Between and more than a thousand ships like the Edward Rich sank off foggy North American coasts. Lighthouses, sometimes fitted with bells and sirens to warn ships in the gloom, could only do so much, and they did nothing to protect ships from underwater obstacles or drifting icebergs. Nor were there accurate maps of the seafloor to help pilots navigate blind. A ship in fog tempted fate. Lighthouse Board showing the network of lighthouses installed along the Massachusetts coast, Their plan, proposed by the American Lighthouse Board 12 years earlier, was to create a system of underwater bells to guide ships around the most dangerous rocks, reefs, and shallows.
Ships outfitted with underwater microphones—called hydrophones—would listen for the bong of the bells and follow the sound to safe harbor. Because sound travels farther in water than in air, the ringing of the heavy underwater bells would have a much greater range than anything projected from a lighthouse. Business was steady but not booming. Within two years the SSC would possess a technology that could prevent another such disaster—a device that used underwater echoes to measure distance.
It was a lucky accident that the bullheaded inventor the SSC chose to build the device knew more about radio than he knew about the sea. Fessenden was a well-known inventor by then, having been the first to successfully transmit voices using radio waves and the first to achieve a two-way transatlantic broadcast in Often a ship would have to come to a complete stop for the hydrophone operator to hear the bell and reckon the way.
While drawn to the spotlight the Titanic disaster had focused on marine safety, Fessenden was leery of business partners, who always seemed to meddle with his inventions. The SSC was committed to improving the hydrophone only. Uncharacteristically, Fessenden compromised. But two years later he found himself jobless when Edison ran into financial trouble. While Fessenden would idolize Edison for the rest of his life, the episode instilled in him a mistrust of private enterprise that only intensified in subsequent years.
After a stint at Westinghouse working on dynamos and one as a professor of electrical engineering, Fessenden was asked by the U. Weather Bureau to take on the then-unproven technology of wireless communication, the 5G of the previous century. He, Helen, and their son moved to an island in the Potomac River where Fessenden demonstrated that voices could be transmitted over wireless through amplitude modulation, better known now as AM.
They jailed him for it. He escaped to Switzerland in , returning as soon as France was liberated. He died in late For example, long-range submarine-seeking sonar systems whose detection range is about ten miles operate at 3 to 40 kilohertz.
In contrast, short-range systems that work at about feet in mine sweepers, for example use kilohertz to 2 megahertz. Modern active sonar has affected military and nonmilitary activities ranging from submarine location to undersea mapping and fish location. In all these uses, two very important goals have been to increase the ability of sonar to identify a target and to increase the effective range of sonar.
Much work related to these two goals has involved the development of new piezoelectric materials and the replacement of natural minerals such as quartz with synthetic piezoelectric ceramics. Efforts have also been made to redesign the organization of sonar systems. One very useful development has been changing beam-making transducers from one-beam units to multibeam modules made of many small piezoelectric elements.
Systems that incorporate these developments have many advantages, particularly the ability to search simultaneously in many directions. The development of the acoustic transducer that converted converting electrical energy to sound waves enabled the rapid advances in SONAR design and technology during the last years of the war. Not all of the advances, however, were restricted to military use.
After the war, echosounding devices were placed aboard many large French ocean-liners. It was the invention of the acoustic transducer and efficient acoustic projectors that made more advanced forms of Sonar possible.
Active sonar creates a pulse of sound, often called a "ping" and then listens for reflections of the pulse.
The pulse may be at a constant frequency or a chirp of changing frequency. If it's a chirp, the receiver correlates the frequency of the reflections to the known chirp. The resulting processing gain allows the receiver to derive the same information as if a much shorter pulse with the same total power were emitted. In general, long-distance active sonars use lower frequencies. To measure the distance to an object, one measures the time from emission of a pulse to reception. Passive sonars listen without transmitting.
They are usually military, although a few are scientific. Passive sonar systems usually have large sonic databases. A computer system frequently uses these databases to identify classes of ships, actions i. Actively scan device characteristics for identification. Use precise geolocation data.
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