A driver is the speaker inside a headphone or earbud that converts electrical signals to sound waves. Larger drivers can move more air, generally producing fuller bass.
Common driver types: Dynamic: traditional voice-coil, most common, good bass. Balanced Armature: small, accurate, used in IEMs, weak bass. Planar Magnetic: thin diaphragm, fast response, found in Hifiman, Audeze. Electrostatic: ultra-thin film, ultra-detailed, very expensive (Stax).
In-ear earbuds typically use 6–13 mm dynamic drivers. Over-ear headphones use 40–53 mm drivers. Multi-driver IEMs combine dynamic for bass + balanced armature for mids/treble.
Driver size alone doesn't determine quality — diaphragm material, magnet strength, and tuning matter more.