Reported in Scientific American, this Week in World War I: October 16, 1915
By October 1915 German submarines had sunk about 750,000 tons of merchant shipping. That large number may give the impression that the seas were swarming with U-boats, but in reality only a handful, three or four, out of the 15 or so submarines available for duty in the North Sea and Atlantic at the start of 1915, were active on any particular day. But the U-boat menace was very real and one of the strongest weapons in the arsenal of the German and Austrian navies. In the years before sonar and radar, it was impossible to know where a submerged submarine was:
The submarine has long depended for its defensive powers on the fact that, traveling below the surface of the water, it was invisible to the crews of surface ships. Thus it can approach its target without attracting attention to itself and, at the opportune moment, after raising the periscope a trifle above the water to secure its bearings and maneuver into the most favorable position for attacking the enemy, fire a torpedo at a fairly close range.
One line of research suggested placing underwater microphones that could detect the noise made by submarine motors. The obvious problem was that the sea was a noisy place with a lot of ship engines and very few submarine motors, so it is difficult to separate the signal from the noise. The article mentions some work by specific researchers:
At this stage of the experimenting William Dubilier, an American electrical engineer with numerous wireless telephone and wireless telegraph inventions to his credit, at that time in France on the mission of installing wireless telegraph apparatus on aircraft, was called upon to aid in the solving of the submarine detector problem. Dubilier went to Cherbourg, an important French port on the English Channel, where he found Prof. [Camille] Tissot of the French Academy of Science hard at work on the detector system.
These two researchers already had brilliant careers in electronics: Dubilier had developed a mica condenser that became a part of all radio and television sets; Tissot was a French naval officer who had first equipped French warships with wireless radios in 1900. There is a limited description of the submarine detection system, but then the veil of secrecy hides the details:
The military value of the submarine detector system now used by France and England naturally prevents the disclosure of important details regarding the apparatus and its exact method of operation. However, it is learned that a number of special microphones are lowered into the water to a depth of several fathoms at each station. They are usually arranged in a semicircle facing out to sea.
During the war, much research was carried out on “ASDIC,” as underwater sound detection came to be called, as well as echo location (eventually called SONAR). Sound detecting devices were apparently installed on at least 3,000 ships by the end of the war. All of these systems, however, were of very limited use in World War 1, and as far as I can tell, they were deployed well after this article was published. So I question why the British and French authorities allowed any of the information in the article to be disclosed during what seems to be just the research phase. Perhaps in 1915 the system gave such poor results in actual tests in the open ocean that it was decided by the powers-that-be to disclose a few snippets of truth, in the hopes that the information would make its way to German submarine commanders and make them less bold in their coastal raiding. It wouldn’t be the first or last time that military intelligence engaged in such creative deception.
Our full archive of the war, called Scientific American Chronicles: World War I, has many articles from 1914–1918 on submarines and the naval technology of the First World War. It is available for purchase at www.scientificamerican.com/products/world-war-i/