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Summary
| Description |
One of the first electronic public address systems from about 1920, used to amplify a speaker's voice when addressing a crowd. It consisted of a microphone (left, then called a "transmitter"), attached to a vacuum tube amplifier (center) to increase the power of the audio signal, which drove a horn loudspeaker (right). The microphone has a metal reflector to concentrate the sound waves, allowing the speaker to stand back from it while speaking so the microphone would not obscure his face. The primitive triode vacuum tubes had very low gain, so the amplifier used six tubes (visible in center). Even so, the output power of the amplifier was only about 10 watts, so the system uses a horn speaker to create enough sound volume to fill a hall. This consists of a small driver unit (below horn) containing a metal diaphragm vibrated by a coil of wire moving in a magnetic field, creating sound waves, which were conducted to the open air by the flaring horn. The horn served to couple the diaphragm more efficiently to the air, increasing the efficiency of the speaker, so it radiated much more sound power from a given audio signal than an ordinary cone speaker. The Magnavox moving-coil loudspeaker, invented in 1915, a later model of which is shown above, made practical loudspeakers and public-address systems possible. It was first tried out before a crowd of 50,000 during the Municipal Christmas Tree Celebration on Christmas Eve, 1915, in San Francisco. |
| Date | before 1922 |
| Source | Downloaded April 23, 2013 from Austin Celestin Lescarboura (1922) Radio for Everybody, Scientific American Publishing Co., New York, p. 197 on Google Books |
| Author | Unknown author |
Licensing
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This media file is in the public domain in the United States. This applies to U.S. works where the copyright has expired, often because its first publication occurred prior to January 1, 1929, and if not then due to lack of notice or renewal. See this page for further explanation.
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This image might not be in the public domain outside of the United States; this especially applies in the countries and areas that do not apply the rule of the shorter term for US works, such as Canada, Mainland China (not Hong Kong or Macao), Germany, Mexico, and Switzerland. The creator and year of publication are essential information and must be provided. See Wikipedia:Public domain and Wikipedia:Copyrights for more details.
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File history
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| Date/Time | Thumbnail | Dimensions | User | Comment | |
|---|---|---|---|---|---|
| current | 20:54, 23 April 2013 | | 900 × 614 (65 KB) | Chetvorno | User created page with UploadWizard |
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