I have built a loading machine for testing material properties. It applies tension at slow rates (ie 1htz) under load control until the specimen fails. One thing I noticed is that if I tap the material as it is loaded, it seems to give off a different pitch noise when it is under higher tension vs. lower tension of the sinusoidal load. Ive been trying to figure out the theory behind this today for my own curiosity. The speed of sound (C) within a material is defined as C = (B/p) we B is the bulk modulus and p is density. I dont think either of those is changing as the load is applied. Bulk modulus is more of a constant determined from the slope of a stress strain curve while the density should not change as it is a material property. Any ideas what is being observed?