Will the sound "explode"?

 Cracking the formation of sonic boom "code"

Sound boom is a physical phenomenon that occurs in nature.

Now let's take a look at the situation of the motorboat sailing in the water. Due to the disturbance of the water during the sailing process of the motorboat, water waves will be generated in the water, and the water waves spread in all directions around the motorboat. At the same time, we can see two triangular water traces on both sides of the sailing direction, pushing them outward.

When the aircraft is flying in the air, disturbances will also occur, and the disturbances will also generate sound waves in the air. At this time, the sound waves in the air will also spread around the aircraft.

If it is flying at subsonic speed (the speed is lower than the speed of sound), the speed of the aircraft cannot catch up with the speed of sound wave propagation. At this time, it will be like a motorboat sailing in the water, the sound waves in the air will spread in all directions, and at the same time, two triangular airflow traces will be produced at the tail (the colored bands on the tail we saw during the flight show are the airflow traces).

When an aircraft flies at the speed of sound or supersonic speed, because the aircraft is flying faster than the speed of sound, the sound waves cannot travel in all directions like subsonic flight. These sound waves will all be superimposed on the head and protrusions of the aircraft. A cone shock wave is formed. In the special case of the aircraft flying at the speed of sound, the cone shock wave degenerates into a plane wave.

 

Whether it is a cone shock wave or a plane shock wave, a large amount of acoustic energy is gathered. When this energy is transmitted to the ears of people on the ground, the eardrum feels a sudden increase in air pressure, which makes people feel a short, extremely strong, thunderous explosion, which is called a sonic boom (sonic boom).

Generally speaking, two shock waves are formed when supersonic vehicles fly: one shock wave wraps the front of the nose, and the other shock wave is at the rear of the fuselage. They propagate to the ground almost at the same time, and reflect on the ground, forming an N-shaped reflected shock wave. Therefore, under normal circumstances, when we encounter a sonic boom, we may often hear two "explosions".

"Fairy Fluttering Skirt" on the Aircraft-Sound Blast

Accompanied by the high-speed flight of the aircraft, the pressure and temperature behind the shock wave on the head of the aircraft increase, but there will be low temperature and low pressure areas behind the tail of the aircraft, which changes the air condition behind the tail and makes the air Water vapor condenses under new conditions and forms clouds and mists. This is what people often call "sonic boom clouds (sonic boom clouds)".

Acoustic blast clouds are similar in shape to cone shock waves, presenting a cone shape, perpendicular to the fuselage of the aircraft, and usually only lasting a few seconds. However, not every aircraft will produce a sound burst phenomenon in flight. The appearance of sonic boom is not only related to the intensity of the shock wave, but also closely related to the meteorological conditions of the surrounding air (humidity, temperature, saturation, condensation nucleus, etc.). Therefore, sound bursts can only appear under certain weather conditions.

 

In addition to the sound boom produced by aircraft, sound booms can also be seen in daily life. For example, the coachman flicks the whip to urge the horse to run quickly, and when people in the park exercise with the whip or spin the top, the little leather whips will make a crackling sound. This is the small sound burst again and again. Although the speed of people's arms is not fast, the whip speed of the whip is very fast, which can break through the speed of sound to produce a sonic boom. This may also be the earliest attempt by humans to break through the speed of sound.

The Enchanting Dragon actually brought its own whip?

In fact, humans are not the first species to produce sonic booms. In the Jurassic period 150 million years ago, there was a kind of dinosaur called fascinating dragon. It was huge, its hips could reach 4.5 meters high, its tail was more than ten meters long, and its root was more than 1 meter thick. However, the tip of the tail looked like a modern man’s little finger Same fine. Such a beautiful tip of the tail makes the tail too fragile, so it can not attack the attacker. So what is the function of its tail? Scientists speculate that the fascinating dragon uses its tail as a whip, which can make loud noises when swung in the air to scare away predators, and even show off in front of potential mates during mating, thereby gaining favor.

According to this conjecture, the scientists used computers and mathematical models to simulate the tail flapping of the Confused Dragon, and also used simulation experiments to verify it. The results proved that the speed of the Confused Dragon's tail might indeed exceed the speed of sound, so it might produce sound. burst.

How harmful is the sonic boom?

The sound burst produced by a supersonic vehicle has a wide spread range and a very large energy. Because the shock wave on the aircraft is attached to the body and flies with the aircraft. Therefore, when the speed and altitude are suitable, you will hear the sound of sonic boom along the flight path of the aircraft. For example, when the flying speed of the aircraft is twice the speed of sound and the altitude is 3000 meters, people can hear the sound of sonic boom within a range of 60 kilometers.

When the aircraft is flying supersonic at low altitude, people on the ground can hear the deafening loud noise, which not only affects people’s lives and work, but can also shatter glass and even damage unsturdy buildings, causing direct economic effects. loss. Due to the short duration of the sonic boom, it has little effect on people working outdoors in the open air. For people working indoors, it is different. Although the pressure of the sonic boom is much smaller in the room, it will resonate after multiple reflections on the wall and the duration will become longer. Therefore, it is very important for people working indoors. People's influence is still great.

If we convert the pressure of the sonic boom into a more intuitive sound intensity: a pressure of 100 Pa is approximately equivalent to 133 decibels. At this time, the noise caused by the sonic boom is still very harmful to the human body, especially to the human eardrum. Although the highest noise that the human body can withstand is 155 decibels, clinical medicine has found that when the human body lives and works in a noise environment exceeding 80 decibels, it will suffer from headaches, anxiety, insomnia and increased blood pressure. According to clinical medical statistics, 50% of people who hear a sound explosion may cause deafness.

In addition, in addition to affecting the body and life of people on the ground, the sound blast can also cause certain damage to the aircraft itself. If the aircraft encounters sonic booms for a long period of time, the performance of the metal structure of the aircraft body will decrease. Therefore, people have been working hard to prevent sonic booms during flight.

 

There are many factors that affect the generation of sonic booms, such as flight speed, altitude, route, and surrounding meteorological conditions. Generally speaking, people can control factors such as flight speed, altitude and route, but they cannot control weather conditions and turbulence near the ground. At present, in order to eliminate sonic booms during flight, scientific and technical personnel mainly focus on the design, manufacture and control of supersonic aircraft by improving the aerodynamic shape of the aircraft, the material performance of key components, improving the performance of the aircraft muffler and installing sound insulation devices.

For people working on open ground, there is basically no good way to prevent and reduce the hazards of sonic booms, because the occurrence and experience of sonic booms are very accidental. But for people living in the range of frequent sonic booms, the best way is to build houses that are soundproofed, strong and have few windows. The staff who command the take-off and landing of carrier-based aircraft on the aircraft carrier must wear earmuffs and protect their entire body.