ESDU 81043 provides essential data for estimating the change in acoustical energy as sound propagates through a ventilating system. From a knowledge of this sound energy the sound pressure level which results in any room served by the system may be estimated. From consideration of the estimated sound pressure levels in the most sensitive areas of the building the amount of silencing required in the system to achieve acceptable sound levels can be assessed. Data are provided, in either graphical or tabular form, for estimating the attenuation of sound in ducts and duct fittings and for estimating the sound generated as air flows through standard duct fittings. A means of estimating the octave band sound pressure levels, due to the air-conditioning system, in a room is also given. Other factors which affect sound in ventilation ducts (e.g. breakout) are discussed.