Magnavox Public_address_system

early public-address system around 1920 using magnavox speaker. microphone had metal reflector concentrated sound waves, allowing speaker stand wouldn t obscure or face. vacuum tubes couldn t produce gain, , 6 tubes amplifier had low power. produce enough volume, system used horn loudspeaker. cylindrical driver unit under horn contained diaphragm, voice coil vibrated produce sound through flaring horn. produced far more volume given amplifier cone speaker. horns used in virtually pa systems, , still used in 2010s in systems, @ least high-range tweeters.

edwin jensen , peter pridham of magnavox began experimenting sound reproduction in 1910s. working laboratory in napa, california, filed first patent moving coil loudspeaker in 1911. 4 years later, in 1915, built dynamic loudspeaker 1-inch (2.5 cm) voice coil, 3-inch (7.6 cm) corrugated diaphragm , horn measuring 34 inches (86 cm) 22-inch (56 cm) aperture. electromagnet created flux field of approximately 11,000 gauss.

their first experiment used carbon microphone. when 12 v battery connected system, experienced 1 of first examples of acoustic feedback, typically unwanted effect characterized high-pitched sounds. placed loudspeaker on laboratory s roof, , claims amplified human voice heard 1 mile (1.6 km) away. jensen , pridham refined system , connected phonograph loudspeaker broadcast recorded music. did on number of occasions, including once @ napa laboratory, @ panama–pacific international exposition, , on december 24, 1915 @ san francisco city hall alongside mayor james rolph. demonstration official presentation of working system, , approximately 100,000 people gathered hear christmas music , speeches absolute distinctness .

the first outside broadcast made 1 week later, again supervised jensen , pridham. on december 30, when governor of california hiram johnson ill give speech in person, loudspeakers installed @ civic auditorium in san francisco, connected johnson s house miles away cable , microphone, delivered speech. jensen oversaw governor using microphone while pridham operated loudspeaker.

the following year, jensen , pridham applied patent called sound magnifying phonograph . on next 2 years developed first valve amplifier. in 1919 standardized 3-stage 25 watt amplifier.

this system used former president william howard taft @ speech in grant park, chicago, , first used current president when woodrow wilson addressed 50,000 people in san diego, california. wilson s speech part of nationwide tour promote establishment of league of nations. held on september 9, 1919 @ city stadium. san francisco installation, jensen supervised microphone , pridham loudspeakers. wilson spoke 2 large horns mounted on platform, channelled voice microphone. similar systems used in following years warren g. harding , franklin d. roosevelt.