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What's with the sonic boom, and only when the sound barrier is broken?

Sonic boom is a very interesting phenomenon

By adalberto alejandrinaPublished 4 months ago 6 min read

There was another loud noise in Xiaoshan, Hangzhou. The windows of the residential buildings were shaking. Many people said they were frightened, and some areas of Shaoxing also heard the loud noise almost at the same time.

The most common loud noises in nature are the sounds of volcanic eruptions, earthquakes and thunder, but the Bureau of Meteorology has made it clear that thunder and earthquakes have not been observed.

Therefore, this sound is likely to be "artificial". The artificial sound that can be heard in the span area of ​​Xiaoshan and Shaoxing in Hangzhou is likely to be the sonic boom caused by the plane breaking the sound barrier.

The sonic boom is a very interesting phenomenon, but why does it have to break the sound barrier? Why is it so loud? And I see a lot of people asking, what happened to the "sonic boom cloud" when the plane broke the sound barrier?

What's with the sonic boom?

The sonic boom is the sound produced when the speed of the object exceeds the speed of sound. We usually see the whip whip, and the top of the whip will make a loud "pop" sound. This sound is actually caused by the supersonic speed of the top of the whip.

There are many cases of sonic boom in daily life. There are many things that can easily exceed the speed of sound in an instant. After all, the speed of sound is only a speed, and it is not very fast.

The speed of sound refers to the propagation speed of the vibration source in a specific medium. When the medium is air, atmospheric pressure, humidity, and temperature will affect the speed of sound. In the case of standard atmospheric pressure and 15°C, the speed of sound at this time is the value we are familiar with. - 340m/s.

It is reasonable to say that it is not difficult for an aircraft to fly at a speed of more than 340m/s, but it is actually quite difficult. Even before the first supersonic flight of human aircraft in 1947, many conservative engineers believed that the speed of sound was the limit of human flight.

This is because to exceed the speed of sound requires breaking through a barrier of dramatically increased resistance, known as the sound barrier or sound barrier. When the aircraft passes through this barrier, due to changes in air pressure and fluid mechanics, it is easy to lose control, and even in the worst case, the aircraft will naturally disintegrate.

Therefore, in order to break the sound barrier, the materials of the aircraft structure, as well as the power system and balance system of the aircraft need to be greatly improved.

Why does the sound barrier exist?

Many people wonder why the plane needs to break the sound barrier before it exceeds the speed of sound, and how does this sound barrier come about?

In fact, the reason is very simple. When an object moves in the air, pressure waves will be generated before and after it. When the speed is faster, the waves in front of the object will be compressed more and more, thus forming resistance.

This process is very similar to the scene when the speedboat is rowing on the lake. When the speedboat travels, there are water waves in the front and back, but the water waves in front are very compressed.

When the plane reaches the speed of sound, the waves in front are compressed to the limit, and the air density and resistance are very large, so it becomes very difficult to go any further.

Of course, our attention is easily drawn to the "sonic boom cloud" of an airplane wing, which is actually the condensation of moisture in the air under low pressure.

The wings are the most prone to "sonic boom clouds", because when the wing provides lift, it creates a low pressure area. When the aircraft accelerates to the speed of sound, the surrounding area of ​​the aircraft will also enter a low pressure state (air is compressed to the nose of the aircraft). ), and eventually the two superimpose, and the reduced pressure causes the water in the air to condense, forming a vapor cloud.

But as the plane continued to accelerate and break the sound barrier, the air in the low pressure area was locally disturbed by the shock wave that was generated, and the vapor cloud disappeared, replaced by a sonic boom.

Taking into account the fast flight of the plane, it is very difficult to take this photo, it must be at the right place and the right people.

It is said that John Guy pressed the shutter momentarily when he heard the sonic boom. Not only that, but the weather for testing supersonic flight that day also had a lot to do with it. It was a very humid day with sufficient water molecules.

So one last question, why is the sonic boom created when the sound barrier is broken, and what makes it sound so loud?

When the plane flies, the pressure wave in front of it hinders the flight and causes compression, and when the plane speed exceeds supersonic speed, the compression reaches the limit value and is broken through, resulting in a shock wave (the constant expansion and contraction of air generates shock waves).

The shock wave generated by the sonic boom of a supersonic aircraft can generally reach about 200 decibels. After it pushes back a certain distance, it continues to advance and weakens at the limit of sound - 194 decibels, until it disappears completely.

Because the plane is at a distance, when a sonic boom occurs, it actually has two pops, one from the pressure wave in front of the plane's compression, and the other when the plane leaves the wave.

It's just that the two pops are so short apart that we may not be able to tell them apart.

The two sonic booms together form a cone-like form that radiates outward, and anyone within the radiating range hears the sound.

Since the shock wave continues to be generated when the plane is traveling at supersonic speed, sonic booms will actually be generated wherever it flies, and the coverage will be very wide.

From the analysis of the situation of the loud noise in Hangzhou - people in Xiaoshan and Shaoxing have heard it, it is very likely that it is the sound produced by supersonic flight.

at last

Some people will also wonder if a pilot flying at supersonic speed can hear sound.

They don't really hear the sonic boom, but they can hear the engine because the engine sound waves travel in the cockpit.


About the Creator

adalberto alejandrina

scientific exploration

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