Abstract: An open-back type headphone comprises a driver unit for operating as a sound generating source, a case accommodating therewithin said driver unit and having a front face on the side to confront and be pressed against an ear of a listener and a back face on the side opposite that of the front face, said back face being acoustically open, an ear pad provided on said front face of the case and forming at the center thereof a front air chamber, and an attachment detachably attached to said back face of the case. The attachment has an open end communicable with the back face of the case and an internal space of a specific volume and is adapted to cover and seal the back face of the case. The attachment and said case have an attaching mechanism by which the attachment can be detachably attached to the case.

Abstract: A system of remote supervising and controlling many console units or equipments associated therewith with a central supervision and control unit. Console units are connected in parallel to the central unit through a single output line comprising a group of wires from the central unit, series signals of clock, data and other pulses are transmitted from the central unit to the respective console units, and signals informing whether the state in the respective console units is normal or not and results of detections of the respective equipments are returned from the console units to the central unit through a signal line in the group of wires.