Sound is a mechanical wave that requires a medium—like air, water, or a solid—to propagate energy through the vibration of particles. Sound cannot travel through a vacuum, depending instead on particle-to-particle interaction to move forward. In the ocean, the speed of sound is high, averaging around 1,500 meters per second (approximately 3,350 miles per hour). This speed is about four to five times faster than the typical speed of sound in air, which is closer to 343 meters per second.
Why Sound Travels So Quickly in Water
The rapid transmission of sound in water is governed by two fundamental properties of the medium: its density and its compressibility. While increased density generally slows sound down, the low compressibility of water has a much greater influence, accelerating the wave significantly.
In a gas like air, molecules are far apart and highly compressible, requiring more time for the vibration to pass from one molecule to the next. Water is a liquid with molecules that are much more tightly packed together, making it highly incompressible. This close proximity means that a vibrating water molecule can transfer its energy to its neighbor almost instantly.
The reduced compressibility of water creates a stiff medium that resists changes in volume when pressure is applied. This stiffness allows the pressure wave of sound to travel with efficiency, overcoming the drag that higher density might otherwise impose. The rigidity of the liquid state makes water an excellent, high-speed conductor for acoustic energy compared to the loose structure of air.
Environmental Factors That Change the Speed
The speed of sound in the ocean is not constant but varies based on environmental factors: temperature, pressure, and salinity. Temperature is the most influential factor, particularly in the upper layers of the ocean. A rise of just one degree Celsius can increase the speed of sound by about 4.5 meters per second because the change in temperature affects the water’s elasticity significantly.
Pressure increases linearly with depth and also raises the speed of sound by compacting the water, making it stiffer. For every 100 meters of depth, the speed increases by approximately 1.7 meters per second, becoming the dominant factor in the deep ocean where temperature is stable. Salinity, or the salt content, has the smallest effect; higher salt concentrations slightly increase the speed by about 1.3 meters per second per practical salinity unit (PSU).
The combined effect of these variables creates a complex vertical structure known as the sound speed profile, which dictates how sound waves travel. An important feature of this profile is the Sound Fixing and Ranging (SOFAR) channel, a deep-ocean layer where the sound speed is at a minimum, typically found around 1,000 meters in depth. Sound waves generated within this channel are continuously refracted back toward the minimum-speed axis, trapping the energy and allowing it to propagate thousands of miles with minimal loss.
Practical Uses of Underwater Acoustics
The predictability of sound’s behavior in water is utilized extensively across several fields, most notably through the technology of Sound Navigation and Ranging, or sonar.
Active sonar systems emit a short acoustic pulse into the water. By measuring the time it takes for the echo to return, scientists calculate the distance and direction of underwater objects or map the seafloor. This process is relied upon for creating nautical charts, locating shipwrecks, and generating high-resolution bathymetric data.
Passive sonar involves “listening” to the ocean environment using hydrophones, which are specialized underwater microphones. This technique is used to detect and track noise generated by vessels or submarines, as well as for scientific monitoring. In marine biology, passive acoustics record and analyze the vocalizations of marine mammals like whales, helping researchers track migration patterns and study communication. The known speed of sound also allows oceanographers to measure water temperature and currents over long distances, a technique called acoustic tomography.