Jets that fly faster than sound compress the air at the front. This forms a head wave. After a certain point of time, the pressure of the air gets discharged like an explosion. As a result, a loud bang takes place and we hear a loud sound. However, ultrasonic sounds are not audible to us. Bats and other animals use ultrasonic waves for orientation. Sound always propagates in the form of waves. Because of this reason we notice a change in the pitch of the siren of the vehicle passing us.
