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Lagging formed of jackets of porous materials such as glasswool and of impervious materials such as metal sheets are widely used to attenuate the sound radiated from pipes. A procedure is described for calculating the insertion lossess produced by such lagging. The procedure yields the insertion lossess corresponding to each of the low order mode types of the pipe. These are breathing, bending and ovalling mode types. Impedance and pressure transfer formulae for the various jacket types are given in terms of the fundamental properties of the jackets. The porous jacket, for example, is defined by its inner and outer diameters and the flow resitivity of the porous material. These formulae enable the frequency dependent insertion loss produced by an arbitrary pipe lagging to be determined. A comparison of the insertion lossess predicted by this procedure with published results is given.