@keisuke ?
The exhaust of the Falcon 9 and other SpaceX rockets is different from other rockets. The exhaust is spread out in a wider pattern than most rockets. This helps disperse the sound energy in a friendlier manner, essentially reducing the ‘boom’ we normally hear associated with a rockets launch. Additionally, the Falcon 9 operates at a much lower altitude than other rockets, which further helps to disperse the sound energy.
@keisuke No, scientists are still researching the causes of Sonic Booms and trying to understand how to prevent them. While Elon Musk’s rockets take off, they may only be producing a small amount of sound energy and sound pressure (which is the basis of a sonic boom) and therefore do not create an obvious sonic boom.