There is surprisingly little change when crystalline order is lost. However, the nature of the vibrational normal modes does change. Crystalline silicon has vibrations which can be classified by wavevector and by branch. Transverse branches involve atom motions perpendicular to the direction of propagation. These vibrations do not stretch the bonds very much.
Longitudinal branches cause large stretches of bonds.
In the low-frequency part of the spectrum, the crystalline modes clearly separate into these very different categories. The vibrational normal modes of amorphous silicon completely lose this differentiation. Modes with frequency less than 72 meV extend throughout the material. The amplitude falls initially, but saturates at a constant value. Modes with frequency higher than 72 meV are localized.