# The Speed of Sound in Air

### After reading this section you will be able to do the following:

• Discuss the relationship between the speed of sound and speed of light.
• Describe what the sound barrier is.

## Sound and speed

If you have ever been to a baseball game or sat far away from the stage during a concert, you may have noticed something odd. You saw the batter hit the ball, but did not hear the crack of the impact until a few seconds later. Or, you saw the drummer strike the drum, but it took an extra moment before you heard it. This is because the speed of sound is slower than the speed of light, which we are used to seeing. The same thing is at work during a thunderstorm. Lightning and thunder both happen at the same time. We see the lightning almost instantaneously, but it takes longer to hear the thunder. Based on how much longer it takes to hear thunder tells us how far away the storm is. The longer it takes to hear the thunder, the farther the distance its sound had to travel and the farther away the storm is.

## The sound barrier

The speed of sound through warm air at sea level has been measured at 346 meters per second or 0.346 km per second. That is the same as a car traveling about 780 miles per hour! Even most jet airplanes do not travel that fast. When a plane does go faster than speed of sound, it is said to break the sound barrier and a sonic boom is produced. On October 14, 1947, Chuck Yeager did just that. In a small plane called the X-1, he was the first person to fly faster than the speed of sound and the listeners on the ground were the first to hear the loud shock wave of a sonic boom.

## Why do we see lightning before the thunder?

The flash of light from lightning travels at about 300,000 kilometers per second or 186,000 miles per second. This is why we see it so much sooner than we hear the thunder. If lightning occurs a kilometer away, the light arrives almost immediately (1/300,000 of a second) but it takes sound nearly 3 seconds to arrive. If you prefer to think in terms of miles, it takes sound nearly 5 seconds to travel 1 mile. Next time you see lightning count the number of seconds before the thunder arrives, then divide this number by 5 to find out how far away the lightning is.

### Review:

1. The speed of sound through warm air at sea level has been measured at 346 meters per second while the speed of light is 3*10^8 meters per second.
2. The sound barrier is broken when something goes faster than the speed of sound.