What Causes a Sonic Boom and Where Does the Sound Come From?

There’s a lot of nonsense out there about sonic booms.

The Columbia Encyclopedia 5th edition (1993) says, “An object such as an airplane generates sound. When the speed of the object reaches or exceeds the speed of sound, the object catches up with its own noise” (I wish some politicians would do that), which causes “piled-up sound.” Ridiculous! Will somebody please tell me what a pile of sound is supposed to be?

On the other hand, many people believe that there is a tangible thing called “the sound barrier,” and that when an airplane passes through it it makes a crashing sound, as if crashing through an invisible wall of glass. That’s wrong too. I guess people have been led to think that way because of the word “barrier.”

It was never meant to imply that there was a physical obstruction up there in the air, but only that the speed of sound posed an obstruction to the development of faster and faster airplanes. It was an aeronautical design barrier, not a physical one. Nevertheless, when an airplane “crosses” the sound barrier there certainly is a lot of physical stress on the plane because of the shock wave, as we’ll see.

The actual barrier to supersonic flight is imposed by the speed of sound itself. (And by the way, supersonic means faster than the speed of sound; ultrasonic refers to sound of a higher frequency than humans can hear.) Unique things do indeed happen when an object approaches the speed of sound in air. Here’s what goes on.

Air, of course, consists of molecules: molecules of nitrogen and oxygen, mainly. In all gases, the molecules are flitting frenetically through space in all directions like a swarm of maniacal bees. At room temperature, for example, the oxygen molecules in the air are zipping around at an average speed of 1,070 miles per hour (1,720 kilometers per hour). The hotter the gas is, the faster the bees are flying.

An airplane flying through the air at a paltry few hundred miles or kilometers per hour gives these sprightly molecules plenty of time to get out of the way and let it through; it’s like a person wending his way slowly through a crowd. But when the plane’s speed becomes comparable to the molecules’ own speed, they don’t have time to get out of the way; they just pile up on the front edges of the plane and get pushed along in front of it like snow before a plow.

This rapid pileup of compressed air constitutes an “air shock” or shock wave, which is, in effect, a loud noise. The sound waves radiate out in all directions and can be heard as a “boom” on the ground below. The plane carries its “circle of boom” along with it, so that people on the ground along the plane’s path will hear it when the plane passes over them. This explains away the popular misconception that there is a single boom as the plane crosses the sound barrier. It is a traveling boom.

What does all that have to do with the speed of sound?

Well, sound is nothing but a series of compressions and expansions in the air. If the air’s molecules are flitting around at some particular speed, there will be a limit to how fast that air can be compressed and expanded, because the molecules can’t be compressed and expanded any faster than they can advance and retreat to and from one another. Thus, the speed of the air’s molecules imposes a limit on how fast they will permit sound to pass through, a limit on the speed of sound through that particular air.

Sound will travel faster in warm air than in cool air, because warmer molecules are moving faster and can collide with one another more effectively.

Example: The speed of sound at sea level is 947 miles per hour (1,524 kilometers per hour) at 80 degrees Fahrenheit (27 degrees Celsius), but only 740 miles per hour (1,200 kilometers per hour) at 32 degrees Fahrenheit (0 degrees Celsius). Sound also travels faster in dense high-pressure air because the molecules are closer together and can better transmit compressions.

Putting it all together, then, the speed of sound is fastest in warm, sea-level air and slowest in cold, thin air. That’s why supersonic aircraft operate best at frigid high altitudes, where they don’t have to go quite so fast to exceed the speed of sound.

At 30,000 feet (9 kilometers) above sea level, the air is cold enough and thin enough that the speed of sound is only 680 miles per hour (1,100 kilometers per hour).