The speed of light is a constant, always traveling at the rate of 186,000 miles (300,000 km) per second. The speed of sound, on the other hand, is much more fickle. For example, sound waves move at 761.2 miles per hour (1,225 km/h) at sea level when the air temperature is 59 degrees F (15 degrees C). However, heat up the air and they move faster; cool the air, and they slow down. That's why breaking the sound barrier high in the sky is relatively easy: Temperatures are colder, so a lower speed will do the job. Interestingly, sound moves very quickly under water, where the molecules are much denser than they are in the air. Their density means that they bump into one another more quickly, which is how sound travels. Sound travels through water at approximately 4,856 feet per second (1,480 meters/sec) -- more than four times faster than its speed through the air. But because the molecules are denser in water, sound needs more energy (ie. loudness) -- to get moving. That's why faint sounds can't be heard under water like they can above it.
- Sound doesn't exist in space because there aren't any molecules for it to travel through.
- The greatest sound ever recorded was the eruption of the Krakatoa volcano in 1883. The explosion could be heard 3,000 miles (4,828 km) away.
- A whip's snap is the sound of the tip breaking the sound barrier.