Work Package 1.1 : Road sources
Objectives
The description of the source in most of the models used today is implicitely semi-empirical: assuming a certain position and directivity of the source, and taking into account the calculated propagation attenuation from the source to the microphone position, the source strength is usually derived from the microphone signal at close distance from the source. The average source strength is then defined as the average result of a statistically sufficient number of measurements. This procedure would lead to the prediction model producing the exact correct result at the microphone position, but deviations may occur at other positions. From a scientific point of view more insight is required in the exact position, the possible mutual coherence and the directivity of the sources involved. This includes a more realistic and theoretically correct description of the sound generation and radiation of the source. As a consequence, the quantity describing the strength of the source would preferrably be physically correct and well defined, viz. the sound power level in dB re 1 pW combined with a 3D (or if sufficient 2D) directivity pattern. When such a quantity can be used as an input, the propagation model becomes independent of the source under concern, which is a sensible requirement.
The noise emission of single vehicles shall be determined in such a way that the data can be used to make accurate predictions of road traffic noise under different road and traffic conditions. To make this possible the road vehicle shall be described as a number of point sources each with a certain one third-octave band sound power level and, if necessary, directivity both in the horizontal and the vertical plane. The location of these point sources shall be determined. Each type of source shall preferably be dealt with separately. The strengths of the sources shall be determined as a function of relevant parameters. Accurate results shall be obtained when combining the source model with point source sound propagation theory.
Task 1 : preparations
This subtask shall do the following:
- define the result required from this work package
- make an overview of current know-how and identify gaps of knowledge
- define measurement and categorization methods to use
- make measurement programmes for subtask 3
Before any measurements are carried out it is necessary to specify test methods, models to describe different vehicle types (passenger cars, vans, lorries with and without trailers, etc) and operating conditions (speed, gear, acceleration, road surface, gradients, etc). This task requires a great deal of coordination and teamwork as it is necessary to collect all experience and to agree on common methods.
Current standards for noise emission measurements on vehicles are not adapted to the needs of prediction methods. They have not considered the consequences of frequency band calculations and long vehicles such as lorries with trailers. It is no longer sufficient to determine maximum sound pressure levels and some standards are too tolerant to excess ground attenuation. We need new standard methods to determine the sound power level (or the sound energy level during passby) and the directivity of the major sources. If these methods are complicated there is a need for a simplified method to determine the combined sound power level of all sources to be used to collect statistically representative data.
The number of parameters important to noise generation is enormous. Available data should be used as much as possible and the experience of all partners has to be used to decide which new data to collect within the project. Because of all the parameters involved it is also essential to identify the most important ones and to describe an efficient model on how to handle the data. Unfortunately there will be very little data available due to limitations in the methods used for the data collection and simply the fact that the data is too old to reflect modern vehicles. Other typical shortcomings of current data are lack of one third octave band data, lack of control of the excess sound attenuation due to the finite acoustic impedance of the road and ground surface and missing information on the temperature of the road surface.. Uniform methods to define vehicles, engines, tyres and road surfaces have to be specified. Some reference cases have to defined, e.g. a reference road surface.
Task 2 : Physical modelling of the sources
This subtask will deal with the location and directivity of the different sources and, to some extent, with the relative strength of the subsources. The final model shall make it possible to determine both maximum and SEL-levels using the same description of the subsources. When combining the source model with point source sound propagation theory it shall be possible to determine ground and screen attenuation accurately for all frequency bands. All major sources, such as tyre/road and power train, shall, if possible, be dealt with separately. Each major source will be described as a subset of one or several point sources at different heights above the road surface.
Task 3 : Measurements and other collection of data
This subtask will deal with the collection of old data, if any, and measurement of new data. Most measurements will take place on real traffic including both tyre/road emission and power train emission. some special measurement series will focuse on each source separately. The measurement programme shall be the one delivered by WP 1.1.1. The main aim of this subtask is to collect data to make it possible to describe the source correctly and not to collect large scale statistical data. Such data will to a large extent have to be collected later.
As to tyre/road emission important parameters will be tyre type, mounting (single or double), road surface, speed and load(static and acceleration). Most important alternative road surfaces shall be included either as a level correction or in some other way. An objective way to describe road surfaces with respect to acoustic properties shall be given. Wet roads and studded tyres shall be considered as far as possible within the budget and time constraints.
As to power train emission important parameters will be engine/fuel type, rpm, speed and load. Both light and heavy vehicles shall be dealt with. Urban driving is included in this subtask.
Task 4 : Validation and feed back
Measurements and calculations in such a way that the validity of the source model and the source strength data is shown. Additional validations are carried out in task .2. At the end of the project, methods developed in the other work packages should be reviewed in view of the experiences gained during the latter part of the project.
References
[1] Hans G. Jonasson, Measurement and Modelling of Noise Emission of Road vehicles for Use in Prediction Methods, SP REPORT 1999:35.
[2] ISO 362:98 Measurement of noise emitted by accelerating road vehicles. Engineering method.
[3] ISO 10844:94 Specification of test tracks for the purpose of measuring noise emitted by road vehicles.
[4] ISO 11819-1:97 Acoustics method for measuring the influence of road surfaces on traffic noise. Part 1: Statistical pass-by method. Part 2: The close-proximity method.
[5] ISO/CD 13325:1997 Tyres – Coast-by test method for measuring tyre road sound emission.
[6] ISO/DIS 13472-1:99 Acoustics – Procedure for measuring sound absorption properties of road surfaces in situ. Part 1: Extended surface method.
[7] ISO 13473:97 Characterisation of pavement texture utilising surface profiles. Part 1 : Determination of mean profile depth. Part 2: Terminology related to pavement texture profile analysis (DIS 99). Part 3 : Specifications and classification of profilometers (DIS 99).
[8] ISO 7188:94 Measurement of noise emitted by passenger cars under conditions representative of urban driving.
[9] Hans G. Jonasson, Source modelling in the new Nordic prediction methods for environmental noise, Internoise, Nice, August 2000.