1-23 Table 1-1.—Comparison of Velocity of Sound in Various Mediums MEDIUM TEMPERATURE VELOCITY ºF ºC (FT/SEC) AIR 32 0 1,087 AIR 68 20 1,127 ALUMINUM 68 20 16,700 CARBON DIOXIDE 32 0 856 FRESH WATER 32 0 4,629 FRESH WATER 68 20 4,805 HYDROGEN 32 0 4,219 LEAD 32 20 4,030 SALT WATER 32 0 4,800 SALT WATER 68 20 4,953 STEEL 32 0 16,410 STEEL 68 20 16,850 Q25.   How does density and temperature affect the velocity of sound? ACOUSTICS The science of sound is called ACOUSTICS. This subject could fill volumes of technical books, but we will only scratch the surface in this chapter. We will present important points that you will need for a better understanding of sound waves. Acoustics, like sound, relates to the sense of hearing. It also deals with the production, control, transmission, reception, and the effects of sound. For the present, we are concerned only with the last relationship—the effects of sound. These same effects will be used throughout your study of wave propagation. Echo An ECHO is the reflection of the original sound wave as it bounces off a distant surface. Just as a rubber ball bounces back when it is thrown against a hard surface, sound waves also bounce off most surfaces. As you have learned from the study of the law of conservation of energy, a rubber ball never bounces back with as much energy as the initial bounce. Similarly, a reflected sound wave is not as loud as the original sound wave. In both cases, some of the energy is absorbed by the reflecting surface. Only a portion of the original sound is reflected, and only a portion of the reflected sound returns to the listener. For this reason, an echo is never as loud as the original sound. Sound reflections (echoes) have many applications in the Navy. The most important of these applications can be found in the use of depth finding equipment (the fathometer) and sonar. The fathometer sends sound-wave pulses from the bottom of the ship and receives echoes from the ocean floor to indicate the depth of the ocean beneath the ship. The sonar transmits a pulse of sound energy and receives the echo to indicate range and bearing of objects or targets in the ocean depths. Refraction When sound waves traveling at different velocities pass obliquely (at an angle) from one medium into another, the waves are refracted; that is, their line of travel is bent. Refraction occurs gradually when one part of a sound wave is traveling faster than the other parts. For example, the wind a few feet above