  We study by exact diagonalization the localization properties of phonons in
mass-disordered harmonic crystals of dimension $d=1,2,3$. We focus on the
behavior of the typical Inverse Participation Ratio $Y_2(\omega,L)$ as a
function of the frequency $\omega$ and of the linear length $L$ of the
disordered samples. In dimensions $d=1$ and $d=2$, we find that the
low-frequency part $\omega \to 0$ of the spectrum satisfies the following
finite-size scaling $L Y_2(\omega,L)=F_{d=1}(L^{1/2} \omega)$ in dimension
$d=1$ and $L^2 Y_2(\omega,L)=F_{d=2}((\ln L)^{1/2} \omega)$ in dimension $d=2$,
with the following conclusions (i) an eigenstate of any fixed frequency
$\omega$ becomes localized in the limit $L \to +\infty$ (ii) a given disordered
sample of size $L^d$ contains a number $N_{deloc}(L)$ of delocalized states
growing as $N_{deloc}(L)\sim L^{1/2}$ in $d=1$ and as $N_{deloc}(L)\sim
L^2/(\ln L)$ in $d=2$. In dimension $d=3$, we find a
localization-delocalization transition at some finite critical frequency
$\omega_c(W)>0$ (that depends on the disorder strength $W$). Our data are
compatible with the finite-size scaling $L^{D(2)}
Y_2(\omega,L)=F_{d=3}(L^{1/\nu} (\omega-\omega_c))$ with the values $D(2)
\simeq 1.3$ and $\nu \simeq 1.57$ corresponding to the universality class of
the localization transition for the Anderson tight-binding electronic model in
dimension $d=3$.
