A. Because sound is produced by the vibration of objects, the sound produced by knocking the porcelain bowl is produced by the vibration of the porcelain bowl, so A is correct;

B. The sound produced by the bowl Sound is transmitted through the air, so B is correct;

C. Bowls of different sizes vibrate at different frequencies, so they emit sounds of different tones, so C is correct;

D. Different frequencies The sound propagation speed in the same medium is the same, so D is wrong.

So choose ABC.