AFessenden oscillator is an electro-acoustictransducer invented byReginald Fessenden, with development starting in 1912 at theSubmarine Signal Company of Boston.[1] It was the first successfulacoustical echo ranging device. Similar in operating principle to a dynamicvoice coilloudspeaker, it was an early kind of transducer, capable ofcreating underwater sounds and ofpicking up their echoes.
The creation of this device was motivated by theRMSTitanic disaster of 1912, which highlighted the need to protect ships from collisions with icebergs, obstacles, and other ships. Because of its relatively low operating frequency, it has been replaced in modern transducers bypiezoelectric devices.
Theoscillator in the name referred to the fact that the device vibrated and moved water in response to a drivingAC current. It was not an electronicoscillator but a mechanical one in that it generated repetitive mechanical vibrations. Electronic oscillators did not yet exist when this device was created. Because the design of the device does not depend on a resonant response, it should not be considered aharmonic oscillator.
The Fessenden oscillator somewhat resembled a modern dynamic microphone or dynamic loudspeaker in overall construction. A circular metal plate, clamped at its edge, in contact with the water on one side, was attached on the other side to a copper tube, which was free to move in the circular gap of a magnet system. The magnet system had a direct-current winding to provide a polarizing magnetic field in the gap, and an alternating current winding that induced currents in the copper tube. These induced currents produced a magnetic field that reacted against the polarizing field. The resulting force was communicated to the membrane and in turn provided acoustic vibrations into the water.
Unlike previous underwater sound sources such as underwater bells, the Fessenden oscillator was reversible; the AC winding could be connected to a head set and underwater sounds and echoes could be heard. Using this device Fessenden was able to detect icebergs at a distance of about 2 miles, and occasionally detected echoes from the sea floor.[2]
The device could also be used as an underwater telegraph, sendingMorse code through the water. TheFessenden underwater signalling apparatus, or more usually just "The Fessenden", was fitted toRoyal Navy submarines inWorld War I.[3]British K-series submarines were equipped with Fessenden oscillators starting in 1915. However, a submarine signalling the surface could be heard by any nearby (enemy)hydrophone, so the system had restricted utility during wartime patrols.[4]
During theFirst World War the Fessenden oscillator was applied to detection ofsubmarines, but its rather low operating frequency of around 1 kilohertz gave it a very broad beam, unsuitable for detecting and localising small targets. In peacetime, the oscillator was used for depth finding, where the lack of directionality was not a concern, and Fessenden designed a commercialfathometer using acarbon microphone as receiver, for the Submarine Signal Company.[5]
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