Boom! It turns out that sound can be seen?

Around 00:07 on November 3

A loud noise suddenly occurred in the urban area of ​​Pingdingshan, Henan

Many sleeping citizens were awakened

3rd morning

Pingdingshan Emergency Management Bureau issued a statement on the relevant situation

The cause was initially determined to be a sonic boom.

What is a sonic boom?

Are sonic booms dangerous?

What exactly is a sonic boom?

Sonic boom is a physical phenomenon that occurs in nature.

We often see that when a motorboat is sailing in the water, water waves are generated in the water due to the disturbance of the water during the sailing process. 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 outwards.

Image source: China Science Expo

In the same way, if the air is disturbed, sound will spread outward in the form of waves like water, which is sound waves. When an aircraft is flying in the air, disturbances will also be generated, and 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 subsonic flight (speed is lower than the speed of sound), the speed of the aircraft cannot catch up with the propagation speed of sound waves. At this time, it will be like a motorboat sailing in the water, the sound waves will spread in all directions in the air, and at the same time, two triangular airflow traces will be generated at the tail (the colored belt at the tail we see during the flight performance is the airflow trace).

Image source: Screenshot of NetEase News video

When an aircraft flies at the speed of sound or supersonic speed, since the aircraft is flying faster than the speed of sound, these sound waves will all be superimposed together to form a cone shock wave at the head and protrusion of the aircraft. In the special case of an aircraft flying at the speed of sound, the cone shock wave degenerates into a plane wave.

Both conical shock waves and plane shock waves gather a lot of acoustic energy. When this energy reaches the ears of people on the ground, the eardrums feel the sudden increase in air pressure, making people feel a short, extremely strong, thunderous explosion, which is a sonic boom.

Image source: China Science Expo

Generally speaking

When a supersonic vehicle flies, two shock waves are often formed.

A shock wave wrapped around the front of the nose

Another shock wave is at the rear of the fuselage

They spread to the ground at about the same time

and reflected on the ground

Forming an N-shaped reflected shock wave

So when we encounter a sonic boom

Often you may hear two "bangs"

Image source: China Science Expo

The aircraft puts on a "wedding dress" - sonic boom cloud

As the sonic boom is produced by the high-speed flight of the aircraft, the pressure and temperature behind the shock wave at the head of the aircraft increase, but a low temperature and low pressure area appears behind the tail of the aircraft. This changes the state of the air behind the tail, causing the water vapor in the air to condense under the new conditions, forming clouds. This is what people often call the "sonic boom cloud".

Image source: China Science Expo

Sonic boom clouds are similar to cone shock waves in shape, in cone shape, perpendicular to the fuselage of the aircraft, and usually only last for a few seconds. However, not every aircraft will produce a sonic boom cloud phenomenon during flight. The appearance of a sonic boom cloud is not only related to the intensity of the shock wave, but also closely related to the meteorological conditions of the surrounding air. Therefore, a sonic boom cloud can only appear under specific weather conditions.

The sonic boom of a supersonic bomber Source: Haining Science

A fighter plane just passed through the sound barrier. Source: Haining Science

Sonic boom in life

In fact, sonic booms often occur in parks. Many elderly people like to play with spinning tops to keep fit. The crisp snapping sounds are small sonic booms. According to observations, the speed of the whip tip exceeds the speed of sound when the whip is cracked.

Source: Northern Morning News

In 2002, Physical Review Letters published an analysis of the cause of whip sonic booms, showing that the whip sheath can reach speeds of Mach 2 (twice the speed of sound). According to the authors' calculations, the tip can reach speeds of more than 30 times the initial speed of the whip.

Image source: Smarter Every Day whip sheath produces shock waves

What are the hazards of sonic booms?

The sonic boom produced by a supersonic aircraft has a wide range of propagation and is very powerful. Under the right conditions of speed and altitude, the sound of the sonic boom can be heard all the way along the flight path of the aircraft.

For example, when an aircraft flies at twice the speed of sound and at an altitude of 3,000 meters, people can hear the sound of the sonic boom within a range of 60 kilometers.

When an aircraft flies at supersonic speed at low altitude, people on the ground can hear a deafening noise. In serious cases, it may shatter glass or even damage fragile buildings.

If we convert the pressure of a sonic boom into a more intuitive sound intensity: 100 Pa of pressure is approximately equivalent to 133 decibels. At this time, the noise caused by a sonic boom is still very harmful to the human body, especially to the eardrum.

How to eliminate sonic booms?

There are many factors that affect the production of sonic booms, such as flight speed, altitude, route, and surrounding weather conditions. Generally speaking, people can control factors such as flight speed, altitude, and route, but cannot control weather conditions and turbulence near the ground.

At present, in order to eliminate the sonic booms produced by aircraft flight, scientific researchers are mainly focusing on the design, manufacture and control of supersonic aircraft by improving the aerodynamic shape of the aircraft, the material properties of key components, improving the performance of the aircraft muffler and installing sound insulation devices.

Source: Institute of Physics, Chinese Academy of Sciences, Haining Science Popularization, China Science Popularization Expo

Compiled by: Dong Xiaoxian

END

Editor: Muzi