Abstract
A sound-velocity variation as a function of the acoustic power is reported. This variation is observed in amorphous PdSi at 10 mK for 500-MHz shear waves. It is shown that the transverse relaxation time T2 of the tunneling defects can be deduced from this effect. A very short value T2=2 nsec is thus obtained in PdSi. It is then confirmed by a hole-burning experiment.