[en] Ultrasonic attenuation and velocity measurements on irradiated Al-Zn alloys (0.01, 0.1, and 0.5 at %) indicate a tunneling relaxation of the predominant mixed dumbbell defect at low temperatures, and mixed dumbbell migration at the Stage II anneal temperature. The effect of an internal strain varying with the zinc concentration on the measured decrement and modulus change is striking. Evaluated in the framework of a six-level system, this reveals the simultaneous action of resonance and nonclassical relaxation processes. Using Fe as a probe atom, it is shown that mixed dumbbell dissociation is in an insignificant component of the annealing of this defect. The decrease of the annealing temperature at higher zinc concentrations provides evidence that the mixed dumbbell migrates as a unit during annealing. The energies associated with dumbbell migration and interstitial escape are derived. Further evidence for the migration mechanism is obtained from successive irradiation and annealing