Radiation of sound waves by a coaxial duct is considered. The coaxial duct has an inner rigid wall which is infinite, while the outer wall is rigid for z < 0 and perforated for z > 0 which makes the problem more interesting. Applying the Fourier transform technique, the mixed boundary value problem is described by a Wiener-Hopf equation and then the solution of the problem is found by means of the Wiener-Hopf (WH) technique. The influence of the parameters of the problem like inner and outer duct radii, frequency and porosity on the sound pressure level is displayed graphically. This study is important because of its applications in noise reduction in modern aircraft jet and turbofan engines. The current problem is also compared with subproblems with and without perforated and very good agreement is observed.