The origin of boson peak -- an excess of density of states over Debyes model in glassy solids -- is still under intense debate, among which some theories and experiments suggest that boson peak is related to van-Hove singularity. Here we show that boson peak and van-Hove singularity are well separated identities, by measuring the vibrational density of states of a two-dimensional granular system, where packings are tuned gradually from a crystalline, to polycrystals, and to an amorphous material. We observe a coexistence of well separated boson peak and van-Hove singularities in polycrystals, in which the van-Hove singularities gradually shift to higher frequency values while broadening their shapes and eventually disappear completely when the structural disorder $eta$ becomes sufficiently high. By analyzing firstly the strongly disordered system ($eta=1$) and the disordered granular crystals ($eta=0$), and then systems of intermediate disorder with $eta$ in between, we find that boson peak is associated with spatially uncorrelated random flucutations of shear modulus $delta G/langle G rangle$ whereas the smearing of van-Hove singularities is associated with spatially correlated fluctuations of shear modulus $delta G/langle G rangle$.
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