Proceedings of the 2nd EAA InternationalSymposium on Hydroacoustics24-27 May 1999, Gdańsk-JurataPOLAND

Underwater Acoustics and its Applications.A Historical Review.

Leif Bjerna, Irina Bjerne

Department of Industrial Acoustics, Technical University of Denmark,

Building 425, DK-2800 Lyngby, Denmark.

l. Introduction

Underwater acoustics is one of the fastest grow-ing fields of research in acoustics. The number ofpublications per year in underwater acoustics is stillincreasing. The relationship to other fields of im-portance for science and technology like oceanogra-phy, seismology and fishery is becoming more close.Every year billions of dollars are spent on the use ofunderwater acoustics by the mineral industry (oiland solid mineral exploration in the sea), by the foodindustry (fishing), by the transportation and recrea-tion industries (navigation and safety devices) andby the worlds navies (undersea warfare). A greatnumber of industrial companies are developing andmanufacturing instrurnents and devices for under-water acoustics, including for instance instrumentsfor inspection and mapping of the seabed, for un-derwater communication, for control of processcs inoff-shore activities, for search and rccovery missionsetc.

This very wide-spread field of undcrwateracoustic activities started many centuries ago withthe humans curiosity about the fundamental natureof sound in the sea. From primitive philosophicaland experimental studies of the velocity of sound inthe sea the development of military technology forundersea warfare during two world wars acceleratedthe underwater acoustics studies. Over the last 45years - strongly supported by the development incomputer technology - a fast increase in experimen-tal and theoretical investigations of all aspects ofunderwater acoustics have taken place and new areasof research like acoustical oceanography andseismo-acoustics have been formed.

2. Studies of underwater acoustics before WorldWar!

The Greek philosopher,. Aristotle (384 - 322B.C.) may have been one of the first to note thatsound could be heard in water as well as in air. In1490 the Italian, Leonardo da Vinci (1452 - 1519)wrote in his notebook: "if you cause your ship tostop, and place the head of a long tube in the waterand place the other extremity to your ear, you willhear ships at great distances". Of course, the back-ground noise of lakes and seas was much lower inhis days than now, when all kinds of ships pollutethe seas with noise. About one hundred years later,Francis Bacon in Narural History supported theidea, that water is the principal medium by whichsounds originating therein reach a human observerstanding nearby.

In the 18th and early 19th century, a few scien-tists became interested in sound transmitted in water,They measured the speed of sound in fresh and saltwater, comparing these with the speed of sound inair already well measured by then. Their soundsources included bells, gunpowder, hunting hornsand human voices. Their own ears usually scrved asreceivers. In 1743, J. A. Nollet conducted a series ofexperiments in order to prove that water is com-pressible. With his head under water, he heard apistol shot, a bell, a whistle and laud shouts. Henated that the intensity of the sound decreased alittle with the depth, thus indicating that the lassmostly occurred at the surface. Alexander Monro,in 1780, tested his ability·to hear sounds underwater.He used a large and a smali bell, which he soundedbath in air and in water. They could be heard in wa-ter, but the pitch sounded lower than in air. He alsoattempted to compare the speed of sound in air and

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in water, and concluded that the two sound speedsseemed to be the same. .

In September 1826 on the Lake Geneva, whenthe water temperature was 8 oC, J.D. Colladon(1802-93), a Swiss physicist, and J.K.F. Sturm(1803-55), a French mathematician, made the first:-videly-known measurements of the speed of soundIn water. A bell hanging down from a boat was usedas transmilter and when striking the bell a flash of

light was made by igniting some gunpowder. Thisflash could be seen by Colladon in a boat situated ata distance of about 10 miles from the transmitter. Hestarted his watch when he saw the flash and stoppedII when he heard the signal about 10 seconds later.His receiver was a trumpet design with one end inthe water and the other in his ear. By means of thisrather primitive set-up they measured the speed ofsound in water at 8 "C to 1435 mis, onIy about 3 misless than accepted today.

t._'=~.l=_~.~~:;f~;:~~, . :.. :- ~.~. ..:~:[~';~~~:.~~~-~~~~-~.=.~~~~

);;''1.0 .• . 0/

Fig. l. I.D. Col/adon and l.K.F. Sums publication of their jirst transmission ofunderwater sound(Annalen der Physik und Chemie, Leiprig, Vol. 12, 1828) . .

During the years 1830 - 1860 the scientistsstarted thinking over some applications of underwa-ter sound. Questions like: "Can the echo of a soundpulse in water be used for determination of the waterdepth or the distance between ships?" Or: ''Can thecommunication between ships be improvcd by un-derwater transmission of sound?" The frustration inrelation to the use of underwater sound for depthmeasurements comes to the surface in chapter 12 ofM.F. Maury's 'Physical Geography of the Sca', 6thEd., 1859, in which he says: "Attempts to fathom theocean; by both sound and pressure, had been made,bul out in 'blue water' every trial was onły a failurerepeated. The most ingenious and beautiful contriv-ances for deep-sea sounding were resorted to. Byexpłoding petards, or ringing bells in the deep sea,when the winds Iw.ere hushed and all was stil!, theecho or reverberation from the bottom might, it beheld, be heard, and the depth determined from therate at which sound travels through water. But,

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though the concussion took place many feet belowthe surface, echo was sileni, and no answer was re-ceived from the bottom ..."

During the last half of the 19th century , when theworld changed from sail to engine driven ships, eon- .cern was expressed about the safety of navigation info.g a~d the danger of collision with other ships orwith icebergs. John Tyndall in the UK and JosephHenry in the USA - in spite of the fact that theyboth In separate investigations found sound in the airto be unreliable - recommended in 1876 to the light-house authorities in both countries, that they shouldadept bigb-power siren warning installations for usein air for all major Iighthouses. This blow to under-water acoustics did not have any serious conse-quences. The possible advantages of signalling bysound in watcr were taken up again in the late 1880sby Lucian Blake and by Thomas Alva Edison inthe USA. Edison invented an underwater device for

communication between ships, but for some un-known reasons the US Government lost the interestin his work.

ReginaJd F essentun

The Fessenden oscillator, circa /9/3.

Ił cross section uj/he worlt.ing pnrts ufthe Fessenden o:rcilfulor:

Fig. 2. Reginald Fessenden and his oscilloscope.

Submerged bells on lightships were introduced toa large extent during the last years of the 19th cen-tury. The noise from these bells could be detected ata great distance through a stethoscope or by meansof simple microphones mounted on a ship's hull.Moreover, when the ship was outfitted with twodetecting devices, one on each side of the hull, itbecame possible to determine the approximatebearing of the lightship by transmitting the soundsseparately to the right and the left ears of the ob-server. Elisha Gray, working with Edison on im-proving the telephone, recognised that the carbon-button microphone in a suitable water-proof eon-tainer could be used as a hydrophone to receive un-derwater bell signals. In 1899, Gray and A.J.Mundy were granted a patent for an electricallyoperated bell for underwater signalling. The com-mercial market now started to motivate the efforts.

The work by Gray and Mundy led in 1901 to theestablishment of the Submarine Signal Cornpany inBoston (now part of the Raytheon .Comp.), but therapid development in radio communication and di-rection finding threatened the commercial market ofundersea acoustic navigation devices.

In 1912, the Submarine Signal Company hiredthe Canadian, R.A. Fessenden to develop a soundsource more efficient than the pneumatically orelectrically operated bell. Fessenden designed andbuild a moving coil transducer for the emission andreception of underwater sound. The Fessenden os-cillator, which was designed somewhat like an elec-tro-dynamic loudspeaker, allowed ships to bothcommunicate with each other by use of the MorseCode and to detect echoes from underwater objects.The power level in water was around 2 kW at theresonance frequency of 540 Hz. By 1914, the echolocation process, known as echo ranging, was farenough developed to locate an iceberg at a distanceof 3.2 Km. This development, unfortunately, cametOGlate to avoid the Titanic disaster.

3. Underwater acoustic studies during WorldWar I.

The outbreak of World War I and the later intro-duced unrestricted submarine warfarę from theGermans side, were the impetus for the developmentof a number of military applications of underwatersound. In France the Russian electrical engineer,Constantin Chilowsky collaborated with PaulLangevin on a project involving a eondenser (elec-trostatic) projector and a carbon-button microphonesituated at the focus of a concave acoustic mirror. In1916 they filed an application on a patent compris-ing the principle of their method and their equip-ment. The same year they had been able to signalunderwater for a distance of 3 Km and to detect ech-oes by reflection from an iron plate at a distance of100 m. As Chilowsky left the project after the filingof the patent, Paul Langevin in 1917 turned his in-terest to the piezoelectric effect - originally discov-ered by Jacques and Pierre Curie in 1880 - in orderto develop transmitters and receiversfor underseause. The newly developed vacuum tube amplifierwas used by Langevin for his quartz receiver and in1918 he completed the development of his steel-quartz-steel transmitter. By means of this transmitterthe range for one-way transmission was increased tomore than 8 Km, and elear submarine echoes wereheard.

A slap of the quartz was sent to Robert W.Boyle in the UK who in 1916 had organised a re-search group to study underwater ultrasonics. Hisexperiments with the quartz receiver were as suc-cessful as Langevin's. The name "ASDIC" for theunderwater detection system was formed during

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these days.

In the USA, Dr. Harvey Hayes had gathered agroup of specialists at the Naval Experimental Sta-tion, New London, with the term of reference: "Todevise as quickly as possible the best of availabletechnology to defeat a U-boat". Hayes and his groupdeveloped the towed hydrophone assembly calledthe "Eel'', and a passive sonar installation using 48hydrophones - hull mounted and towed - weretested. This installation was the most advanced pas-sive sonar system produced during World War L

In Germany, Heinrich Hecht in Kiel developedan e1ectromagnetic membrane transmitter whichduring the war was build into several hundred sur-face ships and submarines and H. Lichte performedrather extensive underwater acoustic studies inwhich he correctly deduced the effects of tempera-ture, salinity and pressure on the speed of sound andhe predicted in 1919 that in deep water the upwardrefraction produced by pressure should produce ex-traordinarily long sound listening ranges. This factwas verified only many years later.

4. Underwater acoustic studies 1918 - 1940.

During the period 1918 - 1940, three uses of un-derwater acoustics based on wartime experienceswere developing extensively, but slowly; namely,echo sounding, sound ranging in the ocean andseismic prospecting. A great practical impetus wasreceived from advances in electronics which madeavailable new devices for amplification, processingand displaying of received underwater signals. M.Marti had in 1919 patented a recorder to be used forecho sounding. This recorder - which turned out tobe of extreme importance to ocean science - eon-sisted of a sheet of paper constraincd to move slowlybeneath a pen writing on the paper, whilc Iraversingthe paper from one side lo the other in a directionperpendicular to the motion of the paper. The penwas driven laterally to its own motion by an electricsignal representing the output from the underwatersound receiver. Viewing side-by-side the successiveechoes, with passage of time, a profile of the seabedcould be produeed. In 1922 the first long echo-sounding profiles were madc while exploring a cableroute between France and Algeria.

The nccd for improved and more rugged high-power transducers instead of the transdueers basedon quarts or Rochelle salt, led G.W. Pierce in theUSA, in 1925, to develop a magnetostrictive oscil-lator operating at 25 kHz with a power level of a fewkW, without the danger of fnłcture of the oscillatingelement as found by the erystal based Iransducers.

During the same period the US Coast and Geo-delie Survey in their attempts to establish geodeticeontrel by henzontal sound ranging was cxperienc-

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ing a strong variability in sound intensity and speedin the sea. Also the Naval Research Laboratory(NRL) seeking to improve submarine hunting,working al 20 - 30 kHz, found the same variability.Some of this variability appeared to show a diurnalcycle, where the equipment in the morning wasworking according to the specifications while it inthe afternoon did not produce any echoes from sub-marines, except at very short ranges. This effeet wasfound in several regions of the ocean. Dr. HarveyHayes and seientists from the young oceanographicinstitution at Woods Hole, including the institutionhead, Columbus Iselin, decided to study these phe-nomena in more depth.

It soon became elear that the upper parts of theoeeans were hcated during the day by the sun leav-ing a layer, 4.5 - 9 meter thiek, with a temperature ofI - 2 "C warrner than the temperature of the moreuniform water layer beneath and with a gradual de-crease in temperature from the surface of the sea. Asthe appearanee of the temperature layer coincidedwith the deterioration of the signal reception, thescientists conc\uded that the warm layer causedsound entering to bend downward, thus producingan acoustic shadow zone in which a submarine couldhide. This discovery in 1937 through the co-opera-tion between acousticians and oceanographers led tothe start of the new field of research called, acousti-cal oceanography. The same year A.F. Spilhaus.build the first bathythermograph, and by the begin-ning of World War II all US naval vessels engagedin antisubmarine work was .equipped with that de-vice.

5. Underwater acoustic studies during WorIdWar II.

The outbreak of the war launched a great activityin underwater acoustics rcsearch in Europe as wellas in thc USA. The hunt for submarines receivedhigh priori ty. The combination of convoys, aircraftpatrol s and the ASDIC gear effectively held off eon-ventional daylight attaeks by the smali number ofGerman submarines. However, the Germans soonlearned to launch night attacks on convoys, using so-ealled wolf-pack techniques. The development of theairborne radar, and in particular the Allied's monop-oly on thc 10 cm radar, beeame a great help in'hunting down the German submarines, of whichGermany during the war lost 781.

The development in Germany of the listeningequipment - in German "Gruppenhorchgerate(GHG)" - with which the cruiser 'Prinz Eugen' forinstance was equipped, gave the Germans some ad-vantages for their surface ships. The crew of 'PrinzEugen' later told, that they were able to track theBritish super-dreadnought 'Hood' over the henzonon thcir passive GHG device ~O miles away for a

long distance and that they had a good range, bear-ing and course on it. They also claimed that they,and not the 'Bismarck', fired the "lucky shot" whiehblew-up the 'Hood'. At any rate, the passive sonarplayed a very important fundamental part in one ofthe great sea battles ofWorld War n.

Apart from the development of underwater armslike the acoustic homing torpedo, the acoustic mineand the seanning sonar, a much better understandingof underwater factors intlueneing sonar performancewas established. Concepts like targct strength, self-noise of ships, reverberation of the underwater envi-ronment etc. were established. Sound propagationunder influence of vertical variation in sound spcedwas studied, for instance using the 'ray theory', bor-rowed from theory of light, which permitted calcu-lations of sound propagation at elevated frequencies.Most of the achievements in underwater acousticsduring World War II were published just after theWar in 23 reports called 'National Defense ResearchCommittee Division 6, Summary Technical Re-ports'. One of these reports - entitled 'Physics ofSound in the Sea' - comprises chapters on deep- andshallow-water acoustic transmission, on intensitytluctuations and on the explosion as a source of un-derwater sound.

6. Underwater acoustic studies after WorldWar II.

Maurice Ewing, profcssor of physics at LehighUniversity in Pennsylvania, was convinced that itwould be possible to propagate sound over hundreds- possibly thousands - of kilometres through theocean if both source and receiver were appropriatelyplaced. World War II had prevented him from test-ing his theory, but in 1945 he propagated soundfrom smali explosions over a distance of more than3000 Km from Eleuthera in the Baharnas to Dakar inWest Africa.

The propagation took place in a ubiquitous per-manent sound channel of the deep ocean. The chan-nel was by Ewing called the 'SOF AR' channel, i.e.the SOund Fixing And Ranging channel. The firstapplication of this discovery was aimed at providinga rescue system for downed-at-sea airmen. From hisintlated rubber boat, the airman should drop smalicartridges over the side, set to explode on the axis ofthe SOFAR channel some 1200 m deep in the NorthAtlantic. Sound from the explosion would be re-fracted back to the channel axis and thc propagationwould only be influenced by cylindrical spreading,Receiving hydrophones positioned on the channelaxis at various positions off the continental shelveswould contribute to the pinpointing of the source. Itshould be noted that also Russian scientists werestudying the undersea sound channel, and in (he late40ties L.M. Brekhovskikh discovered the sound

ehannel in the Pacific Ocean.

Ewing together with J.L. Worzel and severalother colleagues at Woods Hole also studied long-distance sound propagation in shallow water. Basedon their data, Chaim Pekeris eonstructed his normaImode propagation theory. This concept of elasticwave propagation has allowed underwater acousti-cians to model and to understand the complexacoustics of shallow water. Ewings and Worzeisexperiences also formed the basi s of a series of sea-bed geologie structure studies performed mostly inshallow water off the East Coast of the USA. Theco-operation between Ewings group at ColumbiaUniversity and the scientist at Woods Hole tumedout to become very fruitful for underwater seismol-ogy studies. The 'refraction method' and the 'Con-tinuous Seismic Profiler' were developed.

A group around C. F. Eyńng in San Diego hadobserved that diffuse echoes were received from thevolume of (he water. These echoes were arrangedroughly in horizontal layers whose depths were ofthe order of 400 meter at noon, but they migrated tothe surface during twilight and the early evening. Atdawn, they migrated downward to complete a dailycycJe. Due to hel p from marine biologists it waspossible to show, that the responsible scatters weresmali planktonie fish having a swim bladder andliving in the deep water regions of the oceans. Theresearch in the 'deep seattering layers' peaked duringthe period 1949 - 1957. Important contributions. tomarine bio-aeoustics were produced during the sub-sequent years.

The fast developmerit in computer teehnologypermitted a great step forward to be taken in under-water acoustic modelling. The ray, the normaI modeand the parabolic equation methods for underwatersound propagation modelIing were refined and agreat number of computer eodes - range independentand range dependent - were developed. Environ-men tal model s quantifying the boundary conditions(surface and bottom) and the volumetric effects ofthe ocean environment, and ambient noise and re-verberation models have been developed. The de-velopment in acoustical modelIing now seems toaccelerate the model development in fields likeoceanography, seismology and global meteorology.

It is an impossible task in a few lines to givecredit to all the important aspects of underwateracoustics developed during the last 40 years. How-ever, the most important achievements are reportedin several books treating Acoustical Oceanographypublished more recent\y. The list of references com-prises some of the most essential publications on thehistory of underwater acoustics and its applicationsand on the recent developments in acoustical ocean-ography.

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7, Conclusions,

The development over more than 2000 years inunderwater acoustics and its applications has beenbriefly discussed. The importance of the impact ofunderwater warfare and military appIications on thedevelopment in underwater acoustics has been em-phasized. This close relationship has frequentlygiven underwater acoustics a heavy military ele-ment, which sometimes have overshadowed themany important civil applications of underwateracoustic research results. The termination of 'thecoId war' nearly 10 ycars ago has, however, givenbasis for a more peaccful, world-wide co-operationbetween scientists interested in some of the aspectsof great potential in undcrwatcr acoustics as for in-stance an inverse method Iike large seaIe acousticaltomography. which may bring us information onlong term variations in the sea. The sea, which hasprepared so many surprises for us and which is soimportant to us, still earries enough secrets to keepunderwatcr acousticians busy for many, many years.

References.

More extensive historical reviews may be found in:

I. J.B. Hersey, "A chronicie of rnan's use of oceanacoustics", Oceanus, Vol. 20, (2), 1977,8 - 21.

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2. M. Lasky, Review of undersea acoustics to1950. J. Acoust. Soc. Arner., Vol. 61, (2), 1977,283 -97.

3. R.B. Lindsay, "Acoustics - Historical and Phi 10-sophical Development", Douden, Hutchinson &Ross Publ., Stroudburg, PA, 1973.

4. Ziehm, G.H., "Kiel - Ein friihes Zentrum desWasserschalls", Deutsche HydrographischeZeitsehrift, Ergiinzungsheft, Reihe B, Nr. 20,.1988,

More recent textbooks on Acoustical Oeeanographyare for instanee:

5. J. R. Apel, Principles of Ocean Physics, Inter-nat. Geophys. Ser. Vol. 38, Academic Press,1987.

6. L. Brekhovskikh, Yu. Lysanov, Fundamentaisof Ocean Acoustics, Electrophysics Ser., Vol. 8,Springer- Verlag, 1982.

7. C.S. Clay, H. Medwin, Acoustical Oceanogra-phy: Principles and Applications, John Wiley &Sons, 1977.

8. RJ. Urick, Principles of Underwater Sound, 3rdEd. McGraw-Hill, 1983.