ABSTRACT
Viewed from one perspective, one can say that, like life itself, ultrasonics came from the sea. On land the five senses of living beings (sight, hearing, touch, smell, and taste) play complementary roles. Two of these, sight and hearing, are essential for long-range interaction, while the other three have essentially short-range functionality. But things are different under water; sight loses all meaning as a long-range capability, as does indeed its technological counterpart, radar. So, by default, sound waves carry out this longrange sensing under water. The most highly developed and intelligent forms of underwater life (e.g., whales and dolphins) over a time scale of millions of years have perfected very sophisticated range-finding, target identification, and communication systems using ultrasound. On the technology front, ultrasound also really started with the development of underwater transducers during World War I. Water is a natural medium for the effective transmission of acoustic waves over large distances; and it is indeed, for the case of transmission in opaque media, that ultrasound comes into its own.
