For traffic noise studies CPX measurements are used to evaluate the noise-reduction of a road surface.CPX measurements as described in ISO/DIS 11819-2 are carried out at microphone positions close to predefined tyres at constant speed. The dominant CPX sound source is the tyre’s rolling noise as a result of tyre/road interaction, which - apart from the acoustic properties of the road surface - is mainly determined by the properties of the tyre.To ensure continuous quality and comparability of CPX measurements, the variation of acoustically relevant properties of the reference tyres must be taken into account to reduce measurement uncertainties. Thus, to ensure comparability, the standard ISO/TS 11819-3 specifies Shore hardness values to be checked at regular intervals and compared with normative values. According to our experience in CPX-measurements, determining the Shore hardness of the tyres is not sufficient to describe the reference tyres’ acoustically relevant properties. So, as an additional parameter the mechanical impedance of different reference tyres was measured and compared to their Shore hardness and CPX levels on different road surfaces.Based on these results, conclusions are drawn about the usability of the tyre impedance to increase the quality and comparability of CPX measurements
