Asynchronous sound

Asynchronous sound effects are not matched with a visible source of the sound on screen. Such sounds are included so as to provide an appropriate atmosphere. It is used at the beginning of the above clip. We can hear the sound of the ambulance before we know the action is taking place in an ambulance. This could have been done in order to provide suspense for the audience because we associate urgency with the sound of an ambulance siren.