Traffic noise has become an element of stress between economic development and quality of life. For the contact noise between road and tyre, sound sources are located close to the contact path and the tyre-road horn like geometry significantly amplifies the radiated power and its radiation directivity. In this work an analytical method considering the sound diffracted by a sphere placed above a ground surface of finite impedance has been used for the calculation of the horn noise effect. The predicted data has been compared against a set of experimental measurements with the aim of quantifying the accuracy of the model when contrasted with real-time data. This accuracy is fair when excluding the very low and very high frequencies of analysis, that constitutes the horn noise frequency range further emphasised. An application of this procedure is proposed at the end for the estimation of the sound power levels radiated by the rolling sources when circulating in real environments.