small loudspeaker held or worn close to the listener's ear or within the outer ear. Common forms include the hand-held telephone receiver; the headphone, in which one or two earphones are held in place by a band worn over the head; and the plug earphone, which is inserted in the outer opening of the ear. The conversion of electrical to acoustical signals is effected by any of the devices used in larger loudspeakers; the highest fidelity is provided by the so-called dynamic earphone, which ordinarily is made part of a headphone and equipped with a cushion to isolate the ears from other sound sources