Measurement of Bulk Modulus and Associated Loss Factor for Rubber and Plastics

Abstract

To measure the bulk modulus of elasticity and associated loss factor of various rubbers and plastics, an apparatus similar to that described by Meyer and Tamm was constructed. For acoustically absorbent materials a complex elastic modulus of the form Ē = E(1 + iη) may be postulated wherein E is the appropriate elastic modulus and η is a measure of the energy dissipated. The apparatus consists of a water-filled steel tube one meter long, with 38-in. wall thickness. Standing waves are set up in the water column by means of a magnetically driven diaphragm mounted at the bottom of the tube. The sample to be measured is inserted at a pressure antinode and, by considering the water column as a resonant transmission line, the bulk modulus and the loss factor are deduced from the change in Q and shift of resonant frequency when the sample is inserted. The operating frequency is 1500 c.p.s. and a beat-frequency technique is used for measuring the small changes in frequency involved. Temperature is closely controlled at 30°C by surrounding the steel tube with a heated water jacket. Results are given for various rubbers and plastics.