The computation of the wall induced loads on a cylinder by random acoustic waves is often costly. In order to reduce this duration, an approximated model of diffraction has been developed on the basis of the scattering of a plane wave by an infinite cylinder. It leads to simplified expressions of the power spectrum density and the normalized cross-spectrum density, which are functions of the azimuth of the incident wave and of a non-dimensional variable β = k a sin(θ). This model gives excellent results as in the case of the infinite cylinder, as in the one of finite slim bodies. It has been linked to a jet noise sources model to allow an experimental validation in the complex case of a three engines launcher at lift-off. The results point out a satisfactory level of accuracy in different configurations, as on the amplitude, as on the phase.