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What Is the Speed of Sound?

Why does the speed of sound vary? How does sound travel from a place like a speaker to our ears? What is a sonic boom?

By
Sabrina Stierwalt, PhD ,
May 8, 2018
Episode #284

A quick internet search tells me the speed of sound is 343 meters per second or 767 miles per hour. But that speed is far from constant—for example, here on the ground, the speed of sound is much faster than at, say, 35,000 feet, where passenger planes tend to fly. How does sound travel from a place like a speaker to our ears? What happens when you travel faster than the speed of sound? What is a sonic boom?

How Does Sound Travel?

To understand why the speed of sound varies, we first need to look at how sound travels. We can think of the air around us as a collection of particles. Here on Earth, those particles happen to be mostly nitrogen and oxygen. Sound moves through the air by causing those particles to bump into each other. One particle gets bumped, and then it bumps into the next particle it encounters, causing the disturbance, or sound wave, to travel through the air. So the air particles themselves don’t move very far, but the sound wave propagates along them.

A pretty good analogy is “the wave” seen at stadiums. The people may jump out of their seats and throw their arms up, but they don’t move around the stadium, even though the wave does. But without the people, there can be no wave, just like without the air particles there can be no propagation of sound. As we learned from the movie Alien, “in space, no one can hear you scream." This is mostly true—space is predominantly a vacuum or close to it, meaning there are no particles there for sound perturbations to travel along. There are, of course, places in space that are not empty, for example the clouds of dust and gas surrounding newborn stars, where sound could potentially travel.

If you’ve ever listened to an echo, you know that the speed of sound is finite. It takes a noticeable amount of time for sound to leave its source, travel across a canyon, for example, and then return to our ears. At ~60 degrees Fahrenheit (that’s 15.5 degrees Celsius), the speed of sound is around 760 miles per hour (or 1,225 kilometers per hour).

Note that the sound speed depends on the temperature of the air. That is because in colder air, the air particles move more slowly and thus don’t propagate the sound wave as quickly. At 35,000 feet, a typical cruising altitude for passenger planes, the speed of sound is around 295 meters per second or 660 miles per hour. So you don’t have to travel as fast to reach supersonic speeds (i.e. faster than the speed of sound) the higher in the atmosphere you go because air is cooler up there.

The speed of sound also varies depending on the type of gas or medium. For example, the Martian atmosphere is mostly carbon dioxide where the speed of sound is lower than in air. How much lower? You can check out NASA’s online tools that allow you to calculate the speed of sound on different planets and at different altitudes. In liquids, particles are closer together than they are in gases which makes it easier for sound waves to travel. For example, on average, sound travels four times faster in water than it does in air.

Can We Travel Faster Than Sound? How Much Faster?

As sound travels through those air particles causing them to bump into one another, that air is being compressed in what we call a shock wave. Those shock waves don’t only shake the air particles, however, they can also shake whatever may be compressing them, like an aircraft attempting to reach supersonic speeds. Therein lies the challenge in supersonic flight: building an aircraft that is fast and sturdy enough to withstand such shaking.

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