Revising the CPX method for better measurement of road surface noise properties

The ISO 11819-2 standard from 2017 specifies a "CPX method" for measurement of road surface noise properties. The method uses two microphones located at the side of a reference tire, which usually sits in an enclosed trailer with one or two reference tires, towed by a car or van. It has been found that the two microphone positions at the tire side do not properly measure the noise emission from porous road surfaces, thus underestimating the noise reduction of such road surfaces. Therefore, the standard is being revised by adding a microphone behind the tire which reacts more representatively to the noise emission at the trailing edge of the tire on porous surfaces, discussing how near to the tire the microphone should be located, Measurements are being made to verify the performance of the extra rear microphone position. The paper further discusses the options considered in the use of the new microphone position; also, the practical consequences, such as comparability of new and old measurements. Furthermore, a few other revisions of the method based on the experience gathered since its publication are considered, for example, alignment of the test tire(s) and extension of the lower frequency range.