The remainder of this Introduction is devoted to a historical account of the devel- opment of sonar. It is the author’s tribute to the work of Constantin Chilowski,4 Daniel Colladon, Pierre and Jacques Curie, Maurice Ewing, Reginald Fessenden, Harvey Hayes, Paul Langevin, H. Lichte, Leonard Liebermann, J. Marcum, Stephen Rice, and Albert Beaumont Wood. It owes its existence in no small part to the detailed accounts of Hunt (1954), Wood (1965), and Hackmann (1984).
The history focuses on developments in France, Britain, and the U.S.A., as these are the places where the main early advances took place, especially during WW1.
Developments in Germany and the U.S.S.R. are mentioned only briefly, partly due to 1.4 A brief history of sonar 7 Sec. 1.4]
4Zhurkovich (2008) transcribes this name as ‘‘K.V. Shilovsky’’.
the difficulty in finding reliable sources for them (in the case of Russian and Soviet acoustics, corrected recently by the publication of theHistory of Russian Underwater Acoustics, edited by Godin and Palmer, 2008).
1.4.1 Conception and birth of sonar (–1918) 1.4.1.1 Discovery and ingenuity
The concept of echo ranging, by which the distance to an object is determined by measuring the time delay to an echo from that object, originates from at least as far back as the 17th century. More recent origins of sonar can be traced to two seemingly unrelated scientific developments in the 19th century, the first being the measurement of the speed of sound in seawater, ca. 1816, by Franc¸ois Beudant, in the French Mediterranean. Beudant used a crude but effective method (illustrated in Figure 1.1), involving an underwater bell and a swimmer waving a flag. A more precise determina- tion, with improved light–sound synchronization (Figure 1.2), was made in 1826 by Colladon (Figure 1.3) and Sturm, in Lake Geneva.5Both measurements are described by Colladon and Sturm (1827), and in both cases the values obtained (1,500 m/s and
Figure1.1. Sketch of Beudant’s experiment ofca.1816 (reprinted fom Girard, 1877).
5Their purpose was not to measure the speed of sound for its own sake, but to determine the bulk modulus of water, which can be calculated from the sound speed if its density is known.
1.4 A brief history of sonar 9 Sec. 1.4]
Figure1.2. Sketch of the Colladon–Sturm experiment of 1826 (reprinted fom Girard, 1877).
Figure1.3. Inventor Reginald Fessenden (left) and physicist Jean Daniel Colladon (right). The image of Fessenden is reprinted fromhttp://www.ieee.ca/millennium/radio/radio_unsung.html, last accessed October 22, 2009,#RadioScientist.
1,435 m/s) are consistent with modern expectation for the respective measurement conditions.
The second important development is the discovery of piezoelectricity by Pierre and Jacques Curie in 1880. Experiments with certain special dielectric crystals (especially quartz and Rochelle salt) revealed that these materials respond to an applied pressure by developing a small potential difference. The converse effect, whereby an applied electric field distorts the shape of the crystal, was predicted shortly afterwards by Gabriel Lippmann and confirmed by the Curie brothers in 1881.
In the late 1890s and early 1900s, some lightships were fitted with underwater bells, which were rung to alert approaching vessels of danger in conditions of poor visibility. In good visibility these sounds provided an indication of distance as well, by estimating the time delay between light and sound signals, as when estimating the distance from an electrical storm by counting seconds to the thunder following a bolt of lightning. These early underwater signaling systems would eventually mature into what we now call sonar.
1.4.1.2 The Titanic and the Fessenden oscillator
The tragic collision and subsequent sinking of RMSTitanicon the night of April 14/
15, 1912 resulted in a flurry of activity and ideas directed at providing advance warning of nearby icebergs. Lewis Richardson filed patents first for an airborne echolocation system in April 1912 and a month later for an underwater one. Reginald Fessenden (Figure 1.3) patented an electromagnetic transducer in 1913 and demon- strated its use by detecting the presence of an iceberg on April 27, 1914 at a distance of ‘‘nearly two miles’’ (i.e., approximately 3–4 km). This device became known as the Fessenden oscillator(Waller, 1989).
1.4.1.3 WW1: a sense of urgency
It took an even greater tragedy, the loss of life inflicted by U-boats during WW1, to provide the focus of intellect and resources that would lead to the development of a working underwater detection system. French and British efforts began in 1915, with Paul Langevin (Figure 1.4) working in Paris with Russian engineer Constantin Chilowski, while A. B. Wood worked with Harold Gerrard in Manchester. The focus of the French research was on echolocation (‘‘active sonar’’ in modern terminology), while the British team concentrated initially on listening devices known as hydro- phones (‘‘passive sonar’’).
At the outset of WW1, Lord Rutherford had assembled an extraordinary group of physicists at his laboratory at the University of Manchester, including the house- hold names Bohr, Geiger, and Chadwick. In his autobiographical account, A. B.
Wood recalls (Wood, 1965): ‘‘It would be difficult to find anywhere such a galaxy of scientific talent, either before or since, working together in the same physics labora- tory at the same time.’’ Of particular relevance here are the arrivals of Wood himself in 1915 and of the Canadian physicist Robert Boyle (Figure 1.4) the following year.
The Board of Invention and Research (BIR) was established in 1915, with
facilities at Hawkcraig (in Fifeshire, Scotland), and expanded in 1917 to a team of more than 80 scientists and technicians working at Parkeston Quay (Harwich, England) under the leadership of Professor W. H. Bragg. Amongst them were Boyle and Wood from Rutherford’s group, responsible, respectively, for research investi- gating echolocation and passive listening.
Boyle made promising initial progress with the Fessenden oscillator, such that by late 1917 a submarine detection had been reported at a distance of 1,000 yd (910 m) (Hackmann, 1984, p. 75).6Nevertheless, this line of work was abandoned because the frequency of Fessenden’s transmitter (1 kHz) was too low to obtain the necessary resolution in bearing for its intended purpose of locating submarines. A high- frequency transducer was needed to achieve this.
In France, Langevin had begun to experiment with quartz early in 1917 after obtaining a small supply from a Paris optician. Quartz is a piezoelectric material suitable for the radiation of high-frequency sound,7 but the unamplified received 1.4 A brief history of sonar 11 Sec. 1.4]
Figure1.4. Physicists Paul Langevin (left) and Robert William Boyle (right). The image of Langevin is reprinted from Anon. (wp, a) and that of Boyle fromhttp://www.100years.ualberta.
ca, last accessed October 26, 2009.
6The yard (symbol yd) is a unit of length defined as 0.9144 meters (see Appendix B).
7Use here of the term ‘‘sound’’ is not restricted to the audible frequency range, but refers also to ‘‘ultrasound’’, which means that the frequency is above the upper limit of normal human hearing (i.e., 20 kHz). In general, it can also refer to sounds below 20 Hz, known as
‘‘infrasound’’. Langevin’s early experiments with quartz (April 1917) were at a frequency of 150 kHz. The frequency was later lowered to 40 kHz in order to reduce absorption.
signals were found to be very weak. Fortunately, a suitable valve amplifier, designed by Le´on Brillouin and G. A. Beauvais,8was made available to Langevin soon after, enabling him to build a system by November 1917 that ‘‘gave a signalling distance of up to six kilometres’’ (Hackmann, 1984, p. 81).
The real breakthrough came when the French and British teams started sharing their findings after a series of high-level meetings held in Washington, D.C. between May and July 1917. Boyle visited Langevin shortly afterwards, when he would have learnt of the French advances. On his return to England, Boyle started working on quartz transducers, and the French amplifier was made available to the British team at Parkeston Quay. The reliance on quartz was such that, until a suitable supply was identified from Bordeaux, Boyle threatened to ‘‘raid the crystal exhibits in several geological museums’’.
Meanwhile, Langevin continued with his own work in Toulon, and by February 1918 had obtained echoes from a submarine using the high-frequency (40 kHz) quartz transducers. Boyle followed suit a month later with a submarine echo from a distance of 500 yd (about 460 m). The Armistice of November 1918 led to the cancellation of plans to fit both British and French navy ships in early 1919, but asdics (as the technology of high-frequency echolocation was then called) was born.9 The term sonarwas coined during WW2.
The origin of the term asdics as an acronym for Anti-Submarine Division -ics, where the ‘‘ics’’ meant ‘‘activities pertaining to’’ in the same way as in ‘‘physics’’, is recounted by Wood (1965). The alternative explanation (for the term asdic, without the second ‘‘s’’) as an acronym for ‘‘Allied Submarine Detection Investigation Committee’’ appears to be a myth created by the British Admiralty in 1939 in response to a question by Oxford University Press (Hackmann, 1984, p. xxv). During the initial development of the sensor at Parkeston Quay, secrecy was such that even the material quartz was referred to by its codename ‘‘asdivite’’.
On the subject of semantics, it is worth mentioning the change in meaning of the word ‘‘supersonic’’ after the end of WW2. Between the two world wars, this term was used in the U.S.A. to mean ‘‘pertaining to sound whose frequency is too high to be heard by the human ear’’, synonymous with the European term ‘‘ultrasonic’’ (Klein, 1968). Today the European term has been adopted worldwide, presumably as a consequence of the modern use of ‘‘supersonic’’ to describe ‘‘faster than sound’’
flight.
The first working active sonar was built in November 1918 by Boyle, a Canadian scientist working in England. Reading an account of the early history of echo rang- ing, however, one cannot help being struck by a series of key contributions made by
8This work was assisted by a wireless expert, Paul Pichon. Having deserted from the French army he found himself importing some American valve amplifiers to his adoptive Germany early in WW1. Realizing the military value of these, he took them instead to France where he—
though immediately arrested—handed over his equipment to the French authorities. These early valves provided the basis for the Beauvais–Brillouin design (Hackmann, 1984, pp. 80–81).
9Boyle’s quartz system was fitted to a trawler on November 16, 1918, five days after the end of WW1.
French scientists, including:
— the earliest known description of the echo-ranging concept, by Mersenne (1636);
— the measurement of the speed of sound in seawater, by Beudant (ca.1816);
— the discovery of piezoelectricity, by the Curie brothers and Lippmann (1880–
1881);
— the development of the valve amplifier, by Beauvais and Brillouin (ca. 1916);
— pioneering research on the use of quartz transducers, including the first ever detection of an echo from a submarine, by Langevin10(1917–1918).
To this impressive list one can add the work of a remarkable statesman named Paul Painleve´ (Figure 1.5). In January 1915, Chilowski had written a letter urging the French government to develop an underwater echolocation device as a defense against U-boats. Recognizing its importance and urgency, Painleve´ forwarded this letter to Langevin without delay, thus facilitating the early Langevin–Chilowski collaboration. Painleve´ also saw the value
in Anglo-French co-operation, requesting a scientific exchange agreement between France and Britain in December 1915. Despite delays caused by opposition from the Admiralty, the agreement, without which the co-operation between Langevin and Boyle might not have flourished, was eventually approved by the British Government in October 1916 (Hack- mann, 1984, p. 39).
1.4.1.4 Origins of passive sonar
By comparison with active sonar, invented in a race against time between Chilowski’s 1915 letter and the first successful French and Brit- ish tests in 1917, the arrival of passive sonar was a gradual affair that lasted centuries. Its 15th-century conception in Leonardo da Vin- ci’s device able to detect ships ‘‘at a great distance’’ was followed by a 400-year gesta- tion, including the 18th-century observations of Benjamin Franklin (see Section 1.4.3.3), and culminating in the listening equipment fitted to shipping vessels at the end of the
1.4 A brief history of sonar 13 Sec. 1.4]
Figure1.5. French statesman and mathematician Paul Painleve´—rep- rinted from Anon. (wp, b). Painleve´
was Minister for Public Instruction and Inventions during the period 1915–1917, and later served two brief periods as Prime Minister in 1917 and 1925.
10Langevin is one of five sonar scientists after whom the Pioneers of Underwater Acoustics Medal, awarded to this day by the Acoustical Society of America, is named. The others are H. J. W. Fay, R. A. Fessenden, H. C. Hayes, and G. W. Pierce. In 1959, Hayes became the first ever recipient of this medal, which was also awarded to Wood (in 1961) and to Urick (1988).
19th century to notify them of the presence of nearby lightships: in 1889, the U.S.
Lighthouse Board described an invention of L. I. Blake comprising an underwater bell and microphone receiver, and a similar system—patented in 1899 (Hersey, 1977)—was developed a few years later by Elisha Gray and A. J. Mundy (Lasky, 1977).11 In common with the echolocation devices of Langevin and Boyle, it was WW1 that provided the final impetus for the birth of passive sonar. An important difference, though, is that underwater listening equipment was put to practical use well before the end of the war. Portable omnidirectional hydrophones were available as early as 1915, and directional ones followed in 1917. Towed hydrophones were operational before the end of WW1, and in 1918 a prototype passive-ranging system was fitted to an American destroyer.
British listening devices used during WW1, based on early American work, were developed at BIR by Wood and Gerrard (occasionally assisted by Rutherford) at Parkeston Quay and by Captain C. P. Ryan at Hawkcraig. To reduce noise, direc- tional hydrophones could be towed behind the ship in a streamlined capsule known as a ‘‘fish’’, developed by G. H. Nash.
Ryan constructed a network of up to 18 underwater listening stations positioned strategically in British coastal waters. These listening stations, each comprising a field of hydrophones, were manned with shore-based operators, who listened for distinc- tive U-boat sounds and reported their position to the nearest anti-submarine flotilla.
Some minefields were also equipped with special listening devices (magneto- phones), with which it was possible to determine the precise moment at which a U-boat was passing overhead. The mines could then be detonated remotely from a shore-based monitoring facility. According to Hackmann (2000), such minefields were responsible for the destruction of four U-boats towards the end of WW1, the first taking place on August 29, 1918.
Early in WW1, Rutherford had proposed the use of an array of multiple hydro- phones, in theory able to both amplify the signal and provide bearing information.
The Royal Navy considered the proposed device too unwieldy and the idea was dropped in Britain, but American scientists pursued it and by the end of the war had developed the most sophisticated listening devices of that time (Hayes, 1920).
This American research took place at the Naval Experimental Station in New London, under the direction of Harvey Hayes.
The property of sound waves that Rutherford wished to exploit is that they retain their phase coherence over distances of at least several wavelengths. The first Amer- ican device to use this property was the ‘‘M-B tube’’, comprising two groups of eight hydrophones each. The (acoustic) signals from each group were combined coherently by a sequence of equal-length delay lines before being presented (binaurally, one coherently summed group in each ear) to a human listener. The construction was such that coherent reinforcement took place from only one direction at a time, so in order to scan over different bearings it was necessary to rotate this device in the water. The inconvenience of the M-B tube—it needed to be lowered into the sea each time it was
11Gray coined the term ‘‘hydrophone’’ to describe their underwater microphone, while Mundy went on to co-found the Submarine Signal Company (now part of Raytheon) in 1901.
used—was overcome by the introduction of variable-length delay lines, which per- mitted the operator to select the direction of listening without any form of mechanical rotation. This meant that the entire device, known as the ‘‘M-V tube’’, could be fixed to a ship’s hull, and used with the ship in motion. The M-V tube had two groups of six hydrophones (later, two groups of ten), the signals from which were presented binaurally in the same way as for the M-B tube.
The capability to use the M-V tube in motion was a huge advantage, but it came at a price—the din from a ship underway. To counter the noise problem the ‘‘U-3 tube’’ (nicknamed the ‘‘eel’’), was invented. The eel comprised two groups of six hydrophones towed behind the ship, thus benefiting from lower noise levels. The U-3’s streamlined housing gave it the appearance of a snake or eel—hence its nickname. The key advance that made this possible was the use of electrical instead of acoustical delay lines, making the equipment less bulky. An experimental device comprising two towed eels and two ship-mounted M-V tubes was fitted to an American destroyer in April 1918 (Figure 1.6). The combined system was capable of passive ranging by triangulation of the two different bearings (Hayes, 1920). The first working sonar capable of localization in range and bearing was neither a French nor a British invention, but an American one.
1.4.2 Sonar in its infancy (1918–1939) 1.4.2.1 Fathometers and fish finders
In peacetime, the thoughts of sonar engineers turned away from U-boats and back initially to maritime safety, and later to fishing. The principle of acoustic echo ranging was applied to measuring water depth, and Fessenden’s oscillator turned out to be 1.4 A brief history of sonar 15 Sec. 1.4]
Figure1.6. Installation of early U.S. passive-ranging sonar with two towed eels of length 40 ft (12 m), and 12 ft (4 m) apart, and two hull-mounted M-V tubes of the same length. The eel was towed about 300–500 ft (100–150 m) behind the ship (reprinted with permission from Lasky, 1977, copyright 1977 American Institute of Physics).
ideally suited to this purpose. For this new application,12its low operating frequency became an advantage because of reduced absorption, and there was no need for directivity because the direction to the seabed is known in advance. The first patent is attributed by Hersey (1977) to A. F. Wells as early as 1907, while Hackmann (1984) credits the first workable system to Alexander Behm in 1912.13The first commercial echo sounders (called ‘‘fathometers’’) were designed by Fessenden at the Submarine Signal Company, using his electromagnetic oscillator as a transmitter in combination with a conventional carbon microphone receiver. A recording echo sounder, enabling a permanent paper record to be kept of the echo sounder output, was invented by Marti and Langevin in 1922.
The next challenge for echo ranging was to be the detection of fish shoals.
Although these produce weaker signals than the seabed, echoes from fish were recorded by the trawlerGlen Kidstonin the North Sea in 1933 (Cushing, 1973). Even before then, echo sounders were used by Belloc to identify the location of fish shoals in the Bay of Biscay (Belloc, 1929a, b), and by the innovative fisherman Captain R. Balls to find shoals of herring (Hersey and Backus, 1962, p. 499).
1.4.2.2 National research laboratories
The 1920s marked the beginning of nationally co-ordinated peacetime research efforts in both Britain and the U.S.A., with both Wood and Hayes continuing at their respective national research laboratories. In Britain, the Applied Research Laboratory (ARL) was founded in 1921, led first by B. S. Smith (1921–1927) and later by Wood. The achievements of this group include the development of the magnetostrictive transducer in 1928 and of the recording echo sounder used on theGlen Kidston.14
The U.S. Naval Research Laboratory (NRL) followed in 1923. The NRL Sound Division, led by Hayes, was responsible for an oddly named listening device called the
‘‘JK projector’’, installed on U.S. Navy ships in 1931 (Klein, 1968). This listening system made use of Rochelle salt—a more efficient piezoelectric material than quartz—housed in a special material known as ‘‘rho-c rubber’’, providing an impe- dance match with water while keeping the transducer dry (Rochelle salt dissolves in water). The device was later adapted to enable its use for echo ranging also, leading to an early American active sonar known as the ‘‘QB’’, produced commercially by the Submarine Signal Company.
1.4.2.3 Temperature and the ‘‘afternoon effect’’
Soon after the end of WW1, both British and American scientists working on the recently developed asdics sets noticed that their performance was inconsistent. A
12The idea itself was not new, but 19th-century attempts had been unsuccessful (Drubba and Rust, 1954; Maury, 1861; Newman and Rozycki, 1998).
13An entire chapter of Hackmann’s book is devoted to the development of echo sounders.
14Wood’s work is honored by the A. B. Wood Medal, awarded annually by the U.K. Institute of Acoustics.