Earphones, or headphones, work similarly to speakers in that they turn electrical energy into sound by using magnetism and the vibration of air. While a large speaker vibrates the air in the space around it to create sound, a pair of earphones only vibrates the air inside the ear canal. Earphones are usually designed as either closed-back or open-back, which affects how much sound can escape from them.
A pair of closed-back earphones is sealed in the back to limit how much sound escapes when they are being used. Closed-back earphones also minimize the amount of sound that leaks to the outside, reducing sound pollution around the user.
Open-back earphones are open at the back, allowing some air and sound to escape. In some cases, open-back earphones have better sound quality, but it is at the cost of increased sound pollution. Open-back earphones also allow ambient noise to leak into the user's ear canal.
Smaller sizes of earphones can also be fitted with noise-cancelling, soft foam earpieces that completely cover the ear canal. This method of passive noise reduction eliminates any noise escaping from the earphones to the surrounding area while isolating the user from any ambient noise leaking in.