Water affects sound waves in several ways. For example, they move several times faster through water than air, and travel longer distances. However, because the human ear evolved to hear in air, water tends to muffle sounds that are otherwise clear in air. Water can also "bend" sound, sending it on a zigzag path instead of a straight line.
Sound Waves and Water
Sound travels in the form of waves resulting from vibrations emanating from objects. If, by chance, an object is struck or moves, it creates a vibration. These disturbances also cause the surrounding molecules of a medium -- air, liquid or solid -- to vibrate. In turn, the ears receive the tremors of these different substances, which send signals to the brain. These are interpreted as “sounds.”
The production of sound is also the same underwater. When you strike an object, vibrations from the underwater object start to bump surrounding water molecules. The submerged human ear does not hear the sound as easily as above ground. It requires a high frequency or a really loud volume for the human ear to hear it.
Speed of Sound
The speed of sound waves depends on the medium used, not on the number of vibrations. Sound travels faster in solids and liquids, and slower in gases. The speed of sound in pure water is 1,498 meters per second, compared to 343 meters per second in air at room temperature and pressure. The compact molecular arrangement of solids and the closer arrangement of molecules in liquids make these molecules respond more quickly to the disturbances of neighboring molecules than in gases.
Temperature and Pressure
As in gases, the speed of sound underwater is also dependent on density and temperature. In gases, the velocity of molecules increase whenever temperature increases; like gases, sound waves travel faster as temperature increases. Unlike gases, water has a greater density because of its molecular arrangement. Thus, sound waves travel faster underwater as the wave bumps through -- and vibrates with more molecules.
Refraction is a complex phenomenon, involving the bending of sound waves as they speed up and slow down when traveling through different mediums. This goes unnoticed in everyday life, yet scientists consider this property important in underwater oceanic study. The speed of sound in the ocean varies. As the ocean gets deeper, the temperature decreases while pressure increases. Sound travels faster at lower depths than at surface level, no matter how sizable the difference in temperature, due to pressure differences. The change in speed changes the direction of the waves, making it hard to determine where the sound originally came from.
Sound and Salinity
Salinity can also be a factor in determining the behavior of sound. In seawater, sound travels up to 33 meters per second faster than in freshwater. Salinity affects sound speed at the surface, especially at river mouths or estuaries. Sound travels faster in the ocean because there are more molecules -- specifically salt molecules -- for waves to interact with, as well as higher surface temperatures.