The Question
Sound travels through air at about 343 meters per second (767 mph). But in water, it travels at roughly 1,480 meters per second—more than four times faster. In steel, it moves even faster, at about 5,100 meters per second. Why does the medium through which sound travels make such a dramatic difference to its speed?
Detailed Explanation
Sound is not a thing that moves through space—it is a wave of pressure disturbances that propagates through a medium by causing the particles of that medium to bump into each other. When you clap your hands, you compress the air molecules in front of them. Those compressed molecules push into the molecules next to them, which push into the next ones, and so on, creating a chain reaction of compressions and rarefactions (expansions) that travels outward as a sound wave. The speed of this chain reaction depends on two properties of the medium: its elasticity (how quickly it "springs back" after being compressed) and its density (how much mass is packed into a given volume). The relationship is: the higher the elasticity and the lower the density, the faster sound travels. Water is much denser than air—about 800 times denser. This would suggest sound should travel slower in water. However, water is also vastly more elastic (or "stiff") than air—it is about 15,000 times harder to compress. This enormous stiffness more than compensates for the higher density. When a water molecule is pushed, it pushes back almost instantly, passing the disturbance to its neighbor much more quickly than an air molecule would. The net result is that sound travels about 4.3 times faster in water than in air. Steel is even stiffer than water, which is why sound travels fastest of all through solid metals.
Going Deeper
The speed of sound in water is not constant—it varies with temperature, salinity, and pressure. Warmer water is less dense and slightly more elastic, so sound travels faster in warm water than in cold. Increased salinity also increases the speed slightly. Increased pressure (at greater depths) also increases the speed. These variations create a fascinating phenomenon in the ocean called the SOFAR channel (Sound Fixing and Ranging channel). At a depth of about 1,000 meters, the competing effects of temperature and pressure create a minimum in the speed of sound. Sound waves naturally bend toward regions of lower speed, so sound emitted in the SOFAR channel gets trapped and can travel thousands of kilometers with very little loss of energy. Whales exploit this channel to communicate across entire ocean basins. The US Navy used the SOFAR channel during the Cold War to track submarines and to help locate downed pilots at sea. The speed of sound in air also changes with temperature, which is why a symphony orchestra sounds slightly different on a cold night versus a warm one—the instruments are tuned to a specific speed of sound, and if the temperature changes, the pitch of the instruments shifts.
Did You Know?
The fact that sound travels faster in denser solids is why you can hear a train coming by putting your ear to the rail long before you can hear it through the air. Native Americans and early settlers used this technique to detect approaching horses or wagons. Another remarkable application is ultrasound imaging in medicine. Ultrasound machines send high-frequency sound waves into the body and listen for the echoes that bounce back from different tissues. Because the speed of sound in different tissues (muscle, fat, bone, fluid) is known, the machine can calculate the distance to each reflecting surface and build up a detailed image. The same principle is used by bats for echolocation, by dolphins for hunting, and by ships for sonar to map the ocean floor.
