acc. to Master Dydo - 2 Wang Peng qin-making videos

All unfretted stringed instruments need a slight concavity. Silk is usually a bit lower tension than steel on a qin, so the concavity is a bit more. If you take a straight-edge to a violin (high tension strings, short string length) you will see a pronounced concavity. You can see in the Wang Peng qin-building video that he is putting in a good bit of it from the beginning. See http://www.tudou.com/programs/view/dskiDVw7FLw/ and http://www.tudou.com/programs/view/bQ2JYU5D1vQ/ . So you don’t have to ask him! The area of maximum concavity is the area of maximum possible vibration (fattest wave): hui 7.