This question already has an answer here:
When does a person first hear a plane, flying faster than the speed of sound, as it flies directly overhead? Yes, I know there is a sonic boom and I also know the plane will produce sound as it flies away from that person. But what about the sound that the plane produces as it's flying towards the person? Will the sound be different from the sound heard after the plane has passed?
My dad and I have discussed this and looked at a partial explanation on Youtube, but there wasn't anything - as far as we could find - on the sound produced when the plane was coming towards a person.