The control of noise emanating from engines and their exhaust systems is a difficult problem to overcome in a cost effective way. The noise may be reduced by the introduction of a fluid into the exhaust system such that the fluid is heated and expands rapidly producing sound and vibration which is in antiphase to the noise and vibration produced by the engine acting through the exhaust. There is a consequent reduction in the sound and vibration produced by the system. The fluid introduced is conveniently a liquid (water) which enhances the damping effect by virtue of possessing a latent heat of vaporisation. The fluid can be added to the exhaust system by a bleed from a turbo charger or super charger or by alternate means. A primary use would be for the reduction of engine noise emanating from exhaust systems in heavy goods vehicles.