Book - School of Science and Technology

Noise dispersal 46598765432100 1 2 3 4 5 6 7 8 9 10Difference between total and background dBDifference between total and special dB 10Figure 16.25 Substraction of decibel valuesPlant noises are of two kinds:. Acoustic energy created by moving mechanical devices which owing to direct fixing tothe building structure are transmitted throughout the building and thence to occupiedrooms. Vibration is a good example of this structure borne noise transmission.. Airborne noise which is transmitted through the air and attenuated by solid walls,floors and the like. This form of transmission can be troublesome if it is conveyed viaventilation ducts to other rooms.The first category can be dealt with by mounting such plant on anti-vibration supportscomprising springs, rubber and neoprene pads, mats and the like, and by installingflexible connections within the ductwork or pipework. However, misaligned flexibles inductwork may produce turbulence resulting in an increase in sound power level of up to6 dB or more at low frequencies. Figure 16.26 illustrates the various transmission pathsbetween an air handling plant and an occupied space.The second category can be controlled by two principles:Absorption of sound is achieved by the application to surfaces of acoustically absorbingmaterials such as acoustic tiles, and various soft materials such as glass fibre and open cellfoams.Insulation of sound is achieved by interposing a barrier between the space where thenoise is produced and some other space to be kept silent. The fundamental requirement ofa good sound insulator is mass.In the case of the plant room mentioned in Figure 16.26, the sound may be absorbed bycovering the wall, ceiling and other surfaces with suitable material; but this may be friableand liable to damage, hence the use of insulation might be preferable between say twofaces of a hollow partition, or on top of a false ceiling. The difference between the twoapproaches is that absorption will reduce the noise level within the plant room whereasinsulation will reduce the transmission of this noise to adjacent spaces. Usually thisproblem is not intractable and is covered under the general heading of `Building Insulation'dealt with in Chapter 2.