The attention is focused on rotor blade thickness noise (flow subsonic everywhere). The solution of the wave equation is written in a closed form (frequency domain) which emphasizes the essential parameters for this field. On another side, starting from an estimation of the loads on the rotor disk (lifting-line theory), the acoustic emission due to loads is estimated in such a way that we can compare the acoustic efficiency of blade thickness and loads. To compare experimental results and predictions, an original signal processing method was designed, which compensates the Doppler effect due to the helicopter motion.