When measuring noise emission of road vehicles or tyres, regulations such as ECE R51, R30 and R117 and (indirectly) EU regulations (EC) 661:2009 and 1222:2009, require testing to be made on a reference surface defined in ISO 10844. The surface is claimed to:

• produce consistent levels of tyre/road noise emission under various operating conditions, including those appropriate to vehicle noise testing

• minimize inter-site variation

• provide minor sound absorption

Originally, ISO 10844 was developed with the aim to minimize tyre/road noise, to cause as little influence as possible on vehicle noise during full-throttle acceleration, and it served well for this purpose. However, it was soon applied also to testing in conditions when tyre/road noise is the only or dominating source. Then, it appeared that various sites gave site-to-site variations up to 5-8 dB(A), probably as some users tried to produce as low noise as possible within the tolerances. The 2014 version aimed at reducing the site-to-site variation to half; i.e. about 3-4 dB(A) by applying new or updated measurement methods, allowing tightening of some technical requirements. However, even 3 dB variation is too much, which will seriously limit the efficiency of noise limits and tyre labelling system. This paper proposes a calibration procedure for ISO surfaces, by which one can quantify the differences and correct values to a global reference. This is proposed to be based on the use of the SRTT tyre, defined by ASTM F2493:2014. Several tests have shown this tyre to give reproducible values within approximately 1 dB(A). If a number of such tyres are used; say 8, the variation between different sets will potentially be less than ±0.5 dB(A). Testing ISO 10844 surfaces periodically with such SRTT sets can then be used to determine a site-specific calibration level, to be compared to a global average reference. It is proposed to use the CPX method of ISO 11819-2 and a specific type of CPX trailer, and to observe the requirements in ISO/TS 11819-3 about reference tyres.