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The relationship between the change in sound propagation and mechanical properties of a gas hardening polyurethane coldbox bound sand core was studied. A novel test rig was constructed to achieve that goal. The sound propagation properties are quantitatively different in every trial of the experiments. Defining criteria for the start and end of the hardening reaction by the first time derivative render two points that correlate well with the start and end of the development of the mechanical hardness of the sand core. [ABSTRACT FROM AUTHOR]