Closed-back headphones are a type of headphone in which the ear cups or housings completely enclose and seal the ears, acoustically isolating them from the outside environment. The core design principle is that there are no ventilation holes in the rear of the cavity; the air pushed by the driver diaphragm is confined within the ear cups, so sound does not leak outward and external noise is also difficult to enter. In-ear monitors are broadly considered closed-back as well, but the term typically refers more specifically to full-size over-ear headphones.
From the perspective of sound quality and purchasing considerations, the closed-back structure can deliver stronger low-frequency extension and more focused transients, because the internal air pressure is stable and the diaphragm has better control. At the same time, it does not disturb people around you, making it suitable for use in libraries, offices, or at night. However, the trade-off is that the soundstage is usually more "introverted" than that of open-back headphones, with a narrower lateral width, and prolonged wear may cause a sense of stuffiness and pressure. If you need to monitor recordings, block out noise, or frequently use headphones in public settings, closed-back is a reliable choice; if you seek a natural, expansive soundstage and are in a quiet environment, you should weigh the trade-offs carefully.
Practical advice: when purchasing, prioritize the ear pad material and clamping force, and try them on for at least fifteen minutes to confirm that breathability and headband pressure are acceptable—no matter how good the physical isolation of a closed-back headphone is, if it is uncomfortable to wear, the sound quality advantages will be offset by fatigue.