Early in the episode there is an explosion. Shortly afterwards, Rigsby reports to Lisbon that it was a "subsonic explosive, probably C4 or Semtex". C4 and Semtex both have a brisance (speed of burn) of roughly 24,500 feet/second whereas the speed of sound is roughly 1,125fps. That would make the explosion roughly 22 times faster than the speed of sound and definitely transonic, not subsonic.