Abstract: A wireless earbud including a shell housing having an inner shell portion and an outer shell portion. The inner shell portion has an inwardly facing ovate-shaped surface with a forward end portion configured to fit toward the user's ear canal, and a rearward end portion configured to fit in the concha of the user's ear. The outer shell portion has an outwardly facing surface and one or more outwardly protruding abutment surfaces configured to engage other parts of the user's ear to restrict rotation and prevent the earbud from falling out. For example, a first abutment may engage the user's tragus, a second abutment may engage the user's antitragus, and the first and second abutments may together form a convex surface that fits within the user's intertragical notch. Additional protrusions or surfaces may be provided to further improve the self-locking functionality of the earbud in the user's ear.