  We probe the nature of the jamming transition of frictional granular media by
studying their vibrational properties as a function of the applied pressure p
and friction coefficient mu. The density of vibrational states exhibits a
crossover from a plateau at frequencies omega \gtrsim omega^*(p,mu) to a linear
growth for omega \lesssim omega^*(p,mu). We show that omega^* is proportional
to Delta z, the excess number of contacts per grains relative to the minimally
allowed, isostatic value. For zero and infinitely large friction, typical
packings at the jamming threshold have Delta z -> 0, and then exhibit critical
scaling. We study the nature of the soft modes in these two limits, and find
that the ratio of elastic moduli is governed by the distance from isostaticity.
