A sonic boom is usually a phenomenon that accompanies the rapid development of an object such as an airplane when it exceeds the speed of sound (or when it occurs). To explain this phenomenon, it is necessary to have a certain knowledge of fluctuations. We know that sound is a mechanical wave, which is the propagation of density oscillations.
Sound has a certain speed of propagation, and the specific value is related to the medium. Let's take the speed of sound in the air as an example (about 340m s), which is the speed at which the density oscillation travels through the air.
If the velocity of a sound source, such as an airplane, is equal to or exceeds the speed of sound, the density compression caused by the motion of the object will not be able to propagate forward. The result is the formation of a thin compression layer at the contact surface of the object with air, which is dense and has a high temperature, which is known as shock wave.
The compressed layer and the air outside the layer have jumps in density and temperature. When the compression layer passes through ordinary air, the density and pressure of the air will have a process of jumping up and down, and a large amount of energy will be released in this process, making a loud noise. It's called a sonic boom.
Interestingly, during the formation of the shock wave, due to the sudden increase in pressure within its shock layer, the water vapor in the air will condense into small water droplets, forming a beautiful sonic boom cloud. Students who like military or air combat movies must be familiar with this.
In fact, the sonic boom phenomenon is quite common in daily life.
From time to time in the square, we meet someone smoking a spinning top, and we always hear the sound of "popping". This is the sonic boom that the tail end of the whip makes at supersonic speed in an instant.