Early Public Address Systems
Edwin Jensen and Peter Pridham of Magnavox began experimenting with sound reproduction in the 1910s; working from a laboratory in Napa, California, they filed the first patent for a moving coil loudspeaker in 1911. Four years later, in 1915, they built a dynamic loudspeaker with a 1-inch (2.5 cm) voice coil, a 3-inch (7.6 cm) corrugated diaphragm and a horn measuring 34 inches (86 cm) with a 22-inch (56 cm) aperture. The electromagnet created a flux field of approximately 11,000 G.
Their first experiment used a carbon microphone. When the 12 V battery was connected to the system, they experienced one of the first examples of acoustic feedback. They then placed the loudspeaker on the laboratory's roof, and claims say that the amplified human voice could be heard 1 mile (1.6 km) away. Jensen and Pridham refined the system and connected a phonograph to the loudspeaker to be able to broadcast recorded music. They did this on a number of occasions, including once at the Napa laboratory, at the Panama-Pacific Exposition, and on December 24, 1915 at San Francisco City Hall alongside Mayor James Rolph. This demonstration was official presentation of the working system, and approximately 100,000 people gathered to hear Christmas music and speeches "with absolute distinctness".
The first outside broadcast was made one week later, again supervised by Jensen and Pridham. On December 30, when Governor of California Hiram Johnson was too ill to give a speech in person, loudspeakers were installed at the Civic Auditorium in San Francisco, connected to Johnson's house some miles away by cable and a microphone, from where he delivered his speech. Jensen oversaw the governor using the microphone while Pridham operated the loudspeaker.
The following year, Jensen and Pridham applied for a patent for what they called their "Sound Magnifying Phonograph". Over the next two years they developed their first valve amplifier. In 1919 this was standardized as a 3-stage 25 watt amplifier.
This system was used by former US president William Howard Taft at a speech in Grant Park, Chicago, and first used by a current president when Woodrow Wilson addressed 75,000 people in San Diego, California. Wilson's speech was part of his nationwide tour to promote the establishment of the League of Nations. It was held on September 9, 1919 at City Stadium. As with the San Francisco installation, Jensen supervised the microphone and Pridham the loudspeakers. Wilson spoke into two large horns mounted on his platform which channelled his voice into the microphone. Similar systems were used in the following years by Warren G. Harding and Franklin D. Roosevelt.
By the early 1920s, Marconi had established a department dedicated to public address and began producing loudspeakers and amplifiers to match a growing demand. In 1925, George V used such a system at the British Empire Exhibition, addressing 90,000 via six long-range loudspeakers. This public use of loudspeakers brought attention to the possibilities of such technology. The 1925 Royal Air Force Pageant at Hendon Aerodrome used a Marconi system to allow the announcer to address the crowds, as well as amplify the band. In 1929, the Schneider Trophy race at Calshot Spit used a public address system that had 200 horns, weighing a total of 200 long tons.
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