English: 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.
This 1929 issue of Radio World magazine would have the copyright renewed in 1957. Online page scans of the Catalog of Copyright Entries, published by the US Copyright Office can be found here. [1] Search of the Renewals for Periodicals for 1956, 1957 and 1958 show no renewal entries for Radio World. Therefore the magazine's copyright was not renewed and it is in the public domain.
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Este archivo se encuentra en el dominio público en los Estados Unidos porque fue publicado en ese país entre los años 1929 y 1963, y, teniendo un aviso de derechos de autor, no se renovó su protección. A menos que su autor haya fallecido años antes, se mantiene bajo propiedad intelectual en los países o zonas que no aplican la regla del plazo más corto para las obras estadounidenses, como Canadá (70 años p.m.a.), China continental (50 años, a excepción de Hong Kong o Macao), Alemania (70), Suiza (70), México (100) y otros países con reglas particulares. Vea Commons:Hirtle chart como referencia e información.