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What Is a Sonic Boom?

Posted 02-07-2019 at 02:23 AM by Xploit Machine


Ever since humans invented the wheel we have been obsessed with speed. For thousands of years we have been designing and engineering vehicles and techniques to help us move faster. We eventually invented the car which allows us to travel anywhere at speeds of up to a few hundred kilometres per hour. We then quickly realised that this was a grave mistake because all it did was allow us to get to work faster. So we invented even faster airplanes so we can instead go sit on a sunny beach somewhere sipping a Piña Colada.

Ever since humans invented the wheel we have been obsessed with speed. For thousands of years we have been designing and engineering vehicles and techniques to help us move faster. We eventually invented the car which allows us to travel anywhere at speeds of up to a few hundred kilometres per hour. We then quickly realised that this was a grave mistake because all it did was allow us to get to work faster. So we invented even faster airplanes so we can instead go sit on a sunny beach somewhere sipping a Piña Colada.But this incessant pursuit of speed has come with some strange side effects, namely the sonic boom. At sea level sound travels at 340.29 m/s, which is pretty darn fast. But in 1947 some bloke called Chuck Yeager, who was an American test pilot and speed-obsessed lunatic, managed to travel at 428 m/s and break the sound barrier for the very first time, using a Bell X-1 aircraft. Witnesses heard an extremely loud boom and saw something quite strange, a cone like object emanating from the aircraft. This would come to be known as the sonic boom. But what is a sonic boom and what causes it? Let's find out.

Sound travels as a wave, with crests and troughs. If you were to visualise sound waves they would look like sphere's emanating from the source of the sound, with each ring representing a crest of the sound wave. The crests of sound waves appear almost perfectly symmetrical when the object emitting the sound is stationary. However when it starts to move something peculiar happens.The spherical sound wave crests that are traveling in the same direction as the object get squashed together and each wave reaches the observer quicker than the previous wave. Yeah I know, that can be a bit tricky to wrap your head around, but basically, when an object moves, sound gets squashed. The result of this sound wave compression is an increase in the frequency of the sound. This means the sound becomes higher pitched, as the object comes closer to you.

This is why cars approaching you sound higher pitched than when they're driving away from you. Think of it like putting a big burly bloke on a fast roller coaster, his voice may sound deep and full of confidence when he straps himself into that seat. But start that baby up and he starts squealing like a little pig. Okay that's not totally caused by sound wave compression, more a general dislike for roller coasters, but you get the point.

On the flip side however, wave crests that are being emitted in the opposite direction to which the object is traveling, take longer to reach your ears, so you hear the noise at a lower pitch. This is known as the Doppler Effect.But why on Earth am I telling you about all of this? How does it relate to a sonic boom? Well, it's extremely important to understand the doppler effect because a sonic boom is created by exploiting the doppler effect.

Take a high-speed aircraft for example. As it travels through the air it is constantly emitting sound waves. Just like any other sound-emitting object, these waves radiate from the source in a spherical fashion. The sound coming from a jet is mostly a mix of engine noise and air resistance as it pushes air molecules out the way at very high speeds. The latter is what is responsible for that "whooshing" sound you hear as an aircraft passes overhead.The wave crests being emitted in front of the aircraft are being compressed. The faster the aircraft is travelling, the closer together these waves get. But, as long as the vehicle's velocity is not higher than 340.29 m/s, that's the speed of sound, the waves will never touch each other or overlap. Because each wave is traveling fast enough to "get out of the way", before the next wave is emitted.

But what happens when you travel faster than the speed of sound and don't allow each wave enough time to propagate outwards before the next wave is emitted? This is where things start to get funky. When the aircraft exceeds the speed of sound, each sound wave is released ahead of the previous wave, because the object that is emitting the waves is travelling faster than the waves themselves.This causes the waves to get pushed together and create a single sound wave with an enormous amount of energy called a shock wave. If we could see this effect in real life, the sound waves would appear as a cone trailing behind the aircraft. This is known as a "Mach Cone". In the field of aeronautics the speed at which sound travels in known as "Mach One".

The result of this extreme compression of sound waves is a very deep, audible boom, which we refer to as a sonic boom. Any observer watching the aircraft pass by wouldn't actually hear the sonic boom until the very tail of the Mach cone reaches their ears. However, the pilot as well any passengers aboard the aircraft, wouldn't hear the sonic boom at all because they are travelling faster than the actual sound of the boom, so the sound will never catch up to the aircraft and will never reach their ears. They can sit comfortably in their cabin sipping latte's, whilst travelling faster than the speed of sound in relative silence, unbeknown to the fact that they're deafening every poor sod that they pass below.Contrary to popular belief a sonic boom doesn't just happen once, as an object breaks the sound barrier. The Mach cone and the sound of the sonic boom, are present for the entire time that the object is travelling faster than the speed of sound. For as long as the aircraft is travelling at this speed, the boom remains a constant noise that follows the aircraft around.

So that explains the sound, but what about that visual effect of a sonic boom? What causes that weird cone shaped object to appear behind the aircraft when it breaks the sound barrier? This is caused by a rapid change in air-pressure, which is caused by the high velocity of the aircraft. This drop in air pressure causes the temperature in the atmosphere behind the aircraft to drop significantly, causing condensation. So what you're actually seeing is water vapour in the air undergoing rapid condensation.This is not actually directly related to breaking the sound barrier. Condensation clouds from aircraft can be seen at a whole range of velocities. These condensation clouds don't always coincide with a sonic boom. Whether you see this phenomenon or not depends on many factors such as the humidity of the atmosphere and the air temperature.

It's not just high-speed aircraft that can produce sonic booms though. Sonic booms are found in many places in both nature and the man-made world. When you crack a bullwhip, the cracking sound is not caused by the whip hitting itself. The noise you hear is actually a tiny sonic boom, created as the velocity of the whip breaks the sound barrier.

The pistol shrimp is also capable of creating a sonic boom. This tiny little bugger is like the Chuck Norris of the ocean. It can snap its claws together so fast that it creates a sonic boom between its claws. This sound is so loud that it can stun or even kill other sea creatures. That's basically like being able to kill someone by clapping your hands. For goodness sake nature, calm down. That's just a ridiculous amount of power to give to a shrimp. But it just goes to show, anything we can do with technology, nature has probably already beaten us to it.
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