The speed of sound depends on the medium through which the sound waves travel. The properties of a medium that determine the speed of sound are density and compressibility. Density is the amount of material in a unit volume of a substance. Compressibility measures how easily a substance can be crushed into a smaller volume. The denser a medium is and the more compressible it is, the slower the speed of sound is.
In general, liquids and solids are denser than air. But they are also far less compressible. Therefore, sound travels faster through liquids and solids than it does through air. Compared with its speed through air, sound travels about 4 times faster through water and about 15 times faster through steel. The speed of sound through air is commonly measured at sea level at 59 °F. (15 °C). At that temperature, sound travels 1,116 feet (340 meters) per second. However, the speed of sound increases as the air temperature rises. For instance, sound travels 1,268 feet (386 meters) per second through air at 212 °F. (100 °C).
The speed of sound is much slower than the speed of light. In a vacuum, light travels 186,282 miles (299,792 kilometers) per second--almost a million times faster than sound. As a result, we see the flash of lightning during a storm before we hear the thunder. If you watch a carpenter hammering on a distant building, you will see the hammer strike before you hear the sound of the blow.
You may have noticed that the pitch of a train whistle seems higher as the train approaches and lower after the train passes and moves away. The sound waves produced by the whistle travel through the air at a constant speed, regardless of the speed of the train. But as the train approaches, each successive wave produced by the whistle travels a shorter distance to your ears. The waves arrive more frequently, and the pitch of the whistle appears higher. As the train moves away, each successive wave travels a longer distance to your ears. The waves arrive less frequently, producing a lower apparent pitch. This apparent change in pitch produced by moving objects is called the Doppler effect. To a listener on the train, the pitch of the whistle does not change.
Jet airplanes sometimes fly at supersonic speeds. A plane flying faster than the speed of sound creates shock waves, strong pressure disturbances that build up around the aircraft. People on the ground hear a loud noise, known as a sonic boom, when the shock waves from the plane sweep over them.
If you shout toward a large brick wall at least 30 feet (9 meters) away, you will hear an echo. The echo is produced when the sound waves are reflected from the wall to your ears. Generally, when sound waves in one medium strike a large object of another medium--such as the waves in air hitting the brick wall--some of the sound is reflected. The remainder is sent into the new medium. The speed of sound in the two mediums and the densities of the mediums help determine the amount of reflection. If sound travels at about the same speed in both materials and both have about the same density, little sound will be reflected. Instead, most of the sound will be transmitted into the new medium. If the speed differs greatly in the two mediums and their densities are greatly different, most of the sound will be reflected. Sound waves travel much more slowly through air than through brick, and brick is much denser than air. Thus when you shout at the brick wall, most of the sound is reflected.
When sound waves leave one medium and enter another in which the speed of sound differs, the direction of the waves is altered. This change in direction results from a change in the speed of the waves and is called refraction. If sound waves travel slower in the second medium, the waves will be refracted toward the normal. The normal is an imaginary line perpendicular to the boundary between the mediums. If sound travels faster in the second medium, the waves will be refracted away from the normal.
Sound waves can also be refracted if the speed of sound changes according to their position in a medium. The waves bend toward the region of slower speed. You may have noticed that sounds carry farther at night than during a sunny day. During the day, air near the ground is warmer than the air above. Sound waves in the air are bent away from the ground into the cooler air above, where their speed is slower. This bending of the waves results in weaker sound near the ground. At night, air near the ground becomes cooler than the air above. Sound waves are bent toward the ground, enabling sound near the ground to be heard over longer distances.
