The purpose of this project is to contribute to the ongoing international development in the field of acoustical aging of tire/pavement interactions by performing a comprehensive analysis of some Californian and Danish results from noise measurement series on asphalt pavements carried out over long time. The main focus is on asphalt concrete pavements applied on highways. Cement concrete pavements have not been included in this study. An international literature survey has been conducted. It shows that the noise level generally increases as the pavement gets older. For porous pavements (built-in air void content of more than 15 % or so) it is a known phenomenon that air voids tends to clog and that this increases the noise significantly. But for other dense and open graded (but not really porous) pavement types there is not much knowledge on which changes occur in the surface structure causing this increase in noise in the period from when the bitumen film is worn off until the pavements begin to deteriorate with distresses like raveling, cracking, etc. This project focuses on the trend in noise levels measured in the same way - the relative changes of noise over the years - and not on the actual absolute noise levels. The objective is to analyze and compare trends in the development of noise over time. Therefore, it is not so crucial if noise results have been measured by different methods or by the same method applied by different measurement teams/organizations. These factors might influence the actual noise levels and can complicate direct comparison, but when only trends are compared these differences in measurement methods are not so important. The vehicle fleet of California and Denmark may differ, for example, with smaller passenger cars in Denmark. This might influence the comparison of absolute noise levels but will presumably be less important when comparing changes in noise emission over the years measured at the same site. Only changes in the noise levels happening over time are included in this document. Changes in other factors relevant for the description of the development of the physical structure of the pavement surface like texture, porosity, visual signs of wear and tear etc. have not been considered. Two well documented long-time noise measurement series from California and two from Denmark have been analyzed in this project. The results have already been documented in detail in separate national reports. The objective of the current report is to perform a comparison study of the trends for acoustical aging found in these four projects. The University of California Pavement Research Center (UCPRC) finalized in 2009 the third year report on annual On Board Sound Intensity noise measurements on 65 to 76 pavement sections of different ages and mix types in California. Some results from this project are also included.