Road profile texture and tire noise

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1 Road profile texture and tire noise Jean-François Hamet Directeur de Recherches, Laboratoire Transports et Environnement, Institut National de Recherche sur les Transports et leur Sécurité, Bron, France. Philippe Klein Ingénieur des Travaux Publics de l'etat, Laboratoire Transports et Environnement, Institut National de Recherche sur les Transports et leur Sécurité, Bron, France. Fabienne Anfosso xxx, Laboratoire Central des Ponts et Chaussées, Nantes, France Denis Duhamel Ingénieur des ponts et chaussées, CERMMO, Ecole Nationale des Ponts et Chaussées, Champs-sur-Marne, France. Ali Fadavi Doctorant, CERMMO, Ecole Nationale des Ponts et Chaussées, Champs-sur-Marne, France. Bernard Béguet Société MICROdB, Lyon, France RÉSUME Le projet PREDIT Texture & Bruit a pour objectifs de définir et de mettre en œuvre des procédures de mesure et de traitement appropriées à l'évaluation de relations entre les caractéristiques de la chaussée (profil de texture, absorption acoustique; rigidité mécanique) et l émission acoustique du pneumatique dans le domaine des basses et moyennes fréquences. La phase préparatoire présentée ici concerne l'élaboration de modèles et d'algorithmes de calcul relatifs aux processus de contact (enveloppement) et de rayonnement (effet dièdre) dans l'interaction pneumatique/ chaussée, l'élaboration de procédures de mesures. ABSTRACT The main goals of the Texture & Noise PREDIT project are to define and implement measurement procedures and data processing appropriate to the evaluation of the physical relations between the road characteristics (texture profile, acoustic absorption, stiffness) and the tyre acoustic emission in the medium and low frequency domain. The first phase presented here concerns the development of numerical and analytical models relating to the tyre road

2 contact (envelopment) and radiation (horn effect) processes and the specifications of measurement procedures. 1 INTRODUCTION Rolling noise is acknowledged today as a major component of traffic noise. It depends both on the tire and the road. Nevertheless, significant noise reduction has been achieved these past decade by working on road characteristics only. The process followed is generally a qualitative one. It aims basically at reducing the two main mechanisms which are recognised to play a part in the tire noise generation: the air pumping associated to compression and expansion of air volumes enclosed between the tire and the road, the tire vibrations induced by the road surface irregularities. The "low noise" pavements, developed today in Europe have mostly a porous structure and a small chip size rolling surface: it is expected that a high porosity reduces the air pumping mechanism while a low granularity prevents the setting of the tire into vibrations. The generated noise can additionally be reduced through acoustic absorption by the road surface. If the phenomena involved en the tire road noise generation can be considered as being practically identified, they are far from being controlled: the range of SPB levels measured on a given road surface technique is often quite large [1]. It becomes necessary to have at one's disposal measurement tolls and interpretation models enabling one to master the acoustical quality of a road pavement and to evaluate whether a further optimisation of a characteristic could reduce the noise, how much reduction would be achieved and in which frequency domain it would occur. The PREDIT "Texture & Noise" project deals with tire road noise in the low frequency domain. The objectives are to define and implement measurement procedures and data processing appropriate to the evaluation of physical relations between the road characteristics (texture profile, acoustic absorption, mechanical stiffness) and the tyre acoustic emission in this frequency domain. The developments of the procedures are based on theoretical models relating to the tire road contact, to the tire acoustic radiation and to the horn effect. The project is sponsored in the frame of the PREDIT program "noise and nuisance. It involves research centres (INRETS, LCPC, ENPC) and industrials (COLAS S.A, Gerland-Routes, MICROdB). 2 THE ROAD CHARACTERISTICS The road characteristics considered in this project are: the road profile texture, the acoustic absorption, the stiffness. 2.1 The road profile texture The setting of the tire into vibrations is linked to the relative motion of the road profile texture around the tire and thus to the spectrum of this road profile. Experimental research carried out in the 70's have shown that at a sped of 80 km/h the low frequency noise is positively correlated to the 80 mm wavelength texture components (an increase of the texture depth in this 80 mm wavelength domain results in a tire noise increase), while the high frequency noise is negatively correlated to the 5 mm wavelength texture component (an increase of the texture depth in this 5 mm wavelength domain results in a tire noise reduction) [2]. Some authors have further

3 proposed formulations which permit to evaluate the rolling noise in db(a) from the road texture spectrum [3, 4, 5]. Noise evaluations, performed on a set of road texture recently measured [6], are compared Figure 1 to measured SPB values estimated Descornet Huschek Tino 80 km/h [db(a)] measured hamet surf2000 <- underest... (Estim.- Meas.) [db(a)]...overestim -> ISO WG33 light vehicles -10 Descornet Huschek Tino km/h [db(a)] hamet surf2000 Figure 1 Noise estimations from measured texture spectra (given in [6]) Left: estimations vs. measured values.- Right: estimation error vs. measured valued It can be seen that if the previsions follow the same trend than the measurements (left graph), they have a tendency to overestimate the low noise pavements or underestimate the noisy ones (right graph) The tire envelopment Attempts made to correlate texture to tire noise on porous pavements, proved to be deceiving. Porous roads are not included in the most recent formulation [5]. One explanation put forward is that the texture of these pavements show pronounced dips whose amplitude has an influence on the texture spectrum level but has no effect on noise over a certain depth. In order for the texture spectrum to be representative of the road profile, actually 'seen' by the tire, one must beforehand perform the envelopment of this profile (cf. Figure 2). An attempt to make a sort of smoothing did not seem to give true satisfaction to its authors [7]. It seems necessary to simulate, as much as possible, the actual contact. Figure 2 : illustration of the envelopment of the texture profile by the tire Two process are considered: a static one in which the tire is modelled by a semi infinite elastic medium (this PREDIT project), a dynamic one in which the tire is modelled by a plate under tension with the tread rubber represented by independent springs (European project SI.R.U.US.). In both modelling, the envelopment is function of one parameter: the modulus of elasticity of the medium or the stiffness of the rubber.

4 Two illustrations of a static envelopment are given Figure 3, one on a bituminous concrete profile, the other on a drainage asphalt profile. The evaluations are made for three values of the modulus of elasticity : 1, 5 and 20 N/m2. 4 Béton bitumineux Altitude en mm E=20 N/mm E=5 N/mm2 Enrobé drainant E=1 N/mm2 4 Profil brut Altitude en mm Distance en mm Figure 3. Illustrations of two road profile envelopment (static method) for three values of the modulus of elasticity The appropriate value for this modulus of elasticity will be determined empirically, based on a large set of road surfaces: it will be the value which will yield the best correlation between the enveloped texture spectrum and the tire noise spectrum for the ensemble. N.B. This physical approach of the problem solves at the same time the problems of drop out values encountered when the dips of the road are too deep to be measured by the system The measurement of the road texture profile The road texture profile can be measured with static apparatus (each measured length is about one meter) or on board systems (continuous measurement). The road texture measurement systems are still research topics. The PIARC is organising a measurement campaign at an international level (acoustic matters will not be included). The situation is somewhat similar concerning the standardisation: the work addresses the texture characterisation as regards to skidding, not to noise. The texture is mainly evaluated by its mean profile depth (MPD). Attempts to relate this parameter to noise turned out to be unsuccessful The road profile spectrum has to be measured and this implies that the sampling be precise and regular. The sampling rate of an on board system can be perturbed by a vehicle speed variation. Decision was thus taken to use a static system for this research. The drawback is that the measured length being limited to about one meter, the spectral analysis cannot be performed below a wavelength of about 0.5 m.

5 2.2 The acoustic absorption of the road surface No particular research is made here on modelling the acoustic absorption of the road surface: the measurement procedures are standard (in laboratory as well as in situ), the models have been validated (see however 3.1 the horn effect). Absorption measurements will systematically be made in parallel with noise measurements. The procedure consists in an analysis (amplitude and frequency) of the reflection of a controlled signal (MLS type or frequency swept sine) radiated by a loudspeaker and impinging normally on the surface. The method is described in [8]. 2.3 The pavement stiffness A third characteristic to consider is the pavement stiffness. Its potential influence on rolling noise was tackled in the 70's but never thoroughly analysed, although it is often presented as an explanation factor: cement concrete pavements are 'expected' to be noisier than bituminous concrete pavements because there are stiffer; the decrease of rolling noise on 'black' pavements, as temperature increases, would be due to a softening of the binder; inversely, the increase of rolling noise on aging bituminous concrete pavements would be explained by the stiffening of the binder. A local stiffness (force/displacement) of the pavement can be introduced in dynamic models. The main problem is to measure this dynamic stiffness. The work to be undertaken in this project will consist in a feasibility study, and in the design and implementation of an appropriate measurement procedure. 3 THE TIRE ROAD NOISE 3.1 The horn effect The horn effect is a fundamental part in the tire radiation. A tire, per se, is a weak sound source. Tire road noise becomes important because a megaphone effect (horn effect) is created between the reflecting surfaces of the tire and the road Modelling the horn effect The modelling of the horn effect is complex du to the geometry to be considered. The analytical works which represent the tire as a cylinder are two dimensional (infinitely large tire). A correction function is however introduced to account for this width effect [9]. The acoustic absorption is introduced only in a simple, schematic way. The research in the frame of this project is undertaken in four steps: 1) 2D (cylinder) and 3D (spheroid) analytical modelling of the horn effect produced by a tire above a perfectly reflecting road surface [10]. This modelling uses a modal decomposition of the acoustic field, taking as a modal basis the outward propagating modal functions : cylindrical modes for the 2D, spherical modes for the 3D. The perfect acoustic reflection condition is satisfied by the introduction of an image reflector (cylinder or sphere). The sound source is defined through a surface velocity distribution; this distribution can be local (point source) or extended (single mode for instance). The acoustic field created by the vibration of the mode 6 of a cylinder at 2000 Hz is illustrated Figure 4.

6 Figure 4 : Representation of the acoustic field created by the vibration mode 6 of a cylinder at 2000 Hz Specific cases (radiation of a point source, radiation of an extended source) are then defined and evaluated to be used for the tuning and the validation of finite element codes. In the following, the horn effect is characterised by its amplification. This is defined by the ratio between the acoustic pressure created by the source within the horn, and the acoustic pressure in a reference condition. Two reference conditions can be considered: the 'standard' condition in which the source is baffled by the road surface (infinite surface), the 'Kropp' condition in which the source is baffled by the tire surface (finite surface). The reference acoustic pressure field, and consequently the amplification effect are not the same in both cases [10]. 2) 2D modelling by a finite boundary element method (BEM). This has been undertaken using the CESAR-LCPC code. Schematically, the method consists in solving the acoustic problem on the contour only. Classical numerical techniques are used for the discrete contour description. The problem amounts to solve a linear system of equations in which the unknowns are the acoustic pressure at each point of the contour. The acoustic pressure can then be calculated at any point in space. This technique enables to consider various forms of geometry: the contact zone between the cylinder and the road can be a point (rigid cylinder) or a line (soft cylinder). A drawback of the finite element method is the large computation time: the size of the system increases proportionally to frequency. The absorption properties of the road can be taken care of by introducing the road acoustic impedance in the boundary conditions. This acoustic impedance is defined using the phenomenological model [11]. Comparisons were made between the numerical results obtained with the CESAR-LCPC code and those of the analytical model for the case of a cylinder on a perfectly reflecting surface. They enabled the tuning of the mesh size and the validation of the algorithm (cf. Figure 5). Acoustic absorption conditions by the surface (corresponding to porous road surfaces) were then introduced to evaluate their influence on the horn effect. The example given Figure 5 shows that an absorbing surface can dramatically reduce the horn effect in the 1 khz region, where the tire noise usually predominates.

7 Atténuation Kropp (db) Ø=0.6 m R m Fréquence (Hz) CESAR-chaussée réfl. CESAR-chaussée absorb. analytique-chaussée réfl. Figure 5 : 2D modelling of the horn effect amplification, evaluated at point R Comparison of the analytical and the numeric methods. Introduction of an acoustic absorption at the road surface. 3) 3D modelling by the SAMRAY code of the ENPC for a perfectly reflecting road surface. Studies will then be made to see how an acoustical absorption can be introduced for this geometry. The 2D tire geometry is a circle of radius 0.3 m. The source is placed at 10 with respect to the vertical axis passing through the centre of the tire. It has a 10 width (arc length). The 3D tire geometry is a disc having the same radius and a 0.1 m width. The source has a 10 width and is placed at the same position than in the 2D case. Moreover, part of the sides (corresponding to the tire side walls) vibrates with the same amplitude. This is expected to be representative of reality. Figure 6 : Horn amplification, Kropp reference. Comparison of 2D and 3D evaluations.

8 The 2D and 3D evaluations are compared Figure 6. The horn effect amplification is given in the frequency bandwidth 0-3 khz. The reference is Kropp's reference. The horn effect in 3D is quite different than in 2D. This is mainly due to the finite size of the tire: the acoustic waves can go round the tire. The amplification is lower at low frequencies, the interference (reduction of the amplification level) at high frequencies is less pronounced in 3D than in the 2D.. 4) The results obtained using the codes will hopefully be later synthesised in the form of correction functions 2D-3D to be used in the analytical model, more appropriate for operational purposes Experimental studies Experimental controls will be made in laboratory and in situ. Laboratory controls will consist in measuring the acoustic pressure amplification created by a cylinder on a surface. This cylinder will be a concrete pipe, 5 m length, 0.6 m radius. The noise will be generated by a point source (compression chamber) placed in the vicinity of the contact zone. Perfectly reflecting and absorbing surfaces will be considered. In situ controls will use an on board method: the tire noise is measured close to one of a vehicle back tire. For the study of the horn effect, a methodical investigation of the acoustic pressure field will be made in the neighbourhood of the tire. For this, a multi microphone system will be used. The geometry consists of two sets of 4 microphones laid on a 1/4 circle geometry (Figure 7). The signals can be taken individually (for the sound field investigation) or averaged, or even configured so as to focus at a particular zone. Antenne arrière Antenne latérale 2 strates Figure 7 illustration of possible configurations with two 1/4 circle geometries. The knowledge of the acoustic field in the neighbourhood of the tire will enable to define microphone positions adapted to the measurement of noise levels for this texture - noise relation research (cf. below). 3.2 The measurements of noise levels The tire road noise is function of some tire characteristics (treadband width, tread pattern, rubber stiffness, etc.). It is thus not a priori possible to use a unique tire configuration to experimentally characterise a pavement as regards tire road noise, and expect to be representative of what would be measured under real traffic. This is taken into account in the prescriptions of the standard procedures, whether published or under study. The SPB method considers a statistical evaluation of pass by noise levels measured under real traffic [12]. It distinguishes various vehicle categories (light vehicles, heavy vehicles, medium trucks, ). This method is not applicable on test tracks (there is no traffic on these tracks). The

9 tire configurations cannot be controlled, not even known. This method is not adapted to the experimental constrains of this research. Two experimental procedures will be implemented: pass by measurements, on board measurements The controlled pass by method. The pass by measurements will be made according to the CPB standard procedure [13], but for this research, one vehicle only will be used, with two different tire configurations. Eight runs will be made for each of these tire configurations. Each run will be characterised by its maximum pass by level and the vehicle speed (which is measured with a cinemometre). The speeds, different from one run to the other, will be higher than 50 km/h so that tire noise will predominate. A linear regression, performed on the results will enable the evaluation of the tire road noise at 90 km/h. This procedure is close to the one which was used by [2] for establishing the texture noise relationship obtained by both methods. It will be used as a reference for the development of the on board method. The compatibility of both methods will be studied The on board method A major drawback of the pass by methods is that they can only characterise that 10 or 20 m portion length of the road facing the microphone. The on board methods aim at giving a solution to this problem. The CPX project standard [14] is presently in preparation. Two types of vehicles may be used (trailer or self powered vehicle), four tires are to be used at this moment, the noise levels are measured at two microphone positions close to the tire. One of the objectives of this project is to use the knowledge acquired on the acoustic field in the neighbourhood of the tire, to help define an on board procedure whose results will be compatible with those given by the pass by methods, taking into account as much as possible the ISO recommendations. The tire used for the on board measurements will be the same than the ones used for the controlled pass by method. The comparison between the two methods will this be made for each tire. 4 CONCLUSION The Texture & Noise PREDIT project aims at specifying and implementing measurement procedures and data processing appropriate to the evaluation of the physical relations between the road characteristics (texture profile, acoustic absorption, stiffness) and the tyre acoustic emission in the medium and low frequency domain. The project is sponsored in two phases. The first one which covers the period is a preparatory one. It deals with the development of models and computation algorithms relating to the contact (envelopment) and radiation (horn effect) processes involved in the tire road interaction and with the specification of measurement procedures. The second phase will deal with experimental campaigns on a set of road surfaces, hopefully large enough, and on the evaluation of the relation between the road profile texture and the tire noise, taking into account the acoustic absorption and the stiffness of the road surface.

10 BIBLIOGRAPHIE 1. Dulau B., Doisy S., Haettel J., "Mesures au passage du bruit de contact pneumatique chaussée : méthodologie, application à l'évaluation des performances acoustiques des revêtements routiers", Bulletin des Laboratoires des Ponts et Chaussées 224, Sandberg U., Descornet G., "Road surface influence on tire/road noise", Internoise, Miami, Huschek S., "Characterisation of pavement surface texture and its influence on tyre/road noise", 3rd International Symposium on Pavement Surface Characteristics, Christchurch, Descornet G., et al., Traffic noise and road surfaces : State-of-the-Art, SIRUUS.RPT.TE Domenichini L., et al., "Relationship between road surface characteristics and noise emission", 1 International Colloquium on Vehicle Tyre Road Interaction, Rome, Stevens H., et al., International validation test for the "close proximity method" (CPX),,TÜV Automotive GmbH, M+P Raadgevende ingenieurs bv, von Meier A., van Blokland G.J., Descornet G., "The influence of texture and sound absorption on the noise of porous road surfaces.", Second International Symposium on Road Surface Characteristics, Berlin, ISO/CD : "Acoustics - Procedure for measuring sound absorption properties of road surfaces in situ - Part 1: extended surface method" Kropp W., "Ein Model zur Beschreibung des Rollgeräusches eines unprofilierten Gürtelreifens auf rauher Strassenoberfläche",PhD, T. U. Berlin Klein P., Effet dièdre : étude du modèle de Kropp, MMA 9807,INRETS, Hamet J.-F., Bérengier M., "Acoustic characteristics of porous pavements: a new phenomenological model", Internoise, Leuven, ISO : "Acoustics - Method for measuring the influence of road surfaces on traffic noise - Part 1 : the Statistical Pass-By method"" AFNOR NS : "Caractérisation in situ des qualités acoustiques des revêtements de chaussées. Mesurages acoustiques au passage - Procédure "Véhicules Maîtrisés"." ISO CD : "Acoustics - Method for measuring the influence of road surfaces on traffic noise - Part 2 : the close-proximity method""