An embodiment of the present invention provides a sound generating device which comprises a housing, wherein a sound generating unit is mounted in the housing, a closed rear cavity is formed between the sound generating unit and the housing, and the sound generating unit is provided with a rear sound hole in communication with the rear cavity, wherein the sound generating unit comprises a magnetic circuit system, an expandable-capacity cavity being provided on the magnetic circuit system, and the expandable-capacity cavity acting as part of the rear cavity to increase a volume of the rear cavity.