By means of ventilation ducts, fresh air can be delivered and the used one removed. Ventilation ducts are most often made of flexible, thin-walled metal pipes, or elastic synthetic substance. They are used to soundproof the noise and to protect the air against getting cold. The ducts that are used most often are the round ones, but the rectangular ones are used for smaller rooms. The ends of ducts are finished with grilles, or diffusers.