Calibrating the ISO 10844 test surfaces used for vehicle and tyre testing

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 ISO10844. The first version was published in 1994, and the most recent version is from 2014. The surface is claimed to produce consistent levels of tyre/road noise emission under various operating conditions and to minimize inter-site variation. Originally, ISO10844 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 5-6 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 dB(A). 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 in each set, the variation between different sets will potentially be less than 0.5 dB(A). Testing ISO10844 surfaces periodically with such sets can then be used to determine a site-specific reference level, to be compared to a global reference level.