Paul Langevin developed the first active ultrasonic underwater sonar transducer for detecting reflected sound from submerged objects and the seafloor.
The concept of detecting submerged objects using acoustic waves impinging on and reflected from an underwater vessel has been argued as inherent in any detailed understanding of acoustic wave propagation in the ocean, sound velocity in liquids, viscous frictional damping of underwater sound, and sound radiation and diffraction patterns. Sir Isaac Newton’s Philosophiae Naturalis Principia Mathematica (1687; The Mathematical Principles of Natural Philosophy, 1729; best known as the Principia) gives a formula for calculating the approximate depth of a water well from the time it takes for a falling stone to be heard striking the water.
Following the experiments by French physicists Daniel Colladon
As reported by Sir William Henry Bragg,
With underwater sound remaining the readiest expedient with at least some precedent, specific questions remained as to whether the mechanical propeller and flow noises generated by target submarines as signal and by the listening vessel as unwanted background noise could be discriminated reliably to permit estimation of submarine distance, angular bearing, and speed. As onshore coastal listening experiments in the English Channel soon showed, unless more than one listening station was used, passive sonar provided, at best, information only on the probable presence of an underwater acoustic sound source, which detectability could be lost easily if the target submarine reduced speed or the listening vessel increased speed.
In 1914, John Joly
Underwater bells, or vibrators, operated electrically, pneumatically, or by wave action, were located near lighthouses, shoals, and wrecks. Under favorable ocean conditions, their signals were received by ships at ranges of up to 15 kilometers (9.32 miles) using hydrophones fitted to or suspended from the ships’ bows. By about 1912, these technologies were in use in the United States, England, France, and Germany, not only for communication but also for navigational use. All these devices, however, generated acoustic signals of frequencies lower than about 600 cycles per second in a frequency band filled with generally strong competing noises that could not, in general, be screened against or distinguished.
Fessenden’s method, one of the most widely used (for example, by the German navy), was the first electroacoustic transducer based on an oscillator constructed from a reciprocating electromagnetic induction motor moving a steel membrane. The year before its introduction in 1913, the Titanic
Active sonar locates underwater objects by sending out sound pulses and detecting the reflection of those pulses from the objects.
English and French antisubmarine warfare engineers soon realized that considerable improvement in performance and reliability would require greater acoustic output intensity as well as a pulse width smaller than the shortest conceivable two-way acoustic travel time. Although English physicists Lord Rayleigh and E. G. Richardson proposed various ideas after the Titanic sinking in 1912, a Russian engineer working for the French navy, Constantin Chilowski, built the first prototype ultrasonic source unit. The unit’s output intensity, however, was still too weak to permit reliable insonification and detection of reflected signals for objects more than about 100 meters (about 109 yards) distant. After initial work for the French artillery ranging service in developing acoustic detectors for explosions, French physicist Paul Langevin joined Chilowski in mid-1915.
Langevin was well known in Europe for his numerous contributions to the study of X rays (1897 to 1898), of ionization in gases and kinetic theory (1899 to 1912), and, notably, of the electromagnetic properties of dielectric and paramagnetic/diamagnetic solids (1903 to 1913). During World War I, the last of these was still considered esoteric pure research of little applicability, although it proved crucial to Langevin’s sonar transducer work. In 1880, French physicists Jacques Curie and Pierre Curie discovered the change in dimensions of crystals subjected to electromagnetic fields. The effect on quartz and Rochelle salt is one of alternating compression and expansion when subjected to an alternating electric current. As the field oscillates, the crystal slab vibrates longitudinally in the three axes of optical symmetry. When the crystal dimension in either direction is a whole-number multiple of half an acoustic wavelength, an electromechanical resonance is set up, resulting in greatly increased output amplitude.
Although Rochelle salt has a higher Curie constant than quartz and, hence, greater sensitivity to smaller signal voltages, it is readily dissoluble in water. Thus, late in 1915, Langevin had the idea of exciting one or more quartz crystals in ultrasonic resonance using a tuned amplifying circuit of the type invented by William du Bois Duddell in 1900. Langevin’s first experiments with an electrostatic spark generator and carbon button (telephone type) receiver were not successful. After several false starts, an improved thermionic valve amplifier finally became available in France in early 1917. The comparatively weak reflected signals were no longer swamped by amplifier self-noise, and in April, 1917, Langevin finally received the first acoustic echoes from a piezoelectric quartz transmitter and hydrophone receiver.
Sound ranging in seawater is limited by intensity decreases, because of inverse square geometric spreading, and by the thermoviscous absorption of acoustic energy by water molecules, which above 5,000 cycles per second results in roughly a 10-decibel loss per 1,000 meters (about 1,094 yards). Although tests over the 20 to 150 kilocycles band were conducted, based on trade-offs between lower in-water sound attenuation and wider acoustic beams with decreasing transducer frequency, operational frequencies between 30,000 and 40,000 cycles per second were chosen.
Close technical liaison was established between English and French scientists coordinated with, but independent of, Langevin. In 1918, English physicist Ernest Rutherford
The total quartz oscillator area was of the order of 400 square centimeters (62 square inches), permitting interception of considerable reflected acoustic energy by the same surface (in receive mode). In June, 1918, using basically this multicrystal transducer, results were obtained that were superior to those achieved using Rutherford and Boyle’s techniques. Shortly before the end of World War I, a prototype active sonar unit housed in a canvas dome was fitted for secret sea trials aboard a shallow-water trawler. Final operating frequencies could be selected between 20,000 and 50,000 cycles per second; at vessel speeds of 15 knots, the final prototype had an effective underwater echo detection range of more than 2.7 kilometers (1.68 miles).
Concurrent with French-English efforts, several civilian and military research groups in the United States were concentrating on lower-noise thermionic amplifiers (M. I. Pupin at Columbia University), improved transducer reliability and focusing (A. P. Willis, J. Langmuir, A. W. Hull, and C. E. Eveleth at General Electric Company), and collateral prototype system development using water-shielded Rochelle salt crystals (Western Electric Company, Wesleyan University). Shortly after the end of World War I, these transducers were tested successfully at the Naval Experimental Station in New London, Connecticut, with reliable acoustic echoes detected from an armored schooner streaming past a sending/listening ship at distances of more than 400 meters (437.44 yards). Although a joint Allied sonar research conference was held in Paris in October, 1918, to plan further cooperative development of “the Langevin apparatus for supersonic signalling and detection of submarines by echo,” when the war ended, the exchange of scientific information ceased.
During the interwar years, American, English, and French active sonar research progressed along strikingly similar lines. Within about three and one-half years of Langevin’s final prototype, Boyle demonstrated successful ultrasonic detection of submerged icebergs, rocky reefs, shipwrecks, and undersea topographic features, giving birth to what has since been designated acoustic fathometry. Soon after, an improved piezoelectric quartz oscillator was employed, with operating frequencies of up to 500,000 cycles per second, to make the first systematic investigations in vivo of chemical and biological effects of ultrasound—the starting point for the acoustic subdiscipline of ultrasonics.
In 1919, Rochelle salt was used as a more efficient pickup receiver, replacing cactus needles and tungsten styli in gramophone transmitters. By the late 1920’s, routine studies by the U.S. Coastal and Geodetic Survey and the German Oceanographic Meteor Survey were employing active sonar fathometry for comprehensive sounding of the Mid-Atlantic Ridge area, and soon thereafter 14-to-25-kilocycle dual-frequency active sonar units were deployed for commercial whale and fish school location. Although limited by ocean and seafloor medium conditions that have become an increasingly major focus of supporting research, since World War II, active sonar has continued to play a critical role in under-ice navigation, bathymetric mapping, and antisubmarine warfare.
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Bragg, William Henry. The World of Sound: Six Lectures Delivered Before a Juvenile Auditor at the Royal Institution. London: G. Bell & Sons, 1930. Bragg, a major developer of underwater acoustic hydrophones during World War II, presents then declassified aspects of the Royal Navy’s work on directional acoustic detection of ship, submarine, and artillery-generated sounds. -
Crandall, Irving B. Theory of Vibrating Systems and Sound. New York: D. Van Nostrand, 1926. Although intended as an intermediate-level physics text, this is an invaluable reference. Bibliography captures a significant portion of American, English, French, and German literatures from both world wars. -
Hackmann, Willem Dirk. Seek and Strike: Sonar, Anti-submarine Warfare, and the Royal Navy, 1914-1954. London: Her Majesty’s Scientific Office, 1984. Comprehensive study of the specific military requirements and technological possibilities and limits imposed on underwater acoustic transduction in submarine detection. Succeeds in communicating to a general audience the central technical, as well as military-political, issues. -
Hunt, Frederick V. Electroacoustics: The Analysis of Transduction and Its Historical Background. Cambridge, Mass.: Harvard University Press, 1954. Although comparatively technical in focus, this is the acknowledged central reference of choice. Especially convenient is a nearly complete listing of historical English and foreign-language publications, reports, and patents. -
Lurton, Xavier. An Introduction to Underwater Acoustics: Principles and Applications. New York: Springer, 2002. Introductory text provides an overview of the physical phenomena affecting underwater acoustical waves and discusses the features of sonar systems. -
Richardson, Edward G. Sound: A Physical Textbook. 4th ed. New York: Longmans, Green, 1947. Elementary reference. Includes specific discussion of the impact of war on ocean acoustics. -
Urick, Robert J. Principles of Underwater Sound. 3d ed. New York: McGraw-Hill, 1983. A central source for the principles and applications of underwater acoustic transduction and propagation.
Anschütz-Kaempfe Invents the First Practical Gyrocompass
First German U-Boat Is Launched
Germany Begins Extensive Submarine Warfare
German Expedition Discovers the Mid-Atlantic Ridge
Radar Is Developed