With closed headphones, the ear is completely sealed off from outside noise (pressure chamber principle). Typical features of closed headphones are the acoustically sealed housing and the ring-shaped (circumaural) pads that completely surround the ear. The sealing around the ear has a decisive influence on the sound reproduction of closed headphones.
Open headphones are popular for normal headphone use because they sound "more natural". Further, other people can talk to you while you have them on. People who have to concentrate on what they are listening to prefer closed headphones. The problem of sealing does not exist with open headphones, which have foam ear pads that rest on the ears, or ring pads that surround the ears. In this design, the space behind and in front of the diaphragm lets through sound. Therefore, open headphones allow sound to pass straight through the diaphragm without being "muffled", thus resulting in a more transparent and natural sound