Introduction and Historical Background

In the undersea world the sound wave rules supreme, as is evidenced by the highly evolved acoustic physiology and sophisticated auditory processing capability of fish and marine mammals. Humans are relative latecomers to the world of undersea sound. Leonardo da Vinci (1483) developed a device to listen to approaching ships, writing

If you let your ship stop, and dip the end of a long blowpipe in the water and hold the other end to your ear, then you can hear ships which are very far distant from you.

There are also reports that for centuries Inuit whalers have been using acoustic methods to localize their prey. By placing the butt of a dipped oar against one'sjawbone, the underwater vibrations of the vocalizing whales could be sensed.

Curiously the giants of classical physics (e.g., Newton, Euler, Lagrange, Laplace, Helmholtz) seem to have paid little attention to the subject, instead devoting their energies in acoustics toward musical problems. Even the great acoustical authority, Lord Rayleigh, makes scarce mention of underwater sound in his masterpiece publications The Theory of Sound, Parts 1 and 2, in 1887 and 1896. Indeed, the speed of sound in fresh water was not measured until 1826 (Colladon and Sturm, 1827), and the first crude measurements in sea water came roughly a century later (Wood, 1930).

Developments in ocean acoustics seem to be inseparably tied to matters of military importance, the first of which was the problem of knowing the seafloor adequately so as to avoid vessel grounding. This was the genesis of the acoustic fathometer (echo sounder) invented shortly after the turn of the twentieth century, which was also in great favor with the European royals whose fleets suffered many losses due to grounding. The first measurements of the speed of sound in sea water were largely motivated by echo sounding and sound ranging.

The addition of the submarine to the naval arsenal provided a particularly strong catalyst for advancement in the twentieth century. Though there were some significant developments during and after World War I (Wood, 1930), the independent discovery of the ocean sound channel by both US and Soviet scientists toward the end of World War II brought the field to an entirely new level (Ewing and Worzel, 1948; see also the discussion of the Soviet discovery in Munk et al.