INTRODUCTION This Item provides data for estimating the attenuation of sound in lined plenum chambers and ducts@ and through silencers of the types used in ventilation and air-conditioning systems. These data are complementary to those presented in Item No. 81043 which provides a means of estimating the sound pressure levels in rooms that are directly attributable to the ventilation system. Data are provided in graphical or tabular form for the estimation of typical silencer insertion loss@ airflow generated noise and pressure loss. Both rectangular splitter silencers and cylindrical silencers are considered. Also@ equations are provided for the estimation of plenum chamber attenuation. The attenuation of sound in lined ducts is discussed with reference to the estimation procedure given in Item No. 81043. An example is given on the selection and sizing of a silencer for a typical ventilation system@ and the suitable location of silencers within the duct system is discussed. The calculation of noise in ventilation and air-conditioning systems is made in terms of sound power levels which are measured in decibels (dB)*. The reference sound power level used in this Item is 10-12W. To ensure that all frequencies over the range of interest are considered@ the data provided are for each of the octave frequency bands in the range 63 Hz to 8 kHz. During the design stage of a ventilation system there are a number of factors that should be considered as a means of avoiding possible noise problems. Firstly@ the flow velocities should be kept to the practical minimum since even a small reduction in flow velocity can significantly reduce the flow generated noise due to duct fittings. An indication of the reduction of flow generated noise with air velocity may be obtained@ for the case of a silencer@ from Figure 3. Useful gains in low frequency attenuation can be achieved by increasing the number@ and reducing the size of@ the duct outlets into a room. These gains are due to an increase in end reflection loss (see Figure 3 of Item No. 81043). Further reductions can be made by locating grilles at the maximum possible distance from the room occupants so reducing the direct sound pressure levels. However@ before increasing the number of@ or repositioning@ duct outlets it is essential to consider both the economics of increasing duct lengths and the efficiency of air distribution within the serviced space. Another way of controlling sound within a room is by decreasing reverberation time which reduces the reverberant sound pressure levels. The reverberation time in a room may be reduced@ for example@ by the use of carpets and acoustic ceiling tiles. The data provided are intended for use by heating and ventilation engineers when selecting the appropriate noise reduction equipment for a ventilation and air-conditioning system. The data may be used in the initial design of a system or to investigate the appropriate remedial action for an installed air distribution system that is found to be too noisy. This Item@ and the data provided@ is intended for use by engineers who are not acoustic specialists. In cases where very low noise levels must be guaranteed@ for example in an auditorium where the acceptable noise criterion is likely to be less than NC 30@ it is recommended that the advice of an acoustics specialist should be sought. Noise design criteria are discussed in Item No. 82001. In Section 3@ the reduction of sound by duct liners is discussed and reference is made to duct liner attenuation data. The types of purpose-built silencers for air distribution systems are described in Section 4@ and a method of estimating their insertion loss@ airflow generated noise and pressure loss is given. In Section 5 the location of silencers within the duct system is considered and in Section 6 a method is provided for the estimation of plenum chamber attenuation. A worked example that illustrates the selection and sizing of a silencer for a particular application is given in Section 8. * An introduction to noise units and their basic relationships is given in Item No. 82001.