If a fan is provided for forced ventilation, the silencer will usually be placed external to the
fan so that noise generated by the fan will be attenuated as well. When high volumes of air flow
are required, the noise of the fan should be considered very carefully, as this noise source is
quite often a cause of complaint. As fan noise generally increases with the fifth power of the
blade tip speed, large slowly rotating fans are always to be preferred over small high-speed fans.