Scale of the Universe. The-Observable-Universe.jpg (3850×1925) Solar System Interactive. We Choose the Moon: Pre-launch. Onboard Video of Shuttle Launch. Unless I'm misreading the speedo, the word "transonic" is used somewhat inaccurately. If I'm not mistaken, the vehicle went transonic quite a bit before this point. The visible shockwaves only go on for a range of about 100mph, which seems too short for the passage from the initial transonic range to supersonic. What the producer seems to mean is that it is going supersonic (passing the sound barrier).
Transonic means that there is both supersonic and subsonic flow somewhere on the vehicle. The Mach Number calculation is not that easy since Mach Number is True Airspeed divided by the Local Speed of Sound.