Acoustic ducting has an inner and outer layer of flexi-ducting, separated by a layer of insulating fibreglass in between them. As air travels through the ducting it causes the inner layer of the flexi-ducting to vibrate. This would normally create a humming noise. The insulating layer of fibreglass in the acoustic ducting prevents the vibration from getting from the inside layer of flexi-ducting to the outside layer of flexi-ducting. This reduces the hum from the duct vibration and dampens the sound of a ventilation fan.