A single number rating index to qualify the acoustical protection of a noise barrier that accounts for the influence of the ground is developed. This is an extension of a previous index for semi-infinite barrier developed by Pfretzschner [Pfretzschner et al., Acust. Acta Acust.82, 504–508 (1996)]. This index is based on the asymptotic value found for the insertion loss or the acoustic attenuation as physical magnitude, and it is uniquely dependent on the barrier, noise source, and acoustic characteristics of the ground in the source semi-space. The new index agrees well with measured data on a scale model under laboratory conditions, and a direct experimental measuring procedure could be envisaged. In this way the proposed index, which accounts for extrinsic properties (acoustical protection) of barriers, would complete the two previous indices dealing with intrinsic properties (transmission loss and acoustical absorption).