Ultrasonic studies of structural phase transitions

The interaction between the ordering quantity and the strain gives rise to certain characteristic variations in the ultrasonically measured sound velocity and attenuation, from which information concerning the critical behaviour can be extracted. Several topics are discussed as examples where ultrasonic experiments have contributed: dependence of the critical behaviour on the dimensionality of the crystal and order-parameter space, marginal dimensionality, and the microscopic mechanisms of the coupling between strain and ordering quantity.