To protect the privacy of the client, strength ratios are used rather than absolute values. The strength was measured by four-point Modulus-of-Rupture (MOR).
As can be seen in figure 1 above, the torsional frequency measured by the Buzz-o-sonic nondestructive testing system gave a much better correlation to strength, than the traditional relative density. It was then possible to identify and remove the weaker filters from the production batches. For example, a 50% increase of minimum strength was achieved by scrapping 35% (pass rate = 65%)of the filters with the lowest torsional frequencies. If the relative strength was used as a strength indicator instead, then the scrap rate would have increased significantly to 55% (pass rate = 45%)! See figure 2 below.
As can be seen, the torsional frequency measured by the Buzz-o-sonic nondestructive testing system gave a much better correlation to strength, than the traditional relative density.