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Student Reading

Speed of Sound

Sound waves can travel through air, water or even metal. In fact, sound can travel through anything made up of molecules. These molecules carry the sound waves by bumping into each other, like Dominoes knocking each other over. Gases, liquids, and solids are all made up of molecules. So how fast does sound travel? It depends on what is carrying the sound. Sound waves do not travel at the same speed through gases, liquids, and solids. The speed of sound depends on how close together the molecules are.

drawing If they are close together, as in a liquid, they can bump into each other more easily and carry the sound wave. In a gas, the molecules are farther apart, so sound waves are slower. Each molecule of a gas has to travel farther to bump into its neighbor. As you can see, to know the speed of sound, you have to pick a material. Let's pick air.

How fast is the speed of sound in air? Well, as one rises higher in the Earth's atmosphere the air becomes thinner and colder. The atmosphere is thinner because the molecules of air are actually farther apart. With molecules farther apart and/or colder, the sound wave goes slower. Therefore, the speed of sound in air depends on how high up in the sky you are. The chart below shows the relationship of temperature and pressure to the speed of sound:

1,000 ft.
5,000 ft.
10,000 ft.
20,000 ft.
25,000 ft.
30,000 ft.

Let's pick sea level as our height. So, how fast is the speed of sound, in air, at sea level? It is about 760 mph. That means it travels about one mile in five seconds! Have you ever heard a band performing its music live on stage, but you are seated a great distance away? Have you noticed that you can see the drummer hit the drum before the sound of the drum reaches your ears? Or have you noticed that thunder can take a long time to reach you after a lightning bolt hits? This is because, although 760 mph seems fast, it still takes a while for sound to travel to you.

photoIf an airplane is flying slower than the speed of sound, we say it is moving at subsonic speed. If it is flying at the speed of sound, it is traveling at sonic speed. If it is flying faster than the speed of sound, it is traveling at supersonic speed. When we refer to the speed of sound, we measure it in Mach numbers. So, if an airplane is traveling at the speed of sound, we say it is flying at Mach 1. If it is traveling at two times the speed of sound, we say it is flying at Mach 2. The word ``Mach'' comes from the Austrian scientist Ernst Mach who studied airflow and the speed at which sound travels.

drawing When an airplane moves through the air at speeds lower than the speed of sound, the air molecules ahead of the plane can get out of the way and pass around the plane. The air behaves like water does when you swim through it.

However, when an airplane moves faster than the speed of sound, the air doesn't have enough time to get out of the way. Instead, it piles up in front of the airplane and forms a shock wave.


So, if an airplane were traveling toward you at the speed of sound, you would not hear it coming. That's because the sound it makes is traveling at the same speed as the plane itself. If that same plane flew toward you at a speed faster than the speed of sound, you would not hear it until after it had passed.


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