"And the sound of the shofar, very strong": as in (Ibid. 19:19) "And the sound of the shofar grew exceedingly strong." The normal phenomenon, the longer the sound is sustained the weaker it grows. But here, the longer it was sustained, the stronger it grew. Why so? In the beginning, to accustom the ear to (the progressive strengthening of) the sound. "Vayecherad all the people in the camp": They were shaken.