end cars or commercial vehicles.