Book - School of Science and Technology

466 Ductwork design3 64Plant527 8 318Figure 16.26 Transmission paths of noise and vibration. 1, airborne noise via intake louvres; 2, plantvibration via structure; 3, airborne noise via openings; 4, airborne noise via poor seals; 5, airborne noise viaopenings; 6, plant vibration via solid duct supports; 7, ductborne noise to terminals; 8, ductborne noisethrough duct wallsLarge scale boiler and refrigeration plant rooms give rise to special problems and mustbe considered in great detail. High mass walls, floors and ceilings are required betweenmajor plant rooms and occupied spaces, and special attention is needed to ensure thatpenetrations do not present a noise leakage path. Where particularly high levels of soundinsulation are required or structure borne noise transmission could be problematic,consideration should be given to the use of a `floating floor'; that is one in which thewhole floor slab on which the plant sits is supported above the main structure by aresilient layer or mounting.DuctsConsidering now the transmission of noise by ducts, we may assume that the noiseproduced by a fan is conveyed equally via the suction and delivery. The air in passingthrough the system of ducts will lose some of the noise by attenuation, that is by thepressure waves being damped down due to friction on the duct walls, and at changes ofdirection.Secondary noise may be generated in the ductwork by sharp edges, and particularly bydampers.At the terminal, the grille or diffuser may also generate noise, and this is at a pointwhere no treatment is possible; hence correct selection of type and velocity of such items isinherent in good acoustical design.When the air enters the room from the grille, it expands into free space which againaccounts for some attenuation.