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A sharp change of pressure in a narrow region travelling through a medium, especially air, caused by explosion or by a body moving faster than sound is called a shock wave.

Since the jet is moving with a speed greater than the speed of sound, the observer at point A will not hear the sound of the jet, and that is because the jet should reach A before its sound.

The observer at point B will hear a very loud sound which called a sonic boom because he is standing at the position at which the shock wave will be produced, whereas the observer at C will hear a not very loud sound of the jet. This is because, he/she is not in the position at which the shock wave will reach at that moment.

The observer at point A will not hear the sound of the jet, the observer at point B will hear a very loud sound which called a sonic boom and the observer at C will hear the sound coming from the jet but not a very loud sound.