You may have already seen our show on the loudest noises you can hear. Some of those include the sound that follows the pulling of a trigger on a gun; a space shuttle launching into space, and the crack of a volcano erupting. Another of the loudest things we can hear is that of a sonic boom, which can reach 200 decibels. The sound can be compared to an explosion, something some of you old enough to remember may have heard when the fastest passenger plane ever created, Concorde, went overhead. The ‘sound barrier’ had been broken in an aircraft many years prior to that, which is when an object travels faster than 767 miles per hour (1,234 kilometers per hour). Concorde’s top speed was 1,334 mph (2,140 km/h). Today we’ll discuss the topic of speed and sound in this episode of the Infographics Show, What actually is a sonic boom?
Simply put, a sonic boom is the noise we hear when something is travelling at what we call supersonic speed. That is why we also sometimes call a sonic boom a supersonic boom. When we hear a plane travelling faster than the speed of sound, it causes us to hear something that sounds like an explosion. Not all sonic booms are alike, though, as we will see.
Planes that don’t break the sound barrier, meaning those that travel at subsonic speeds, emanate their sound in all directions. As you will know, when a plane is approaching you, it makes a higher pitched noise the closer it gets, and then as it recedes into the distance that noise becomes lower in pitch. This might seem normal to us, but it was something science needed to figure out in the past. We now call it the Doppler Effect, after an Austrian physicist in 1842 worked out why it happens. He didn’t use planes, but a vehicle blowing its horn as it approached people, sounding higher in pitch as it reached the listener and fading to a lower pitch as it passed. The reason? As something gets closer, the waves take less time to reach the observer, which increases the frequency. It all sounds obvious to us now.
Now, when Concorde flew overhead that wasn’t the case. The Doppler Effect was no more. When a plane is travelling slower than the speed of sound, the sound wavelets are both compressed at the front and back of the plane, much like a boat in water when it makes waves at both the front and the back. When a plane is travelling, you can just imagine pressure waves forming at the front and the back of the plane, but as the plane picks up speed, the waves become compressed. At supersonic level, they are virtually squeezed together and they can’t escape. This pressure forms a cone (known as a Mach cone) from the front to the back of the plane, and this cone contains all the pressure. When the plane travelling at supersonic speed travels above you, at first you hear nothing, as all the sound is concentrated in this cone, but as it travels past, you hear the wake of this pressure in the form of a boom. One person put it like this: Think of the boat traveling through the water again. As it sails through the water, it takes time for the waves to hit you at the shore. You are actually not hearing the plane break the speed of sound, but the crack of thunder is when your ears hear the cone. The cone, as we say, extends from the back of the plane and takes some time to hit you, just as that water takes time to reach the shore.
These booms can even shatter windows if the plane is flying low to the ground, and the noise that they emit all depends on the size and shape of the plane, as well as the weather conditions. If you want to see the power of a sonic boom, look no further than the viral video clip of two Brazilian Air Force planes shattering windows at the country’s supreme court. This happened in 2012 when two Mirage 2000s showed off their speed during a public ceremony, to shocking effect.
Of course, not only planes break the sound barrier, and believe it or not a human being has done it. That was when Felix Baumgartner jumped out of a balloon in 2012 from New Mexico. He jumped from a distance of 25 miles (38.6km) and reached a maximum speed of 843.6 mph (1357 kph). On forums it is discussed whether he made a sonic boom or not. Some say yes, some say no because the atmosphere was so thin. However, some videos do reveal a sound that could be a sonic boom.
Many things break the sound barrier, from whips, to ping pong balls fired from cannons, to bullets fired from guns. You’ve probably also heard of the Super Sonic Car, a jet-propelled car also known as the ThrustSSC. In 1997, it became the fastest car on land and the only land vehicle to break the sound barrier. It travelled at a speed of 763 mph (1,228 km/h).
But do all these things each create their own sonic boom?
The super sonic car certainly did. As one writer put it, “The ThrustSSC was silent in its menacing and deliberate approach, until it finally passed you with a wallop of a double BANG as it crushed and broke through the sound barrier.” The reason we might hear a double boom is because we first hear the boom from all that built-up pressure and then a second boom when the pressure returns to normal.
But what about a bullet, do we really hear the sonic boom of a bullet. The answer is yes, but these are just miniature booms, similar to the crack of a whip. In fact, some people say the whip was the first human invention to create a sonic boom. The firer of the gun won’t hear the boom, and only the explosion the gun makes. Another source we found says that as a bullet passes over your head, God forbid, you will hear a crack if it is travelling at supersonic speeds. The same source said that even if that sound is a zipping sound, it is just a miniature boom. This became the subject on one science website, and a poster who said he had military training stated that if the bullet passes over you, you are likely to hear the boom. It’s just not an experiment one would want to conduct!
We hope you enjoyed today’s show, and if you have any experience with supersonic speeds and sonic booms, please let us know in the comments! Also, be sure to check out our other video called How Fast Can Humans Go!