The efficiency in reducing road traffic noise of a pavement is often expressed as “noise reduction” when the pavement is in new condition. Far too often, the longevity of the noise reduction is neglected. But what the noise exposed residents along the road perceive is the noise during its life-cycle. Especially porous asphalt pavements change its properties substantially with time. Moreover, the speed of this change depends on the construction. For example, single-layer porous pavements mostly get clogged earlier than double-layer porous pavements, given a certain air voids and maximum aggregate size; something which is due to the much greater air voids volume under the surface for the double-layer version. When evaluating overall performance over the life-cycle, therefore, it is important to consider the noise reduction year by year over the lifetime. The author argues that fair descriptors of the acoustic efficiency of low noise pavements are the “Time-integrated noise reduction (TINR)” and the “Time-averaged noise reduction (TANR)”. In the paper, some examples from measurements as well as hypothetical examples are presented. Using this descriptor, it is shown that single-layer porous pavements often come out far less efficient than double-layer pavements; and the latter often offer better cost-benefit ratios. It is also suggested how one should average noise properties of different wheel tracks, lanes, and directions of highways and motorways.