... or 0.9 cm of plywood, the middle is a sponge, a thickness of 1 cm, 3 cm, 5 cm, ... the spongeLittle effect on the appearance of a sponge, but with a long time will have deep feelings, good ... but the poor quality of the sponge will be a long time, subsidence caused by the soft bag, ... more expensive than ordinary sponge, (how to identify good and bad sponge,if it's to press down hard, if quickly rebound ...