Early Paratone cone loudspeaker from advertisement in 1929 radio magazine. Loudspeakers that used a paper cone for a diaphragm replaced horn loudspeakers in the mid-1920s. Before the driver used in modern speakers, the "electrodynamic" moving-coil (voice coil) mechanism, was invented in 1924 by Chester Rice and Edward Kellogg, earlier moving-iron drivers were used, as in this early speaker. The audio signal is applied to a stationary coil, which vibrates an iron vane between the poles of a magnet, attached to the speaker cone. This type of driver had inherently higher harmonic distortion than the modern moving-coil, and with large excursions the vane could collide with the magnet poles, producing a buzzing sound. The speaker sold for $6.00, while the driver unit alone sold for $3.50 to allow the user to build his own speaker by attaching the driver to a light diaphragm of wood or paper.