Sound travels more than four times faster in water than in air.

Sound is a mechanical disturbance passed through a material. In a fluid, small pressure changes compress and expand neighboring regions, carrying energy without transporting the entire fluid from source to listener. The speed depends on how strongly the fluid resists compression and how much mass must respond to that restoring force.

For a fluid, sound speed is related to the square root of bulk modulus divided by density. A larger bulk modulus means greater resistance to compression and tends to increase speed; greater density, considered alone, tends to decrease it. Water is denser than air, but its much greater stiffness more than offsets that difference.

Near 20 °C, sound travels at about 343 meters per second in air and about 1,480 meters per second in fresh water. Their ratio is about 4.3. These are representative values, not universal constants: temperature changes both, while seawater's salinity and the pressure associated with depth also influence underwater sound speed.

An echo provides a practical way to measure travel time. If a pulse returns from a known distance, its speed follows from the total path length divided by elapsed time. The total path matters because a reflected signal travels outward and back. Ignoring that return journey would introduce a factor-of-two mistake.

Sonar reverses the reasoning. With a suitable estimate of sound speed, the time between transmitting a pulse and receiving its echo gives a distance. Real systems may need a sound-speed profile rather than one value because the water column can have changing temperature, salinity and pressure, altering both timing and the path traveled.

NOAA explains that sound can bend, or refract, as its speed changes with ocean depth. Cooling water can reduce speed through part of the water column, while increasing pressure raises it at greater depths. Under suitable conditions, these opposing trends create a region where sound speed reaches a minimum.

Sound traveling above and below that minimum can bend back toward it, helping form a deep sound channel. Some signals can then propagate over very long distances. This channeling is an important part of the explanation for long-range underwater listening, rather than a simple consequence of water transmitting sound faster than air.

A fast signal does not necessarily travel farther before becoming too weak to detect. Listening range depends on source strength, frequency, absorption, spreading, background noise and the shape of the sound path. Low-frequency whale calls can travel far under favorable conditions, but it is inaccurate to attribute every long-range detection to speed alone.

Crossing between air and water introduces another complication. Much sound energy is reflected at the boundary because the two media have very different acoustic impedances. That helps explain why hearing a person speaking above the surface while you are submerged is not simply a faster version of ordinary listening. Transmission, speed and intelligibility are separate properties.

Speed and loudness answer different questions. Sound can travel rapidly in water while an underwater conversation remains difficult to understand. How efficiently a source couples vibration into the water, how a receiver detects it, and how the path bends all influence the actual listening experience.