Kenneth D. Rolt
Dept. of Ocean Eng., MIT, Cambridge, MA 02139
The first practical man-made sonar oscillator, conceived and designed by the Canadian Reginald A. Fessenden, was a 540-Hz air-backed electrodynamically driven clamped-edge circular plate. Work on the oscillator started in 1912 while Fessenden was working for the Submarine Signal Company, Boston, MA. In January 1914, in Boston Harbor, underwater communication was first shown by using a Morse code carrier to modulate the oscillator, thus demonstrating a means of ship--submarine acoustic communication. In March 1914, the oscillator was later tested aboard the U. S. Coast Guard cutter Miami on the Grand Banks, off Newfoundland Canada, where echo ranging from a 3200-m distant iceberg and depth sounding were demonstrated. In 1915, the oscillator was even tested at 100 kHz. The Fessenden oscillator models (ca. 500, 1000, and 3000 Hz) were so successful that they were even used until, and during, World War II for sonar and mine detection purposes. Despite these landmark achievements, at present no oscillators are known to exist, and no modern acoustic measurements have ever been made to establish the acoustical performance. To partially fill in this gap, the Fessenden oscillator, will be described and an electroacoustic model will be used to predict the acoustic performance.