  Acoustic signatures of the single-site quadrupolar Kondo effect in
Y$_{0.966}$Pr$_{0.034}$Ir$_2$Zn$_{20}$ are presented. The elastic constant
($C_{11}-C_{12}$)/2, corresponding to the $\Gamma_3$(E)-symmetry
electric-quadrupolar response, reveals a logarithmic temperature dependence of
the quadrupolar susceptibility in the low-magnetic-field region below $\sim$0.3
K. Furthermore, the Curie-type divergence of the elastic constant down to
$\sim$1 K indicates that the Pr ions in this diluted system have a non-Kramers
ground-state doublet. These observations evidence the single-site quadrupolar
Kondo effect, as previously suggested based on specific-heat and electrical
resistivity data.
