2.2. Measuring setup
High-pressure measurements of the small signal sound speed were performed using the setup designed and constructed by the authors, see Fig. 1. High pressure was generated in a thick-walled cylinder chamber (length = 23 cm, diameter = 1.7 cm, capacity = 52 cm3) with a simple piston and Bridgman II sealing system. Pressure was applied by a manually driven hydraulic press. Volume changes were measured by observation of piston displacement in the chamber. The piston displacement was measured by a digital caliper gauge, with an accuracy of 0.01 mm.