A new methodology to assess sound power level of tyre/road noise under laboratory controlled conditions in drum test facilities

Tyre/road interaction is the main source of noise emission caused by road traffic when cruising at speeds over 30 km/h. Several methods such as the Coast-By, the Close-Proximity, the Statistical Pass-By or the Controlled Pass-By have been used over recent decades to measure noise emission. However, since Regulation (EC) No. 1222/2009 on the labelling of tyres was published, only the method described in UNECE Regulation 117 concerning the approval of tyres with regard to rolling sound emissions, can be used in order to obtain tyre/road noise emission approved values. All these conventional methods have several disadvantages such as the lack of repeatibility, the influence of environmental factors or the different results that can be obtained depending on the test track or the vehicle upon which the tests are carried out. A new methodology based on drum tests and the ISO 3744:1994 has been developed in order to avoid these limitations. This paper describes the new method including the positioning of microphones, calculating correction factors, characterising the background noise caused by the drum and obtaining the sound power level of a tyre when rolling against a drum.     

Fundamentals and New Frontiers of Bose–Einstein Condensation

A vehicle skids whenever the friction between tyre and road is insufficient to meet the demands of the driver in accelerating, braking, or cornering. In wet weather, when the water film on the road acts as a lubricant, the friction between tyre and road may be greatly reduced. The magnitude of the friction between a tyre and a wet road depends on the physical characteristics of both the road surface and the tyre.

Recent work has shown that when well-lubricated rubber slides over a hard surface, as in the case of a tyre on a wet road, a large part of the frictional resistance may arise from energy losses in the rubber as it is deformed by projections in the hard surface and then recovers. These are the so-called hysteresis losses. Evidence suggests that if the associated practical problems can be solved very worthwhile improvements in skidding friction may be obtained by the use of tyres in which the rubber of the tread has much higher hysteresis losses than the normal tyre tread rubber.     

Statistical tyre/road noise modeling in Hong Kong on friction course

Based on the close proximity (CPX) method specified in ISO/DIS 11819-2, we recorded and analyzed the instantaneous tyre/road sound pressure levels with 9 road sections that are constructed with the same pavement surfacing materials, that is, friction course. A total of 1320 segments were made in urban areas with a pair of SRTT (Standard Road Test Tyre). We tried to relate the tyre/road noise with the instantaneous acceleration, speed, air temperature, road temperature, road gradient, road surface age to develop a multi-variants model. It was subsequently found that a simple tyre/road noise model linking driving speed and acceleration is the best model. The model provides an easy way to estimate the instantaneous tyre/road noise level. As the tyre/road noise is becoming more dominant component of the road traffic noise, our proposed model has the potential to improve the current practice in estimating the road traffic noise.     

ON THE HORN EFFECT OF A TYRE/ROAD INTERFACE, PART II: ASYMPTOTIC THEORIES

In Part I, it was shown that boundary element method calculations could successfully be applied to determine sound amplification by a tyre/road geometry. However, the computations are expensive, limited to frequencies below 2500 Hz, and provide little physical insight. In Part II, two supplementary asymptotic approaches are developed; a ray theory for high frequencies and a compact body scattering model for low frequencies. When tested on a representative tyre geometry, these methods are found to have excellent predictive capabilities, at frequencies above 3k Hz and below 300 Hz respectively. Furthermore, the ray theory shows that the neglect of curvature in Ronneberger's wedge model (1989 Workshop on Rolling Noise Generation, Institut fur Technische Akustik, Technische Universitat, Berlin) leads to erroneous amplification levels and interference effects, and the scattering model intriguingly predicts that low frequency amplification increases with belt width independently of the tyre diameter. Lastly, this work confirms the importance of numerical calculations for the intermediate frequencies, where tyre noise is most significant.     

Importance of tread inertia and damping on the tyre/road contact stiffness

Predicting tyre/road interaction processes like roughness excitation, stick-slip, stick-snap, wear and traction requires detailed information about the road surface, the tyre dynamics and the local deformation of the tread at the interface. Aspects of inertia and damping when the tread is locally deformed are often neglected in many existing tyre/road interaction models. The objective of this paper is to study how the dynamic features of the tread affect contact forces and contact stiffness during local deformation. This is done by simulating the detailed contact between an elastic layer and a rough road surface using a previously developed numerical time domain contact model. Road roughness on length scales smaller than the discretisation scale is included by the addition of nonlinear contact springs between each pair of contact elements. The dynamic case, with an elastic layer impulse response extending in time, is compared with the case where the corresponding quasi-static response is used. Results highlight the difficulty of estimating a constant contact stiffness as it increases during the indentation process between the elastic layer and the rough road surface. The stiffness–indentation relation additionally depends on how rapidly the contact develops; a faster process gives a stiffer contact. Material properties like loss factor and density also alter the contact development. This work implies that dynamic properties of the local tread deformation may be of importance when simulating contact details during normal tyre/road interaction conditions. There are however indications that the significant effect of damping could approximately be included as an increased stiffness in a quasi-static tread model.     

On the sound radiation of a rolling tyre

The sound radiation from rolling tyres is still not very well understood. Although details such as horn effect or directivity during rolling have been investigated, it is not clear which vibrational modes of the tyre structure are responsible for the radiated sound power. In this work an advanced tyre model based on Wave Guide Finite Elements is used in connection with a contact model validated in previous work. With these tools the tyre vibrations during rolling on an ISO surface are simulated. Starting from the calculated contact forces in time the amplitudes of the modes excited during rolling are determined as function of frequency. A boundary element model also validated in previous work is applied to predict the sound pressure level on a reference surface around a tyre placed on rigid ground as function of the modal composition of the tyre vibrations. Taking into account different modes when calculating the vibrational field as input into the boundary element calculations, it is possible to identify individual modes or groups of modes of special relevance for the radiated sound power. The results show that mainly low-order modes with relative low amplitudes but high radiation efficiency in the frequency range around 1 kHz are responsible for the radiated sound power at these frequencies, while those modes which are most strongly excited in that frequency range during rolling are irrelevant for the radiated sound power. This fact is very essential when focusing on the design of quieter tyres.     

An experimental procedure to obtain sound power level of tyre/road noise under Coast-By conditions

The rolling noise from tyre–pavement interaction represents the greatest sound contribution from a vehicle when cruising at a high speed. To evaluate the sound levels from this source, existing standardized methods that establish different measurement procedures in both the immediate tyre surroundings, for example the Close-Proximity method, as well as at greater distances, as the Coast-By method. A fundamental parameter that can quantify the sound generation of a source is its sound power level. The standardized methods establish procedures to measure the sound pressure level but not the power level of a tyre as a noise source. For this reason, this paper presents a novel methodology based on sound pressure measurements to obtain the sound power level that a vehicle emits in Coast-By conditions, where noise is generated at tyre/road interaction. The paper describes the testing procedure used to obtain the sound power level, and it is accompanied by a mathematical simulation that studies the feasibility of the proposal. Finally, the proposed methodology is further validated through a field study.     

Towards a model for electric vehicle noise emission in the European prediction method CNOSSOS-EU

The CNOSSOS-EU method is recommended in Europe for environmental noise prediction. In regards to road traffic, it includes vehicle noise emission models implicitly referring to internal combustion vehicles. The development of electrically driven vehicles calls for the future consideration of these vehicles in prediction models. On the basis of experimental data, the study reported in this paper proposes a noise emission model for extending CNOSSOS-EU to light electric vehicles. Correction terms to be applied to the propulsion noise component are determined. Investigations on a sample of tyres with good rolling resistance performance, which is a main tyre selection criterion on these vehicles, indicated that no correction is required for the rolling noise component. Differences between the noise emission from conventional vehicles and electric vehicles are discussed for several road surfaces. Owing to the limited vehicle sample as well as transitional statements, this new model for electric vehicles running at constant speed over 20 km/h should be considered as a first step towards the definition of this vehicle technology in CNOSSOS-EU.     

Validation of a complex urban noise model close to a road

The adequacy of a model for the sound level close to a road is investigated by comparing resulting predictions for the sound level over a building façade with measurements. The road model involves the road geometry (the number and positions of traffic lanes), the traffic structure (vehicle flow rates and their average speeds in each lane) and equivalent omnidirectional point sources representing the vehicles. It is found that the assumed road traffic noise source model is adequate only for predicting levels over the higher part of the façade. However the investigation has allowed definition of what improvements are needed in the road source modelling to enable adequate predictions over the whole of the building façade.     

Investigating generation mechanisms of tyre/road noise by speed exponent analysis

The purpose of this study is to clarify the influence of air-pumping related noise sources on typical tyre/road noise. The aim is to increase the understanding of noise generation mechanisms and catalyse the development of existing tyre/road noise simulation tools. The speed dependency of measured and simulated tyre/road noise is analysed and the results show that a large part of the noise can be explained by a high speed exponent traditionally connected with air-pumping mechanisms. Surprisingly, this is also the case for rough road surfaces which are expected to mainly generate noise by tyre vibrations. It is also found that vehicle wind noise may have a strong influence on the pass-by noise and care must be taken when analysing measurement data of quiet tyre/road combinations. Even simulated tyre/road noise shows higher speed exponents than what is anticipated without the inclusion of any type of air-pumping mechanism in the model. It is concluded that it is unfeasible to separate noise created by tyre vibrations from noise created by air-pumping with a speed exponent analysis due to the overlap in the speed exponents connected with the different generation mechanisms.     

ON THE HORN EFFECT OF A TYRE/ROAD INTERFACE, PART I: EXPERIMENT AND COMPUTATION

Near the tyre/road contact area, the road surface and the tyre belt form a horn-like geometry, which provides a significant amplification mechanism for sound sources. Measurements have been carried out on a stationary tyre placed on a plane surface in an otherwise anechoic chamber. Following the reciprocal theorem a microphone was placed in the road surface near the contact patch and a white noise source was used in the far field. The amplification by the horn effect can then be determined as a function of frequency for an array of microphone positions relative to the contact patch and the centre of the tyre. These experimental measurements show that the horn effect is responsible for about 10-20dB increase in noise level. The amplification function shows a distinct interference pattern for higher frequencies and is independent of the longitudinal source position for low frequencies and source positions close to the contact patch. Numerical calculations using the indirect boundary element method have been carried out. These show excellent agreement with the measurements in the frequency regime of the BEM, i.e., up to 2500 Hz. The dependence of the horn effect on primary geometrical parameters such as the effect of the radius of curvature of the shoulders, the load and the width of the tyre has been investigated experimentally and numerically. The broad features of the horn effect are given by the cylindrical geometry of the tyre. The rounded edges of the tyre tend to increase the levels of the minima and shift them to higher frequencies, while slightly decreasing the levels of the maxima. Shape variations due to load can be accounted for by correcting the source distance to the edge of the formed contact patch. The amplification at low frequencies increases with width, the results collapsing onto a single curve as a function of the dimensionless widthω /λ.     

Commercial road vehicle noise

A survey of the characteristics of the noise emitted by commercial vehicles has been made. The most important single parameter determining the noise of a modern diesel-engined vehicle is the engine speed. All of the other parameters such as load, road speed, etc., have only a secondary effect.The sources of noise on the vehicle are reviewed and it is shown how the characteristics of these sources determine the overall noise characteristics of the vehicle. It has been found that a simple model of the vehicle as a number of coincident point sources predicts the overall noise characteristics of the vehicle to ±2 dB(A). It is shown that there are two extremes of behaviour, the rolling noise controlled vehicle and the power unit noise controlled vehicle; the engine is currently the controlling noise source.Tyre noise has been investigated in some detail as comparatively little has been published previously on this source. Empirical relationships between the tyre noise and speed, tyre size and road surface roughness are given. It is concluded that tyre noise is generated by impacting between elements of the tyre tread and elements of the road surface.Modifications have been made to the engine, exhaust, intake and cooling fan of a 9 ton, 6 litre diesel engined truck which have reduced its ISO test noise level from 88 dB(A) to 80 dB(A). However, it is concluded that 80 dB(A) commercial vehicles are not yet feasible for production. In particular insufficient is known about cooling fan design.Finally cab noise has been investigated and it has been found to originate from the same source as the exterior noise, power unit airborne noise. Therefore any modifications to the power unit to reduce exterior noise will have a similar effect on interior noise. This is confirmed by the vehicle modifications mentioned above which reduced the maximum cab noise from 87 dB(A) to 79 dB(A).     

Experimental study of the noise generated by a passenger automobile equipped with studded and regular snow tyres

Studded snow tyres are used to prevent automobiles from slipping on ice-covered roads. In this paper, the noise generated by a passenger automobile equipped with studded snow tyres is studied and compared with the noise generated when regular snow tyres are used. One-third octave band spectrum analysis reveals the existence of two distinct frequency regions, one above, and the other below, 200 Hz. In the higher frequency region, the spectral levels are dependent on the tyre type. In contrast, in the lower frequency region, the spectral levels are independent of tyre type.     

Definition of road roughness parameters for tire vibration noise control

Road roughness plays an important role in the generation of tire vibration noise. However, it is unclear which kinds of road roughness parameters should be controlled to reduce the noise. In this paper, we define the essential road roughness parameters that govern tire tread vibration and provide information on tire/road noise abatement. The detailed effects of road roughness parameters on tire tread vibration are estimated using a tire/road contact model. The results reveal that pavement asperity height itself is not an essential parameter, but asperity height unevenness, asperity radius, and asperity spacing are important for the abatement of tire vibration noise.     

The identification of sound generating mechanisms of tyres

Tyre noise is generated by several mechanisms. With a modern tyre, wall vibration, air pumping and air resonant radiation are all considered to be important. But tyre noise generating mechanisms are still not clear due to the complication of tyre vibration behaviour. Vibrations of the tyre shell are the combination of several different wave types which appear at different frequencies. In a low frequency range, where the tyre behaves like an elastically supported beam, the circular ring model is used to analyze the dispersion relations. Above 300 Hz, which is the transition point from one-dimensional to two-dimensional waveguide properties of the passenger car tyre, a cylindrical shell model is used to analyze flexural waves propagation. Two important features on the wave propagation, wave-guide behaviour and the curvature effect of the tyre wall are analyzed. In consideration of noise radiation from tyre waves, most of the tyre waves observed in this study are inefficient sound radiators since their wavenumbers are larger than the acoustic wave number. As a result, It is observed that one of the most important features in sound radiation of a tyre shell is acoustically excited wave motion of the tyre wall.     

Developing a Hong Kong based speed correction factor by CPX method

A systematic procedure for developing speed correction factor to adjust measured tyre/road noise is proposed in this paper. The draft ISO/CD 11819-2 on the Close-Proximity method to measure tyre/road noise stipulates that speed correction is needed to convert the tyre/road noise level at the sampling speeds to that at the reference speeds. However, the ISO standard does not mention the exact methodology for developing the speed correction factor development, although it recommends local authorities to develop their own. In this study, the tyre/road noise and speed data over 59 runs on a high speed porous asphalt surface is recorded by our CPX trailer in Hong Kong. The instantaneous noise and speed data are then analysed using this proposed procedure and the set of speed correction factor developed is found to be more stable and repeatable.     

A numerical investigation of curve squeal in the case of constant wheel/rail friction

Curve squeal is commonly attributed to self-excited vibrations of the railway wheel, which arise due to a large lateral creepage of the wheel tyre on the top of the rail during curving. The phenomenon involves stick/slip oscillations in the wheel/rail contact and is therefore strongly dependent on the prevailing friction conditions. The mechanism causing the instability is, however, still a subject of controversial discussion. Most authors introduce the negative slope of the friction characteristic as a source of the instability, while others have found that squeal can also occur in the case of constant friction due to the coupling between normal and tangential dynamics. As a contribution to this discussion, a detailed model for high-frequency wheel/rail interaction during curving is presented in this paper and evaluated in the case of constant friction. The interaction model is formulated in the time domain and includes the coupling between normal and tangential directions. Track and wheel are described as linear systems using pre-calculated impulse response functions that are derived from detailed finite element models. The nonlinear, non-steady state contact model is based on an influence function method for the elastic half-space. Real measured wheel and rail profiles are used. Numerical results from the interaction model confirm that stick/slip oscillations occur also in the case of constant friction. The choice of the lateral creepage, the value of the friction coefficient and the lateral contact position on the wheel tread are seen to have a strong influence on the occurrence and amplitude of the stick/slip oscillations. The results from the interaction model are in good qualitative agreement with previously published findings on curve squeal.     

A waveguide finite element aided analysis of the wave field on a stationary tyre, not in contact with the ground

Although tyre/road noise has been a research subject for more than three decades, there is still no consensus in the literature as to which waves on a tyre are mainly responsible for the radiation of sound during rolling. Even the free vibrational behaviour of a stationary (non-rotating) tyre, not in contact with the ground, is still not well understood in the mid- and high-frequency ranges. Thus, gaining an improved understanding of this behaviour is a natural first step towards illuminating the question of which waves on a rolling tyre contribute to sound radiation. This is the topic of the present paper, in which a model based on the waveguide finite element method (WFEM) is used to study free wave propagation, on a stationary tyre, in the range 0-1500 Hz. In the low-frequency region (0-300 Hz), wave propagation is found to be rather straightforward, with two main wave-types present. Both have cross-section modes involving a nearly rigid motion of the belt. For higher frequencies (300-1500 Hz) the behaviour is more complex, including phenomena such as ‘curve veering’ and waves for which the phase speed and group speed have opposite signs. Wave-types identified in this region include (i) waves involving mainly sidewall deformation, (ii) belt bending waves, (iii) a wave with significant extensional deformation of the central belt region and (iv) a wave with a ‘breathing’ cross-section mode. The phase speed corresponding to found waves is computed and their radiation efficiency is discussed, assuming free-field conditions. In a future publication, the tyre model will be used in conjunction with a contact model and a radiation model to investigate the contribution of these waves to radiated sound during rolling.