Abstract
A recovery with time of the amplitude of an ultrasonic third harmonic wave in single crystal NaCl after applying a static deforming stress has been investigated. The recovery is found to be temperature dependent with an activation energy of 0·32 ev. Simultaneous measurements of the attenuation of the fundamental wave revealed a similar recovery for which an activation energy has also been measured. A recovery of the third harmonic amplitude has also been observed following the eventual removal of the deforming stress. The kinetics of the recovery of the third harmonic amplitude and also of the fundamental wave attenuation are found to obey a t ageing law.