Noise pollution caused by vehicle tires has been an enduring problem for decades. Because all production car tires are inevitably a compromise to meet widely differing ambient conditions (temperature span, extreme weather variations, rough surfaces, model-specific ride and handling requirements), tackling noise has always been a tough call.
But improvements are in prospect. Tire testing in Europe is set for changes next year as legislation continues to demand reduced noise levels. New surface specifications will come into effect to make significant changes to requirements.
Darren Carter, Principal Engineer, Vehicle Measurement Group at Millbrook Proving Ground in the U.K., says: “The new noise specification ISO 10844:2011 is the international standard governing the specification of test tracks for measuring noise emitted by road vehicles and their tires. In 2014, these new surface specifications will be mandated into legislative type approval requirements for ECE Regulation 51; all test facilities must comply with this new standard.”
Millbrook’s tire test facility has recently been upgraded to meet ISO 10844:2011 requirements and is described as currently the only commercially available facility in the U.K. to provide access to this new noise surface.
“ISO 10844 is the mandatory specification for test tracks used for evaluating pass-by noise to European Type Approval standards, “ explains Carter. “The objective of this standard is to produce consistent levels of reproducibility between different test tracks, ensuring noise transmission is not significantly affected by the surface and to minimize the variability of tire-to-surface noise generation. This new specification aims to more accurately achieve the objective.”
Legislation on pass-by noise levels means that future noise testing will be equally driven by the need to understand both the contribution to overall noise levels from tire noise and from powertrain derived noise, he explains. “The changes in the ISO surface specification mean that Millbrook’s engineers can more easily isolate tire-generated noise from the overall pass-by noise of the test vehicles and help manufacturers to understand the various areas that are contributing to the total noise of their vehicles.”
Most significant amendments from the previous (1994) version to the 2011 standard incorporate three categories: composition of the test track surface, geometry, and surface properties.
“Composition of the surface relates to specifications that span surface texture, smoothness, and noise. The new ISO specification mandates very stringent requirements for the quality and composition of the top surface of the site, whilst allowing a degree of flexibility in the construction of the track’s substrate,” states Carter.
Geometry relates to size and shape of the site, proximity of large reflecting objects, gradient and cross-fall (longitudinal and transverse slope), and surface dips and bumps (irregularities). The physical changes to the effective length of Millbrook’s noise site will enable engineers to extend testing capabilities to help manufacturers conduct the newly introduced “off-cycle” higher speed testing requirements, says Carter.
Surface Properties have been updated in the new standard, with surface texture changing from mean texture depth (MTD) to mean profile depth (MPD). This means that there are now lower and upper bounds placed on the MPD (previously the standard had a lower limit only). Sound absorption now has a requirement for each one-third-octave band from 315 to 1600 Hz and separate requirements for the drive lane and propagation area.