The design of the headphones is a fundamental aspect that affects the way the headphones reproduce sound, as well as how the headphones sit on or around the ears. The main types of headphone designs include closed, open and semi-closed. Closed headphones provide sound isolation and are ideal for listening in noisy environments. Open-back headphones, on the other hand, allow air to pass through the headphone drivers, resulting in a more natural sound. Semi-closed headphones are a compromise between the two types, offering balanced sound and a medium level of isolation.