Silencers are commercially available devices that allow the passage of air while restricting the passage of sound generated from air distribution equipment. They subdivide the airflow into several passages each lined with perforated sheet backed by mineral wool or glass fibre. A silencer usually has a cross section greater than the duct in which it is installed such that noise induced by high air flow velocity passing through the silencer can be avoided. Silencers are available for circular or rectangular ducts, as shown in Figure 54, and are fabricated in modular form in cross section, and in lengths of 0.6, 0.9, 1.2 and 1.5 m, etc. They are generally specified by the insertion loss in decibels (dB) in each octave band, so that the degree of match with the sound power distribution of the noise source over the frequencies may be judged. The other important parameter associated with silencers is the resistance to airflow. The use of silencer will inevitably increase the load of the fan and it is essential for engineers to consider both the acoustic and air flow performances during the design stage.