When a noise reduction of 20 dB(A) or more is required, it is generally necessary to use a complete enclosure if the noise problem is a result of air-borne noise transmission. The enclosure should be internally lined with 50 mm thick sound absorbing material (e.g. fibre glass). A variety of materials can be utilized for fabricating an enclosure. The sound transmission loss for enclosures using different materials are given in Table 15. Ventilation of enclosures should not be overlooked as most equipment, such as motors, requires an adequate air supply either to prevent overheating or to enable them to function efficiently. A silenced ventilation system incorporating silencers at the air intakes and discharge openings should be employed (see Figure 16 and Figure 28).