Low-noise road surfaces
In-situ measurement and mathematical modelling
The emission and propagation of road traffic noise is highly dependent on the characteristics of the structure of the surface layer of the road pavement. For example, the propagation of sound above the road surface is influenced by the values of the acoustic impedance of the surface as a function of frequency. From the acoustic impedance, which is a complex quantity, another characteristic can be derived: the sound absorption coefficient, which is a real positive quantity. Apart from frequency, the sound absorption coefficient is mainly a function of the internal porosity of the layer. That's why the knowledge of the sound absorption coefficient has become important in view of the increased development of porous road surfaces. It is important in the intrinsic characterisation of the road surface and test track properties according to the ISO 10844 International Standard, and also in order to arrive at a quantitative ranking of road surfaces as a function of their sound absorption qualities. Another application to which this test method could be useful is the determination of the drainage properties of open graded road surfaces by measuring their sound absorption coefficient. Thus, there is a strong need for a test method aiming to satisfy the need of road planners, road administrators, manufacturers of road pavements and other people concerned with the prediction and control of road traffic noise requiring a knowledge of the sound absorption coefficient of road surfaces. The ISO/TC 43/SC 1/WG 38 working group is developing a set of test methods for measuring, in situ, the sound absorption coefficient of road surfaces as a function of frequency. This is representative of the sound absorption properties of road surfaces as far as they are relevant to the traffic noise levels measured at the road side.
The Group of Applied Acoustics of the University of Bologna carried out a specific
research project for the measurements of the sound absorption coefficient of road
surfaces in situ. The research project exploited the general techniques developed for
testing the acoustic properties of materials using MLS
The method has been selected by WG38 to become the part 1 of the international standard ISO 13472-1 with the title "Acoustics - Procedure for measuring sound absorption properties of road surfaces in situ - Part 1: Extended surface method".
Qualification tests were succesfully performed in close cooperation with other research teams, like: LCPC, Nantes (France); ARPAT, Firenze (Italy); ARPA, Modena (Italy).
Here is an example of results obtained by DIENCA on the LCPC test track. The road surface characteristics are as follows. Lifetime : 8 years. Actual conditions : surface layer in good condition. Composition : porous asphalt 0/10, porosity : 20 %, thickness : 0.04 m. Road surface dryness: partial water infiltration from adiacent zones. Road surface temperature: 5°C.
Sketch of the measurement set up
Graph of the results
The results obtained on the same porous pavement by the LCPC of Nantes, using different test signal and post-processing technique, show an encouraging good reproducibility. In general, the measured values compare well on a wide frequency range with the predictions of a theoretical model of the acoustical behaviour of porous surfaces. For further details see the paper:
GARAI M., BERENGIER M., GUIDORZI P., L'HERMITE PH. (1998)
Procedure for measuring the sound absorption of road surfaces in
Proc. Euro-Noise '98 (invited paper), Munich. (PDF, 35 KB)
- AFNOR S 31-089 (1990) Code d’essai pour la détermination de caractéristiques acoustiques d’écrans installés en champ libre, AFNOR, Paris.
- BERENGIER M. (1995) An in-situ method to evaluate the absorption coefficient of road pavements, paper presented to ICA '95, Trondheim.
- GARAI M. (1993) "Measurement of the sound-absorption coefficient in situ: the reflection method using periodic pseudorandom sequences of maximum-length", Appl. Acoust., 39, 119-39.
- GARAI M., BERENGIER M., GUIDORZI P., L'HERMITE PH. (1998) Procedure for measuring the sound absorption of road surfaces in situ, Proc. Euro-Noise '98 (invited paper), Munich.
- ISO 362:1994, Acoustics - Measurement of noise emitted by accelerating road vehicles - Engineering method.
- ISO 7188:1994, Acoustics - Measurement of noise emitted by passenger cars under conditions representative of urban driving.
- ISO 10844:1994, Acoustics - Specification of test tracks for the purpose of measuring noise emitted by road vehicles.
- ISO 11819:1996, Acoustics - Method for measuring the influence of road surfaces on traffic noise - Part 1: The Statistical Pass-By Method.
- MOMMERTZ E. (1995) "Angle-dependent in-situ measurements of reflection coefficients using a subtraction technique", Appl. Acoust., 46, 251-263.
- WILMS U., HEINZ R. (1991) "In-situ Messung komplexer Reflexionfaktoren von Wandflächen", Acustica, 75, 28-39.