Submarine signals had a specific, even proprietary, meaning in the early 20th century. It applied to a navigation aid system developed, patented and produced by the Submarine Signal Company of Boston. The company produced submarine acoustic signals, first bells and receivers then transducers, as aids to navigation. The signals were fixed, associated with lights and other fixed aids, or installed aboard ships enabling warning of fixed hazards or signaling between ships. ATLAS-Werke, at the time Norddeutsche Maschinen und Armaturenfabrik, of Germany also manufactured the equipment under license largely for the European market. The system used more reliable underwater sound to project acoustic signals from a shore station or an undersea hazard on which a signal was placed. The signals were usually associated with a lightvessel, a bell buoy or hung on a tripod frame on the sea floor connected to a shore stations by cable. At first the system depended on bells operated by electric strikers. Receivers aboard ships could detect the acoustic signal and when equipped with receivers on each side the ship could determine approximate direction from which the signal came. A ship-to-ship system was also produced allowing ships so equipped to detect each other and estimate direction in fog. The company collected data from ships including ranges at which the signals of specific stations were detected. The collected data formed an early base of ocean acoustical properties. The original bells were quickly replaced by the Fessenden oscillator, a transducer, after its invention by Reginald Fessenden with development starting in 1912 at the Submarine Signal Company. That transducer allowed both sending and receiving leading to major advances in both submarine signals and extension into submarine telegraphy and experiments with underwater telephone communication and eventually sonar. Ships, commercial or naval, equipped with submarine signaling capability had that equipment noted as one of the ship's navigation capabilities in registry information from the first decade of the century until nearly mid century. In 1907 the information was important to insurance underwriters and American Bureau of Shipping required that ships so equipped by indicated by the note "Sub. Sig." in ship's registry information. Commercial lines advertised the capability as a safety measure. Submarine signaling was made obsolescent and overtaken by advances during World War II. In 1946 the Submarine Signal Company was acquired by and merged with Raytheon, becoming Raytheon's Marine Division, after having become the national leader in underwater sound, sonar and other work with the Navy during the World Wars and branching into other marine systems.
Early research In 1826 Jean-Daniel Colladon and Jacques Charles François Sturm used a submerged bell for experiments in Lake Geneva. Lucian I. Blake in association with the United States Lighthouse Service did similar work in 1883 using a submerged bell with the explicit purpose of using sound as an aid to navigation. Experiments in England and the United States occurred independently afterward. Reception problems related to ship noise were partially solved when A. J. Munday, who had worked with Dr. Elisha Gray on signaling by underwater bells to include actual messages, found that a microphone placed in a metal box filled with water and attached to a ship's skin from inside allowed clear reception. In further experiments placement of such microphones on each side of a ship allowed finding the direction of the source. Intensity on one side showed the source to that side of the ship and equal intensity showed the source to be directly ahead. A direction indicator box allowed the selection of receivers individually for comparison of signal strength for direction. Experiments determined modifications to bells used in air that optimized them for underwater use. Electrical striking systems allowed the bells to be connected to surface aids. Canadian experiments showed the practicality of determining direction by comparison of the reception by two receivers mounted on each side of a vessel's bow.
Commercial production
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