US Patent 8712087 Earphone

The earphone includes a driver unit; a housing accommodating the driver unit, the housing having a front face serving as a sound emitting surface and a bowl shaped rear face; a hollow casing provided separately from the housing, the casing being configured to increase an internal volume adjacent to the rear side of a vibrating plate and to reduce the back pressure of the vibrating plate; and a connecting channel connecting the rear face of the housing and the casing such that the housing is in communication with the internal space of the casing, in which the housing includes a sound emitting tube protruding from the front face thereof so as to be fitted into an external auditory meatus.