Sound is a form of mechanical energy, which is the movement of energy through a medium caused by vibrations. These vibrations transfer energy between interconnected particles, which is why sound cannot travel through the vacuum of space. Sound can travel through water, and water serves as an excellent medium for this transmission.
How Sound Waves Propagate in Liquid
Sound requires a physical medium, such as a gas, liquid, or solid, for its propagation. In water, sound energy travels as a longitudinal wave, which is a pressure disturbance moving through the substance. This wave is characterized by the molecules of the medium vibrating parallel to the direction the wave is traveling.
The process begins when a vibrating source pushes on the surrounding water molecules, creating an area of high pressure called a compression. This compression transfers energy by pushing on the next layer of molecules. Following the compression is an area of low pressure, or rarefaction, as the molecules momentarily spread apart before the process repeats.
Water is a liquid, and fluids only transmit these longitudinal pressure waves. Water molecules are much closer together than those in a gas like air, allowing for a highly efficient transfer of vibrational energy. This close proximity and strong intermolecular bonds mean that one water molecule can quickly affect its neighbor, enabling the disturbance to move rapidly through the liquid.
Velocity: Sound Speed Comparison
The speed of sound is significantly faster in water than in air. At 20 degrees Celsius, sound travels through air at approximately 343 meters per second. In fresh water, sound moves at about 1,481 meters per second, which is more than four times faster.
This difference in speed is due to the medium’s physical properties, specifically its density and its resistance to compression, known as stiffness. While water is denser than air, its much greater stiffness is the dominant factor. The speed of sound is calculated using a formula that relates stiffness to density, and for water, high stiffness far outweighs the effect of density.
Water is far less compressible than air, making it a stiffer medium for sound waves. Molecular bonds act like stiff springs, snapping back almost instantaneously after a disturbance and accelerating the transfer of the pressure wave. The close spacing of the molecules ensures rapid particle-to-particle interaction, transmitting energy quickly.
Within the ocean, the speed of sound varies with changes in temperature, pressure, and salinity. Increasing pressure, occurring with greater depth, also causes the speed to rise. These factors combine to create complex sound speed profiles.
Practical Uses of Underwater Acoustics
The effective transmission of sound through water has led to numerous practical applications in both human technology and the natural world. Humans utilize the phenomenon through sonar, which stands for sound navigation and ranging. Active sonar transmits acoustic pulses and listens for returning echoes to map the ocean floor, detect shipwrecks, or locate submerged objects like submarines.
Hydrophones, specialized underwater microphones, allow for passive acoustic monitoring in the marine environment. Researchers use these devices to track the movement and population size of marine animals and to study the effects of human-generated noise on ocean life. Hydrophone arrays can pinpoint the location of a vocalizing whale by measuring differences in the arrival time of the sound at each sensor.
Marine mammals, such as whales and dolphins, rely heavily on sound for survival. They use acoustic signals for long-distance communication, navigation, and hunting. Dolphins and toothed whales use echolocation, or “biosonar,” by emitting clicks and listening for returning echoes to form a picture of their surroundings.