Developing an idea of Laville and Soedel [ J. Sound Vib., 60 (1978) 273], the noise radiated by the exhaust system of a four cylinder engine is calculated, analytically and numerically, and compared to experiment. The basic idea is as follows: the effect of the exact shape of the volume velocity signal produced by a cylinder during opening can be considered as a second order effect on the result. The amplitude of every harmonic of the radiated noise is shown to be proportional to the first order of certain quantities, the zeroth order being zero. On the one hand, the harmonics H 2 , H 4 , … are proportional to the deviation of the signal during opening from a square signal and to the ratio of the overlap time of opening of two cylinders to the valve opening duration. On the other hand, the other harmonics are proportional to quantities like the ratios of the length difference between two parallel tubes of the manifold to the wavelength. The analysis of the amplitude of the radiated noise exhibits the role of the resonances of the admittance of the whole exhaust as viewed from outside when the cylinders are closed, and of the resonances of the manifold. Comparison with experiment confirms the basic hypothesis and the result of the theory, at least qualitatively, but losses seem to cancel the manifold resonances, which are related to the interferences between parallel tubes.