Improving the CPX Method by Specifying Reference Tyres and Including Corrections for Rubber Hardness and Temperature

Recently completed work in the EU project ROSANNE and in working group ISO/TC43/SC1/WG33 has allowed substantial improvements to measurements made by the Close-Proximity (CPX) method, intended for measurement of noise properties of road surfaces. The paper summarizes the experimental work and describes the conclusions and implementation in ISO/FDIS 11819-2 (CPX method) and in the supporting ISO (draft) Technical Specifications 11819-3 about Reference Tyres and 13471-1 about Temperature Corrections. In the Reference Tyre Specification, two reference tyres are defined, of which one is the 16" SRTT specified in ASTM 2493-14 and the other is a commercial light truck tyre. Both tyres have been tested with respect to correlations with SPB (pass-by) measurements and found to represent car and truck tyres, respectively, in this particular application. To account for reasonable ageing and minor production differences, a correction for rubber hardness is specified, in order to normalize measurement results to tyres having reference hardness. The Temperature Correction Specification includes temperature coefficients that can be used for normalizing noise levels to an air reference temperature of 20 degrees C. The coefficients relate to driving speed by a simple equation which is different for three categories of road surfaces, which are specified in the document.