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In the question, it is given that the aircraft has a speed that is greater than Mach 1.

A sonic boom is a sound that can be heard when the velocity at which a moving object is moving crosses a limit. That limit is the value of the velocity of the sound wave. 1 Mach is equal to the speed of sound, which is 343 m/s.

When the aircraft moves through a medium which is air in this case, it will create waves in the medium.

The waves, which are known as pressure waves, will be in front and behind the aircraft.

When the aircraft increases its speed and crosses a limit, it won't allow the waves to move fast enough to give space to the next wave; the waves will just combine and generate a wave whose properties are similar to the shock wave.

That limit is the speed of the sound wave (1 Mach).

For the sonic boom, it needs to cross that limit. It is definitely not a one-time incident. As long as the speed of the aircraft is greater than it will keep generating those waves which are responsible for that loud sound which is almost similar to an explosion.

Thus, an aircraft makes a sonic boom as long as its speed exceeds Mach 1. It is not a one-time occurrence, or it won't make a sonic boom only at an instant if its speed exceeds Mach 1.