The fan is usually the main source of noise generation in a ventilation system. Rotating fans always constitute a source of noise generation. However, aerodynamically designed fan blades may reduce the noise generated. Another important source of noise is the bearings of various kinds inside the fan motor. Defective bearings add directly to the ventilation noise. Another common rea son for high noise levels may be the imbalances which easily arise in a fan system or ductwork. Imbalances give rise to vibration and hence to noise generation. The noise generated by a fan may also be due to poor impact sound insulation in the fan mountings and duct connections. Noise from drive motors for the fan may cause strong radiation from a fan room, especiallv if it has poor sound insulation.
The noise from the fan is propagated in the duct to the openings inwards or outwards in the premises. The ventilation noise often propagates into the surrounding area from the supply air and exhaust air terminal units in the rooms. The ducts in themselves may also be important sources of noise, particularly if they are poorly insulated or otherwise designed in such a way that noise generation may occur.
Noise generation in the ducts may occur as a result of unfortunately posi tioned bends, inappropriate geometry, or imbalances. Supply and exhaust air terminal devices may themselves create noise if the air velocity is high or the devices are designed in such a way as to cause air turbulence. The higher the air velocity, the greater the risk of noise-generating turbulence occurring in the ducts.
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