Hans G. Jonasson Measurement and Modelling of Noise Emission of Road Vehicles for Use in Prediction Models Nordtest Project 1452-99 KFB Project 1998-0659/1 997-0223 -.. SP Swedish National Testing and Research Institute SP Acoustics SP REPORT 1999:35 .— 2 Abstract The road vehicle as sound source has been studied within a wide frequency range. Well defined measurements have been carried out on moving and stationary vehicles. Measurement results have been checked against theoretical simulations. A Nordtest measurement method to obtain input data for prediction methods has been proposed and tested in four different countries. The effective sound source of a car has its centre close to the nearest wheels. For trucks this centre ,seems to be closer to the centre of the car. The vehicle as sound source is directional both in the vertical and the horizontal plane. The difference between SEL and LpFn,aduring a pass-by varies with frequency. At low frequencies interference effects between correlated sources may be the problem. At high frequencies the directivity of tyre/road noise affects the result. The time when LPF.,Uis obtained varies with frequency. Thus traditional maximum measurements are not suitable for frequency band applications. The measurements support the fact that the tyreh-oad noise source is very low. Measurements on a stationary vehicle indicate that the engine source is also very low. Engine noise is screened by the body of the car. The ground attenuation, also at short distances, will be significant whenever we use low microphone positions and have some “soft” ground in between. Unless all measurements are restricted to propagation over “hard” surfaces only it is necessary to use rather high microphone positions. The Nordtest method proposed will yield a reproducibility standard deviation of 1-3 dB . depending on frequency. High frequencies are more accurate. In order to get accurate results at low frequencies large numbers of vehicles are required. To determine the sound power level from pass-by measurement requires a proper source and propagation model. As these models may change it is recommended to measure and report both SEL and LpFnlanormalized to a specified distance. Key words: Noise, emission, road vehicles, measurement method, source modelling Swedish National Testing and SP Research Institute SP Rapport 1999:35 SP Report 1999:35 ISBN 91-7848-794-3 ISSN 0284-5172 Bor5s 2000 Postal address: BOX 857, SE-501 15 BO~S, Sweden Telephone +46 33165000 Telefax +4633 135502 e-mail: info @sp.se http:llwww.sp.se DISCLAIMER Portions of this document may be illegible in electronic image products. Images are “ produced from the best available original document. Contents Abstract 2 ,,” Contents 3 Preface 5 Conclusions 6 1 Introduction 7 1.1 Aim and background 7 1.2 List of symbols 7 1.3 Some basic theory 7 2 Some preliminary considerations 10 2.1 General 10 2.2 Source height 10 2.3 Ground interference and instantaneous sound pressure levels 10 2.4 Interference effects and SEL 13 2.5 Meteorology 14 2.6 Discussion 14 3 Some pass-by measurements on single vehicles 15 3.1 General description of the measurements 15 3.2 Vertical directivity and interference effects 17 3.3 Distance dependence 20 3.3.1 Sound exposure level 20 3.3.2 Maximum sound pressure level 22 3.4 Engine noise versus tyre/road noise 24 3.5 Integration time 26 3.6 Time history 26 3.6.1 Car 26 3.6.2 Truck 28 3.7 Ground attenuation 29 3.7.1 Sound exposure level 29 3.7.2 Maximum sound pressure level 33 3.8 Aerodynamic noise 36 4 Some measurements with parabola 38 5 Some further measurements 41 5.1 Measurement site 41 5.2 High exhaust 41 5.3 Screening of engine noise 42 5.4 Measurements with a barrier 43 5.5 More examples at another test site 45 6 Measurements on stationary vehicles 48 6.1 Description of measurements 48 6.2 Analysis of the results 48 7 Determination of SEL, LpFmaxand Lw 51 51 7.1 Difference between SEL and LpF.a . ..-., . __Q...L2.2. .~. —.. .- ?.. 4 7.2 Calculation of Lw and LpFmx 52 8 Discussion and conclusions 55 9 Comparison measurements using Nordtest method 56 9.1 Introduction 56 9.2 Results 57 10 References 61 Annex Proposal for Nordtest method 63 5 Preface The work accounted for in this report have been financed by 3 different projects: Nordtest project 1452-99 Measurement of noise emission of road vehicles has financed the comparison measurements and the elaboration of the Nordtest method, Swedish Transport & Communications Research Board (KFB) project 1998-0659 New Nordic prediction method for road trafic noise - Acoustic source modelling of road vehicles and KFB project 1997-0223A new test method for the noise emission of cars .,, have financed the other measurements and the theoretical work. In addition everything ,.- .- has been discussed and planned within the frame of the current Nordic project Nerd 2000 !’ aiming at making a new generation of prediction methods for environmental noise. The following people have been actively involved in the projects: Steind6r Gudmundsson, Icelandic Building Research Institute Jorgen Kragh, Birger Plovsing, Delta Acoustics & Vibration, Denmark Svein Storeheier and Truls Berge, SINTEF, Norway Juhani Parrnanen, Technical research Centre of Finland Hans Jonasson, Tomas Strom, Geir Andresen and Xuetao Zhang, SP Swedish National Testing and Research Institute. Volvo Truck Corporation supplied a truck with driver for some of the tests. The help of the above individuals and organizations are gratefully acknowledged. Most of the work was earned out in 1999 but the report was not finalized until December 2000. Bor%, December 2000 Hans Jonasson . — —— ,. e-r, . .,. - ..- 6 Conclusions The effective sound source of a car has its centre close to the nearest wheels. For trucks this centre seems to be close to the centre of the car. The vehicle as sound source is directional in the vertical plane. Between 100 and 800 Hz there seems to be some decrease of sound at all positions above the bottom of the car body. This is probably due to screening of the engine. At high frequencies there seems to bean increased directivity upwards. Both effects seem to be less than about 2 dB for distances and heights practical to use for emission measurements. The vehicle is also directional in the horizontal plane. The difference between SEL and LPF.Uvaries with frequency. The time histories of pass-bys verify such a frequency dependence. At low frequencies interference effects between correlated sources maybe the problem. At high frequencies the directivity of tyre/road noise affects the result. The time when LPF.U is obtained varies with frequency. Thus traditional maximum measurements are not suitable for frequency band applications. The measurements support the fact that the tyre/road noise source is very low. Measurements on a stationary vehicle indicates that the engine source is also very low. It seems that a source model using three different point sources yields reasonably good results. For passenger cars the three sources can be used throughout the frequency range. For trucks, however, the lowest source should only be included above 2000 Hz. At high frequencies there are large statistical variations. The significant frequency dependence of the difference between SEL and LPF~mmakes it difficult to measure only one of the quantities. Nor is it easy to calculate one quantity from the other. In order to be able to do so we need an accurate source model. Such a model is also required to calculate the sound power level. The ground attenuation, also at short distances, will be significant whenever we use low microphone positions and have some “soft” ground in between. Unless all measurements are restricted to propagation over “hard” surfaces only it is necessary to use rather high microphone positions. To determine the sound power level from pass-by measurement requires a proper source and propagation model. As these models may change it is recommended to measure and report both SEL and LPFn,mnormalized to a specified distance. The Nordtest method proposed will yield a reproducibility standard deviation of 1-3 dB depending on frequency. High frequencies are more accurate. In order to get accurate results at low frequencies large numbers of vehicles are required. 7 1 Introduction 1.1 Aim and background . ,.- The aim of Nordtest project 1452-99 Measurement of noise emission of road vehicles is to define a measurement method suitable to use to obtain input data for road vehicles in prediction methods. The aim of Swedish Transport& Communications Research Board (KFB) project 1998- 0659 New Nordic prediction method for road trafic noise - Acoustic source modelling of road vehicles is to describe the road vehicle as one or more point sources which may either be omnidirectional or have a specified directivity. In combination with point source sound propagation theory traffic noise can the be calculated accurately. KFB project 1997-0223A new test method for the noise emission of cars aims at analyzing problems with the current noise emission method ISO 362, [3], in relation to practical trafllc noise conditions. The results of this project reflects the fact that the original budget was cut by 50%. The first two projects are essential for the Iongterm Nerd 2000 project which aims at new prediction methods for environments noise, including road traffic noise. 1.2 List of symbols a, d= the shortest distance to source (m); C(v) = calculated difference between sound exposure level and sound power level h,, height above ground of receiver; h$,height above ground of source; LE, sound exposure level (dB); LPF~u,maximum sound pressure level with time weighting F, Lw, sound power level, in dB; n, number of sources; p, sound pressure (Pa); t,time (s); P, sound power (W); v, speed (m/s); ALi = the increase in sound pressure level due to the presence of a sound reflecting ground surface (dB); cz open angle (radians); Z time constant (s); 1.3 Some basic theory Assume that each vehicle has n different omnidirectional sources, each emitting a different sound power Pi.