Flooring Hardness Guide
Submitted by: Woody Wall & Wikipedia on March 26, 2012
The Flooring Hardness Guide used the Janka hardness test to measures the resistance of a type of wood to withstand denting and wear. It measures the force required to embed an 11.28 mm (0.444 in) steel ball into wood to half the ball's diameter. This method leaves an indentation. It is commonly used for flooring hardness ratings.
The hardness of wood varies with the direction of the wood grain. Testing on the surface of a plank, perpendicular to the grain, is said to be of "side hardness." Testing the cut surface of a stump is called a test of "end hardness."
Referenced from http://en.wikipedia.org/wiki/Janka_hardness_test