When people buy porcelain, they knock and listen with their hands to distinguish the quality.
The main problem encountered in firing porcelain is the internal cracks in the firing process. Such pottery is fragile, and the porcelain with few cracks is homogeneous when struck, and the sound transmission is very clear. However, if there are cracks, the sound will be different, which is mainly due to the fact that the speed of sound transmission by air is far less than that by solid.